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Department Of Electronics and Communication Engineering NOTES ON LESSON CLASS: III YEAR ECE SU!ECT: CO"#UTER NET$OR%S

CODE: EC&'(&

AI":

To introduce the concept ,terminologies and technologies used in modern data communication and computer networking. OBJECTIVES:  To introduce the students the unctions o dierent la!ers.  To introduce IEEE standard emplo!ed in computer networking.  To make students to get amiliari"ed with dierent protocols and network n etwork components 



Data Co Com mmun muniicat catiion System System Compo mponent nent

Net)or* Tec+nologies There is no generall! accepted ta#onom! into which all computer networks it, $ut two dimensions stand out as important: Transmission Tec+nolog, and Scale. The classiications $ased on these two $asic approaches are considered in this section.

Classification ased on Transmission Tec+nolog,

 

Computer networks can $e $roadl! categori"ed into two t!pes $ased on transmission technologies: Broadcast networks %oint&to&point networks Figure 2-2

nt-to-Point to-Point Lin Line Configurati ratio on Point-

Figure 2-3

Multi ultipoint point Line Line Confi Configu gurat ratio ion n

roadcast Net)or*s

Broadcast network ha'e a single communication channel that is shared $! all the machines on the network as shown in (igs.).).* and ).).+. ll the machines on the network recei'e packets certainrecipient. conte#ts, -pon sent $! an! machine. n address ield short withinmessages, the packetcalled speciies the in intended recei'ing a

 

 packet, machine checks the address ield. I packet is intended or itsel, it processes the  packet i packet is not intended or itsel it is simpl! simpl! ignored. This s!stem generall! also allows possi$ilit! o addressing the packet to all destinations/all nodes on the network0. 1hen such a packet is transmitted and recei'ed $! all the machines on the network. This mode o operation is known as Broadcast 2ode. Some Broadcast s!stems also supports transmission to a su$&set o machines, something known as 2ulticasting. %oint&to&%oint 3etworks  network $ased on point&to&point communication is shown in (ig. ).).4. The end de'ices that wish to communicate are called stations. The switching de'ices are called nodes. Some 3odes connect to other nodes and some to attached stations. It uses (52 or T52 or node&to&node communication. There ma! e#ist multiple paths $etween a source&destination pair or $etter network relia$ilit!. The switching nodes are not concerned with the contents o data. Their purpose is to pro'ide a switching acilit! that will mo'e data rom node to node until the! reach the destination. s a general rule /although there are man! e#ceptions0, smaller, geographicall! locali"ed networks tend to use $roadcasting, whereas larger networks normall! use are point&to&  point communication.

Figure 2-5

Mesh Topology

 

Figure 2-6

Star Topol olog ogy

Figure 2-7

Tree Topology

Classification -ased on Scale

lternati'e criteria or classi!ing networks are their scale. The! are di'ided into 6ocal 6oca l rea /630, 2etropolitan rea 3etwork /230 and 1ide rea 3etworks /130.

 

Figure 2-16-continued

Figure 2-17

Local Area Network

Metropolitan Area Network Network

 

Figure 2-18

Wie Area Network

Local Area Net)or* .LAN/

63 is usuall! pri'atel! owned and links the de'ices in a single oice, $uilding or campus o up to ew kilometers in si"e. These are used to share resources /ma! $e hardware or sotware resources0 and to e#change inormation. 63s are distinguished rom other kinds o networks $! three categories: their si"e, transmission technolog! and topolog!. 63s are restricted in si"e, which means that their worst&case transmission time is $ounded and known in ad'ance. 7ence this is more relia$le as compared to 23 and 13. 8nowing this $ound makes it possi$le to use certain kinds o design that would not otherwise $e possi$le. It also simpliies network management. 63 t!picall! used transmission technolog! consisting o single ca$le to which all machines are connected. Traditional 63s run at speeds o )9 to )99 2$ps /$ut now much higher speeds can $e achie'ed0. The most common 63 topologies are $us, ring and star. 2etropolitan rea 3etworks /230 23 is designed to e#tend o'er the entire cit!. It ma! $e a single network as a ca$le TV network or it ma! $e means o connecting a num$er o 63s into a larger network so that resources ma! $e shared as shown in (ig. ).).. (or e#ample, a compan! can use a 23 to connect the 63s in all its oices in a cit!. 23 is wholl! owned and operated  $! a pri'ate compan! or ma! $e a ser'ice pro'ided $! a pu$lic compan,0 "etropolitan  rea 3etworks /230

The main reason or distinguishing 23s as a special categor! is that a standard has  $een adopted or them. It is 5;5B /5istri$uted ;ueue 5ual Bus0 or IEEE <9*.. 1ide rea 3etwork /130 13 pro'ides long&distance transmission o data, 'oice, image and inormation o'er large geographical areas that ma! comprise a countr!, continent or e'en the whole world. In contrast to 63s, 13s ma! utili"e pu$lic, leased or pri'ate communication de'ices, usuall! in com$inations, and can thereore span an unlimited num$er o miles as shown

 

  13 that is wholl! owned and used $! a single compan! is oten reerred to as enterprise network. The Internet Internet is a collection o networks or network o networks. Various networks such as 63 and 13 connected through suita$le hardware and sotware to work in a seamless manner. Schematic diagram o the Internet is shown in (ig. ).).<. It allows 'arious applications such as e&mail, ile transer, remote log&in, 1orld 1ide 1e$, 2ultimedia, etc run across the internet. The $asic dierence $etween 13 and Internet is that 13 is owned $! a single organi"ation while internet is not so. But with the time the line  $etween 13 and Internet is shrinking, and these terms are sometimes used interchangea$l!. pplications In a short period o time computer networks ha'e $ecome an indispensa$le part o  $usiness, industr!, entertainment as well as a common&man=s lie. lie. These applications ha'e changed tremendousl! rom time and the moti'ation or $uilding these networks are all essentiall! economic and technological. Initiall!, computer network was de'eloped or deense purpose, to ha'e a secure communication network that can e'en withstand a nuclear attack. ter a decade or so, companies, in 'arious ields, started using computer networks or keeping track o in'entories, monitor producti'it!, communication $etween their dierent $ranch oices located at dierent locations. (or e#ample, >ailwa!s started using computer networks $! connecting their nationwide reser'ation counters to pro'ide the acilit! o reser'ation and en?uir! rom an! where across the countr!. nd now ater almost two decades, computer networks ha'e entered a n new ew dimension the! are now an integral part o the societ! and people. In )@@9s, computer network started deli'ering ser'ices to pri'ate indi'iduals at home. These ser'ices and moti'ation or using them are ?uite dierent. Some o the ser'ices are access to remote inormation,  person&person communication, and interacti'e entertainment. So, some o the applications o computer networks that we can see around us toda! are as ollows: 2arketing and sales: Computer networks are used e#tensi'el! in $oth marketing and sales organi"ations. 2arketing proessionals use them to collect, e#change, and anal!"e data related to customer needs and product de'elopment c!cles. Sales application includes teleshopping, which uses order&entr! computers or telephones connected to order processing network, and online&reser'ation ser'ices or hotels, airlines and so on. o n. (inancial ser'ices: Toda!=s inancial ser'ices are totall! depended on computer networks. pplication includes credit histor! searches, oreign e#change and in'estment ser'ices, and electronic und transer, which allow user to transer mone! without going into a  $ank /an automated teller machine is an e#ample o electronic und transer, automatic  pa!&check is another0. 2anuacturing: Computer networks are used in man! aspects o manuacturing including manuacturing process itsel. Two o them that use network to pro'ide essential ser'ices are computer&aided design /C50 and computer&assisted manuacturing /C20, $oth o which allow multiple users to work on a proAect simultaneousl!.

 

5irector! ser'ices: 5irector! ser'ices allow list o iles to $e stored in central location to speed worldwide search operations. Inormation ser'ices:  3etwork inormation ser'ice includes $ulletin $oards and data  $anks.  1orld 1ide 1e$ site oering technical speciication speciication or a new product is an inormation ser'ice. Electronic data interchange /E5I0: E5I allows $usiness inormation, including documents such as purchase orders and in'oices, to $e transerred without using paper. Electronic mail: pro$a$l! it=s the most widel! used computer network application. Teleconerencing: Teleconerencing allows conerence to occur without the participants  $eing in the same place. pplications include simple te#t conerencing /where  participants communicate through their normal ke!$oards and monitor0 and 'ideo conerencing where participants can e'en see as well as talk to other ellow participants. 5ierent t!pes o e?uipments are used or 'ideo conerencing depending on what ?ualit! o the motion !ou want to capture /whether !ou want Aust to see the ace o other ellow  participants or do !ou want to see the e#act acial e#pression0. Voice o'er I%: Computer networks are also used to pro'ide 'oice communication. This kind o 'oice communication is prett! cheap as compared to the normal telephonic con'ersation. Video on demand: (uture ser'ices pro'ided $! the ca$le tele'ision networks ma! include 'ideo on re?uest where a person can re?uest or a particular mo'ie or an! clip at an!time he wish to see. Summar!: The main area o applications app lications can $e $roadl! classiied into ollowing categories:

 

Scientiic and Technical Computing Client Ser'er 2odel, 5istri$uted %rocessing %arallel %rocessing, Communication 2edia Commercial d'ertisement, Telemarketing, Teleconerencing 1orldwide (inancial Ser'ices  3etwork or the %eople /this is the most widel! used application nowada!s0 Telemedicine, 5istance Education, ccess to >emote Inormation, %erson&to&%erson Communication, Interacti'e Entertainment

Open S,stem Interconnection Reference "odel Figure 3-1

Moe oel !S" M

Figure 3-2

!S" Layers

 

Figure 3-3

An #$change %sing %sing the !S" Moel

Figure 3-4

Physical Layer

 

Figure 3-5

Link Layer Data Link

Figure 3-7

Net etwork work Layer

Figure 3-9

Transport Layer

 

Figure 3-12

Presentation on Layer Presentati

Figure 3-13

Applicati ication on Layer

The Open S!stem Interconnection /OSI0 reerence model descri$es how inormation rom a sotware application in one computer mo'es through a network medium to a sotware application in another computer. The OSI reerence model is a conceptual model composed o se'en la!ers, each speci!ing particular network unctions. The model was de'eloped $! the International Organi"ation or Standardi"ation /ISO0 in )@<4, and it is now considered the primar! architectural model or inter&computer communications. The OSI model di'ides the tasks in'ol'ed with mo'ing inormation  $etween networked computers into se'en smaller, more managea$le task groups.  task or group o tasks is then assigned to each o the se'en OSI la!ers. Each la!er is reasona$l! sel&contained so that the tasks assigned to each la!er can $e implemented

 

independentl!. This ena$les the solutions oered $! one la!er to $e updated without ad'ersel! aecting the other la!ers. The OSI >eerence 2odel includes se'en la!ers: 10 Application Layer : %ro'ides pplications with access to network ser'ices. 20 Presentation Layer : 5etermines the ormat used to e#change data among networked

computers. . Session 6a!er: llows two applications to esta$lish, use and disconnect a connection  $etween them called a session. %ro'ides or name recognition and additional unctions like securit!, which are needed to allow applications to communicate o'er the network. 4. Transport 6a!er: Ensures that data is deli'ered error ree, in se?uence and with no loss, duplications or corruption. This la!er also repackages data $! assem$ling long messages into lots o smaller messages or sending, and repackaging the smaller messages into the original larger message at the recei'ing end. +. 3etwork 6a!er: This is responsi$le or addressing messages and data so the! are sent to the correct destination, and or translating logical addresses add resses and names /like a machine name (62E0 into ph!sical addresses. This la!er is also responsi$le or inding a path through the network to the destination computer. *. 5ata&6ink 6a!er: This la!er takes the data rames or messages rom the 3etwork 6a!er and pro'ides or their actual transmission. t the recei'ing computer, this la!er recei'es the incoming data and sends it to the network la!er or handling. The 5ata&6ink 6a!er also pro'ides error&ree deli'er! o data $etween the two computers $! using the  ph!sical la!er. It does this $! packaging the data rom the 3etwork 6a!er into a rame, which includes error detection inormation. t the recei'ing computer, the 5ata&6ink 6a!er reads the incoming rame, and generates its own error detection inormation $ased on the recei'ed rames data. ter recei'ing the entire rame, it then compares its error detection 'alue with that o the incoming rames, and i the! match, the rame has $een recei'ed correctl!. ). %h!sical 6a!er: Controls the transmission o the actual data da ta onto the network ca$le. It deines the electrical signals, line states and encoding o the data and the connector t!pes used. n e#ample is )9BaseT. C+aracteristics of t+e OSI La,ers

The se'en la!ers o the OSI reerence model can $e di'ided into two categories: upper la!ers and lower la!ers as shown in (ig. ).*.*. The upper la!ers o the OSI OS I model deal with application issues and generall! are implemented onl! in sotware. The highest la!er, the application la!er, is closest to the end user. Both users and application a pplication la!er processes interact with sotware applications that contain a communications component. The term upper la!er is sometimes used to reer to an! la!er a$o'e another la!er in the OSI model. The lower la!ers o the OSI model handle h andle data transport issues. The ph!sical la!er and the data link la!er are implemented in hardware and sotware. The lowest la!er, the  ph!sical la!er, is closest to the ph!sical network medium /the network ca$ling, or e#ample0 and is responsi$le or actuall! placing inormation on the medium .

 

  #ROTOCOL The OSI model pro'ides a conceptual ramework or communication $etween computers,  $ut the model itsel is not a method o communication. ctual communication is made  possi$le $! using communication protocols. In the conte#t o data networking, a protocol is a ormal set o rules and con'entions that go'erns how computers e#change inormation o'er a network medium.  protocol implements the unctions o one or more o the OSI la!ers.  wide 'ariet! o communication protocols e#ist. Some So me o these protocols include 63  protocols, 13 protocols, network protocols, and routing protocols. 63 protocols operate at the ph!sical and data link la!ers o the OSI model and deine communication o'er 'arious 63 media. 13 protocols operate at the lowest three la!ers o the OSI model and deine communication o'er the 'arious wide&area media. >outing protocols are network la!er protocols that are responsi$le or e#changing e#chang ing inormation $etween routers so that the routers can select the proper p roper path or network traic. (inall!, network  protocols are the 'arious upper&la!er protocols that e#ist in a gi'en protocol suite. 2an!  protocols rel! on others or operation. (or e#ample, man! routing protocols use network  protocols to e#change inormation $etween routers. This concept o $uilding upon the la!ers alread! in e#istence is the oundation o the OSI model. OSI 2odel and Communication $etween S!stems Inormation $eing transerred rom a sotware application in one computer s!stem to a sotware application in another must pass through the OSI la!ers. (or e#ample, i a sotware application in S!stem  has inormation to transmit to a sotware application in S!stem B, the application program in S!stem  will pass its inormation to the application la!er /6a!er 0 o S!stem . The application la!er then passes the inormation to the presentation la!er /6a!er 0, which rela!s the data to the session la!er /6a!er 0, and so on down to the ph!sical la!er /6a!er )0. t the ph!sical la!er, the inormation is placed on the ph!sical network medium and is sent across the medium to S!stem B. The ph!sical la!er o S!stem B remo'es the inormation rom the ph!sical medium, and then its ph!sical la!er passes p asses the inormation up to the data link la!er /6a!er *0, which passes it to the network la!er /6a!er +0, and so on, until it reaches the application la!er /6a!er 0 o S!stem B. (inall!, the application la!er o S!stem B passes the inormation to the recipient application program to complete the communication  process. Interaction $etween OSI 2odel 6a!ers  gi'en la!er in the OSI model generall! communicates with three other OSI la!ers: the la!er directl! a$o'e it, the la!er directl! d irectl! $elow it, and its peer la!er in o other ther networked computer s!stems. The data link la!er in S!stem , or e#ample, communicates with the network la!er o S!stem , the ph!sical p h!sical la!er o S!stem , and the data link la!er in S!stem B. (igure).*.+ illustrates this e#ample. Ser'ices and ser'ice access points One OSI la!er communicates with another la!er to make use o the ser'ices pro'ided $! the second la!er. The ser'ices pro'ided $! adAacent la!ers help a gi'en OSI la!er

 

communicate with its peer la!er in other computer s!stems. Three $asic elements are in'ol'ed in la!er ser'ices: the ser'ice user, the ser'ice pro'ider, and the ser'ice access  point /S%0. In this conte#t, the ser'ice user is the OSI la!er that re?uests ser'ices rom an adAacent OSI la!er. The ser'ice pro'ider is the OSI la!er that pro'ides ser'ices to ser'ice users. OSI la!ers can pro'ide ser'ices to multiple ser'ice users. The S% is a conceptual location at which one OSI la!er can re?uest the ser'ices o another OSI la!er. OSI 2odel 6a!ers and Inormation E#change The se'en OSI la!ers use 'arious orms o control co ntrol inormation to communicate with their  peer la!ers in other computer s!stems. This control inormation inormation consists o speciic re?uests and instructions that are e#changed $etween peer OSI la!ers. Control inormation t!picall! takes one o two orms: headers and trailers. 7eaders are  pretended to data that has $een passed down rom upper la!ers. Trailers are appended to data that has $een passed pa ssed down rom upper la!ers. n OSI la!er is not re?uired to attach a header or a trailer to data rom upper la!ers. 7eaders, trailers, and data are relati'e concepts, depending on the la!er that anal!"es the inormation unit. t the network la!er, or e#ample, an inormation unit consists o a 6a!er + header and data. t the data link la!er, howe'er, all the inormation passed down  $! the network la!er /the 6a!er + header and the data0 is treated as data. In other words, the data portion o an inormation unit at a gi'en OSI la!er potentiall! can contain headers, trailers, and data rom all the higher la!ers. This is known as encapsulation. (igure )& shows how the header and data rom one la!er are encapsulated into the header o the ne#t lowest la!er. Inormation E#change %rocess The inormation e#change process occurs $etween peer OSI la!ers. Each la!er in the source s!stem adds control inormation to data, and each la!er in the destination s!stem anal!"es and remo'es the control inormation rom that data. I s!stem  has data rom sotware application to send to S!stem B, the data is passed to the application la!er. The application la!er in S!stem  then communicates an! control inormation re?uired $! the application la!er in S!stem B $! pre&pending a header to the data. The resulting inormation unit /a header and the data0 is passed to the presentation la!er, which pre&pends its own header containing control inormation intended or the  presentation la!er in S!stem B. The inormation unit grows in si"e as each la!er pre&  pends its own header /and, in some cases, a trailer0 that contains control inormation inormation to $e used $! its peer la!er in S!stem S !stem B. t the ph!sical la!er, the entire inormation unit is  placed onto the network medium. The ph!sical la!er in S!stem B recei'es the inormation unit and passes it to the data link la!er. The data link la!er in S!stem B then reads the control inormation contained in the header pre&pended $! the data link la!er in S!stem . The header is then remo'ed, and the remainder o the inormation unit is passed to the network la!er. Each la!er perorms the same actions: The la!er reads the header rom its peer la!er, strips it o, and passes the remaining inormation unit to the ne#t highest h ighest la!er. ter the application la!er

 

 perorms these actions, the data is passed to the recipient sotware sotware application in S!stem B, in e#actl! the orm in which it was transmitted $! the application in S!stem . 3unctions of t+e OSI La,ers

(unctions o different la,ers of t+e OSI model are presented in t+is section0 #+,sical La,er

The ph!sical la!er is concerned with transmission o raw $its o'er a communication channel. It speciies the mechanical, electrical and procedural network interace speciications and the ph!sical transmission o $it streams o'er a transmission medium connecting two pieces o communication e?uipment. In simple terns, the ph!sical la!er decides the ollowing: )  3um$er o pins and unctions o each pin o the network connector /2echanical0 Signal 6e'el, 5ata rate /Electrical0 1hether simultaneous transmission in $oth directions Esta$lishing and $reaking o connection 5eals with ph!sical transmission There e#ist a 'ariet! o ph!sical la!er protocols such as >S&*+*C, >s&44@ standards de'eloped $! Electronics Industries ssociation /EI0. ).*.4.* 5ata 6ink 6a!er The goal o the data link la!er is to pro'ide relia$le, eicient communication $etween adAacent machines connected $! a single communication channel. Speciicall!: ). Droup the ph!sical la!er $it stream into units called rames. 3ote that rames are nothing more than packets== or messages==. B! con'ention, con'en tion, we shall use the term rames== when discussing 566 packets. *. Sender calculates the checksum and sends checksum together with data. The checksum allows the recei'er to determine when a rame has $een damaged in transit or recei'ed correctl!. +. >ecei'er recomputes the checksum and compares it with the recei'ed 'alue. I the! dier, an error has occurred and the rame is discarded. 4. Error control protocol returns a positi'e or negati'e acknowledgment to the sender.   positi'e acknowledgment indicates the rame was recei'ed without errors, while a negati'e acknowledgment indicates the opposite. . (low control pre'ents a ast sender rom o'erwhelming a slower recei'er. (or e#ample, a supercomputer can easil! generate data aster than a %C can consume it. . In general, data link la!er pro'ides ser'ice to the network la!er. The network la!er wants to $e a$le to send packets to its neigh$ors without worr!ing a$out the details o getting it there in one piece.

 

Design Issues elo) are t+e some of t+e important design issues of t+e data lin* la,er:

a0. >elia$le 5eli'er!: (rames are deli'ered to the recei'er relia$l! and in the same order as generated $! the sender. Connection state keeps track o sending order and which rames re?uire retransmission. (or e#ample, recei'er state includes which rames ha'e $een $ een recei'ed, which ones ha'e not, etc.  $0. Best Eort: The recei'er does not return acknowledgments to the sender, so the sender has no wa! o knowing i a rame has $een successull! deli'ered. 1hen would such a ser'ice $e appropriateF ). 1hen higher la!ers can reco'er rom errors with little loss in perormance. That is, when errors are so inre?uent that there is little to $e gained $! the data link la!er  perorming the reco'er!. It is Aust as eas! to ha'e higher la!ers deal with occasional loss o packet. *. (or real&time applications re?uiring $etter ne'er than late== semantics. Old data ma!  $e worse than no data. c0. 5eli'er! Thecknowledged recei'er returns an acknowledgment rame to the sender indicating that a data rame was properl! recei'ed. This sits somewhere $etween the other two in that the sender keeps connection state, $ut ma! not necessaril! retransmit unacknowledged rames. 6ikewise, the recei'er ma! hand o'er recei'ed packets to higher la!er in the order in which the! arri'e, regardless o the original sending order. T!picall!, each rame is assigned a uni?ue se?uence num$er, which the recei'er returns in an acknowledgment rame to indicate which rame the C8 reers to. The sender must retransmit unacknowledged /e.g., lost or damaged0 rames. d0. (raming The 566 translates the ph!sical la!er=s raw $it stream into discrete units /messages0 called rames. 7ow can the recei'er detect rame $oundariesF Various techni?ues are used or this: 6ength Count, Bit Stuing, and Character stuing. e0. Error Control Error control is concerned with insuring that all rames are e'entuall! e'en tuall! deli'ered /possi$l! in order0 to a destination. To achie'e ach ie'e this, three items are re?uired: cknowledgements, Timers, and Se?uence 3um$ers. 0. (low Control (low control deals with throttling the speed o the sender to match that o the recei'er. -suall!, this is a d!namic process, as the recei'ing speed depends on such changing actors as the load, and a'aila$ilit! o $uer space. 6ink 2anagement In some cases, the data link la!er ser'ice must $e opened== $eore use: The data link la!er uses open op en operations or allocating $uer space, control $locks, agreeing on the ma#imum message si"e, etc.

 

S!nchroni"e and initiali"e send and recei'e se?uence num$ers with its peer at the other end o the communications channel. Error 5etection and Correction In data communication, error ma! occur $ecause o 'arious reasons including attenuation, noise. 2oreo'er, error usuall! occurs as $ursts $u rsts rather than independent, single $it errors. (or e#ample, a $urst o lightning will aect a set o $its or a short time ater the lightning strike. 5etecting and correcting errors re?uires redundanc! /i.e., sending additional inormation along with the data0. There are two t!pes o attacks against errors: Error 5etecting Codes: Include enough redundanc! redundan c! $its to detect errors and use C8s and retransmissions to reco'er rom the errors. E#ample: parit! encoding. G Error Correcting Codes: Include enough redundanc! to detect and correct errors. E#amples: C>C checksum, 25. Net)or* La,er

The $asic purpose o the network la!er is to pro'ide an end&to&end communication capa$ilit! in contrast to machine&to&machine communication pro'ided $! the data link la!er. This end&to&end is perormed using two $asic approaches ap proaches known as connection& oriented or connectionless network&la!er ser'ices. 3our issues:

). Interace $etween the host and the network /the network la!er is t!picall! the $oundar!  $etween the host and su$net0 *. >outing +. Congestion and deadlock 4. Internetworking / path ma! tra'erse dierent network technologies /e.g., Ethernet,  point&to&point links, etc.0 Net)or* La,er Interface

There are two $asic approaches used or sending packets, which is a group o $its that includes data plus source and destination addresses, rom node to node called 'irtual circuit and datagram methods. These are also reerred to as connection&oriented and connectionless network&la!er ser'ices. In 'irtual circuit approach, a route, which consists o logical connection, is irst esta$lished $etween two users. 5uring this esta$lishment  phase, the two users not onl! agree to set up a connection $etween them $ut also decide upon the ?ualit! o ser'ice to $e associated with the connection. The well&known 'irtual& circuit protocol is the ISO and CCITT H.* speciication. The datagram is a sel& contained message unit, which contains suicient inormation or routing rom the source node to the destination node without dependence on pre'ious message interchanges  $etween them. In contrast to the 'irtual&circuit method, where a i#ed path is e#plicitl! set up $eore message transmission, se?uentiall! transmitted messages can ollow completel! dierent paths. The datagram method is analogous to the postal s!stem and the 'irtual&circuit method is analogous to the telephone s!stem.

 

O4er4ie) of Ot+er Net)or* La,er Issues:

The network la!er is responsi$le or routing packets rom the source to destination. The routing algorithm is the piece o sotware that decides where a packet goes ne#t /e.g., which output line, or which node on a $roadcast channel0. (or connectionless networks, the routing decision is made or each datagram. (or connection&oriented networks, the decision is made once, o nce, at circuit setup time. Routing Issues:

The routing algorithm must deal with the ollowing issues: ) Correctness and simplicit!: networks are ne'er taken down indi'idual parts /e.g., links, routers0 ma! ail, $ut the whole network should not. Sta$ilit!: i a link or router ails, how much time elapses $eore the remaining routers recogni"e the topolog! changeF /Some ne'er do.0 (airness and optimalit!: an inherentl! intracta$le pro$lem. 5einition o optimalit! usuall! doesn=t consider airness. 5o we want to ma#imi"e channel usageF 2inimi"e a'erage dela!F 1hen we look at routing in detail, we=ll consider $oth adapti'e&&those that take current traic and topolog! into consideration&&and non&adapti'e algorithms. Congestion T+e net)or* la,er also must deal )it+ congestion:

1hen more packets enter an area than can $e processed, dela!s increase and  perormance decreases. I the situation continues, the su$net ma! ha'e no alternati'e $ut to discard packets. I the dela! increases, the sender ma! /incorrectl!0 retransmit, making a $ad situation e'en worse. O'erall, perormance degrades $ecause the network is using /wasting0 resources  processing packets that e'entuall! get discarded. Internetworking (inall!, when we consider internetworking && connecting dierent network technologies together && one inds the same pro$lems, onl! worse: %ackets ma! tra'el through man! dierent networks Each network ma! ha'e a dierent rame ormat Some networks ma! $e connectionless, other connection oriented Routing

>outing is concerned with the ?uestion: ? uestion: 1hich line should router J use when orwarding a packet to router 8F There are two t!pes o algorithms: dapti'e algorithms use such d!namic inormation as current topolog!, load, dela!, etc. to select routes. In non&adapti'e algorithms, routes ne'er change once initial routes ha'e $een selected. lso called static routing.

 

O$'iousl!, adapti'e algorithms are more interesting, as non&adapti'e algorithms a lgorithms don=t e'en make an attempt to handle ailed links. Transport La,er

The transport le'el pro'ides end&to&end communication $etween processes e#ecuting on dierent machines. lthough the ser'ices pro'ided $! a transport protocol are similar to those pro'ided $! a data d ata link la!er protocol, there are se'eral important dierences  $etween the transport and lower la!ers: ). -ser Oriented. pplication programmers interact directl! with the transport la!er, and rom the programmers perspecti'e, the transport la!er is the network==. Thus, the transport la!er should $e oriented more towards user ser'ices than simpl! relect what the underl!ing la!ers happen to pro'ide. /Similar to the $eautiication principle in operating s!stems.0 *. 3egotiation o ;ualit! and T!pe o Ser'ices. The user and transport protocol ma! need to negotiate as to the ?ualit! ?u alit! or t!pe o ser'ice to $e pro'ided. E#amplesF  user ma! want to negotiate such options as: throughput, dela!, protection, priorit!, relia$ilit!, etc. +. Duarantee Ser'ice. The transport la!er ma! ha'e to o'ercome ser'ice deiciencies o the lower la!ers /e.g. pro'iding relia$le ser'ice o'er an unrelia$le network la!er0. 4. ddressing $ecomes a signiicant issue. That is, now the user must deal with it $eore it was $uried in lower le'els. Two solutions: -se well&known addresses that rarel! i e'er change, allowing programs to wire in== addresses. (or what t!pes o ser'ice does this workF 1hile this works or ser'ices that are well esta$lished /e.g., mail, or telnet0, it doesn=t allow a user to easil! e#periment with new ser'ices. -se a name ser'er. Ser'ers register ser'ices with the name ser'er, which clients contact to ind the transport address o a gi'en ser'ice. In $oth cases, we need a mechanism or mapping high&le'el ser'ice names into low&le'el encoding that can $e used within packet headers o the network protocols. In its general orm, the pro$lem is ?uite comple#. One simpliication is to $reak the pro$lem into two parts: ha'e transport addresses $e a com$ination co m$ination o machine address and local  process on that machine. . Storage capacit! o the su$net. ssumptions 'alid at the data link la!er d do o not necessaril! hold at the transport 6a!er. Speciicall!, the su$net ma! $uer messages or a  potentiall! long time, and an old== packet ma! arri'e at a destination at une#pected times. . 1e need a d!namic low control mechanism. The data link la!er solution o reallocating $uers is inappropriate $ecause a machine ma! ha'e hundreds o connections sharing a single ph!sical link. In addition, appropriate settings or the low control parameters depend on the communicating end points /e.g., Cra! supercomputers 's. %Cs0, not on the protocol used. 5on=t send data unless there is room. lso, the network la!erdata link la!er solution o simpl! not acknowledging rames or which the recei'er has no space is unaccepta$le. 1h!F In the data link case, the line is not $eing used or an!thing else thus

 

retransmissions are ine#pensi'e. t the transport le'el, end&to&end retransmissions are needed, which wastes resources $! sending the same packet o'er the same links multiple times. I the recei'er has no $uer space, the sender should $e pre'ented rom sending data. . 5eal with congestion control. In connectionless Internets, transport protocols must e#ercise congestion control. 1hen the network $ecomes congested, the! must reduce rate at which the! insert packets into the su$net, $ecause the su$net has no wa! to pre'ent itsel rom $ecoming o'erloaded. <. Connection esta$lishment. Transport le'el protocols go through three phases: esta$lishing, using, and terminating a connection. (or data gram&oriented protocols, opening a connection simpl! allocates and initiali"es data structures in the operating s!stem kernel. Connection oriented protocols oten e#changes messages that negotiate options with the remote peer at the time a connection are opened. Esta$lishing a connection ma! $e trick!  $ecause o the possi$ilit! o old or duplicate packets. (inall!, although not as diicult as esta$lishing a connection, terminating a connection  presents su$tleties too. (or instance, $oth ends o the connection must $e sure that all the data in their ?ueues ha'e $een deli'ered to the remote application. Session La,er

This la!er allows users on dierent machines to esta$lish session $etween them.  session allows ordinar! data transport $ut it also pro'ides enhanced ser'ices useul in some applications.  session ma! $e used to allow a user to log into a remote time& Sharing machine or to transer a ile $etween two machines. Some o the session related ser'ices are: ). This la!er manages 5ialogue Control. Session can allow traic to go in $oth direction at the same time, or in onl! one direction at one time. *. Token management. (or some protocols, it is re?uired that $oth sides don=t attempt same operation at the same time. To manage these acti'ities, the session la!er pro'ides tokens thatThis can $e e#changed. side thatinto is holding token canin perorm thes!stem critical operation. concept can $eOnl! seen one as entering a critical section operating using semaphores. +. S!nchroni"ation. Consider the pro$lem that might occur when tr!ing to transer a 4& hour ile transer with a *&hour mean time $etween crashes. ter each transer was a$orted, the whole transer has to start again and again would pro$a$l! ail. To Eliminate this pro$lem, Session la!er pro'ides a wa! to insert checkpoints into data streams, so that ater a crash, onl! the data transerred ater the last checkpoint ha'e to $e repeated. #resentation La,er

This la!er is concerned with S!nta# and Semantics o the inormation transmitted, unlike other la!ers, which are interested in mo'ing data relia$l! rom one machine to other. (ew o the ser'ices that %resentation la!er pro'ides are: ). Encoding data in a standard agreed upon wa!.

 

*. It manages the a$stract data structures and con'erts rom representation used inside computer to network standard representation and $ack. Application La,er

The application la!er consists o what most users think o as a s programs. The application does the actual work at hand. lthough each application is dierent, some applications are so useul that the! ha'e $ecome standardi"ed. The Internet has deined standards or: )G (ile transer /(T%0: Connect to a remote machine and send or etch an ar$itrar! ile. (T% deals with authentication, listing a director! contents, SCII or $inar! iles, etc. G >emote login /telnet0:  remote terminal protocol that allows a user at one site to esta$lish a TC% connection to another an other site, and then pass ke!strokes rom the local host to the remote host. 2ail /S2T%0: llow a mail deli'er! agent on a local machine to connect to a mail deli'er! agent on a remote machine and deli'er mail. G 3ews /33T%0: llows communication $etween a news ser'er and a news client. G 1e$ /7TT%0: Base protocol or communication on the 1orld 1ide 1e$.

Transmission 2edia Introduction Transmission media can $e deined as ph!sical ph !sical path $etween transmitter and recei'er in a data transmission s!stem. nd it ma! $e classiied into two t!pes as shown in (ig. *.*.). 5uided: Transmission capacit! depends criticall! on the medium, the length, and whether the medium is point&to&point or multipoint /e.g. 630. E#amples are co&a#ial ca$le, twisted pair, and optical i$er. electro&magnetic signals $ut do not guide Unguided: pro'ides a means or transmitting electro&magnetic them. E#ample wireless transmission. Characteristics and ?ualit! o data transmission are determined $! medium and signal characteristics. (or guided media, the medium is more important in determining the limitations o transmission. 1hile in case o unguided media, the $andwidth o the signal produced $! the transmitting antenna and the si"e o the antenna is more important than the medium. Signals at lower re?uencies are omni&directional /propagate in all directions0. (or higher re?uencies, ocusing the signals into a directional $eam is  possi$le. These properties determine what kind o media one should use in a particular application. In this lesson we shall discuss the characteristics o 'arious transmission media, $oth guided and unguided. 5uided transmission media In this section we shall discuss a$out the most commonl! used guided transmission media such as twisted&pair o ca$le, coa#ial ca$le and optical i$er. T)isted #air In twisted pair technolog!, two copper wires are strung $etween two points:

 

G The two wires are t!picall! twisted== together in a heli# to reduce intererence $etween the two conductors .Twisting decreases the cross&talk intererence $etween adAacent adAace nt pairs in a ca$le. T!picall!, a num$er o pairs are $undled together into a ca$le $! wrapping them in a tough protecti'e sheath. ctuall!, the! carr! onl! analog signals. 7owe'er, the analog== signals can 'er! closel! correspond to the s?uare wa'es representing $its, so we oten think o them as carr!ing digital data. d ata. 5ata rates o se'eral 2$ps common. Spans distances o se'eral kilometers. 5ata rate determined $! wire thickness and length. In addition, shielding to eliminate intererence rom other wires impacts signal&to&noise ratio, and ultimatel!, the data rate. Dood, low&cost communication. Indeed, man! sites alread! ha'e twisted pair installed in oices && e#isting phone lines T,pical c+aracteristics: Twisted&pair can $e used or $oth analog and digital communication. The data rate that can $e supported o'er a twisted&pair is in'ersel!  proportional to the s?uare o the line length. 2a#imum transmission transmission distance o ) 8m can $e achie'ed or data rates up to ) 2$s. (or analog 'oice signals, ampliiers are re?uired a$out e'er!  8m 8 m and or digital signals, repeaters are needed or a$out * 8m. To reduce intererence, the twisted pair can $e shielded with metallic $raid. This t!pe o

wire is known as Shielded Twisted&%air /ST%0 and the other orm is known as -nshielded Twisted&%air /-T%0. -se: The oldest and the most popular pop ular use o twisted pair are in telephon!. In 63 it is commonl! used or point&to&point short distance communication /sa!, )99m0 within a  $uilding or a room. ase -and Coa6ial 1ith coa#==, the medium consists o a copper core surrounded $! insulating material and a $raided outer conductor as shown in (ig. *.*.+. The term $ase  $and indicates digital transmission /as opposed to $road$and analog0. %h!sical connection consists o metal pin touching the copper core. There are two common wa!s to connect to a coa#ial ca$le: )). 1ith 'ampire taps, a metal pin is inserted into the copper core.  special tool drills a hole into the ca$le, remo'ing a small section o the insulation, and a special connector is screwed into the hole. The tap makes contact with the copper core. *. 1ith a T&Aunction, the ca$le is cut in hal, and $oth hal'es connect to the T&Aunction.  T&connector is analogous to the signal splitters used to hook up multiple TVs to the same ca$le wire. C+aracteristics:  Co&a#ial ca$le has superior re?uenc! characteristics compared to twisted&pair and can $e used or $oth analog and digital signaling. In $ase$and 63, the data rates lies in the range o ) 87" to *9 27" o'er a distance in the range o ) 8m. Co& a#ial ca$les t!picall! ha'e a diameter o +<K. Coa#ial ca$les are used $oth or $ase$and and $road$and communication. (or $road$and CTV application coa#ial ca$le o )*K diameter and  L impedance is used. This ca$le oers $andwidths o +99 to 499 27" acilitating high&speed data communication with low $it&error rate. In $road$and signaling, signal propagates onl! in one direction, in contrast to propagation in $oth

directions in $ase$and signaling. Broad$and ca$ling uses either dual&ca$le scheme or single&ca$le scheme with a headend to acilitate low o signal in one direction. Because

 

o the shielded, concentric construction, co&a#ial ca$le is less suscepti$le to intererence and cross talk than the twisted&pair. (or long distance communication, repeaters are needed or e'er! kilometer or so. 5ata rate depends on ph!sical properties o ca$le, $ut )9 2$ps is t!pical. -se: One o the most popular use o co&a#ial ca$le is in ca$le TV /CTV0 or the distri$ution o TV signals. nother importance use o co&a#ial ca$le is in 63.

Broadband Coaxial The term $road$and reers to analog transmission o'er coa#ial ca$le. /3ote, howe'er, that the telephone olks use $road$and to reer to an! channel wider than 4 k7"0. The technolog!: )G T!picall! $andwidth o +99 27", total data rate o a$out )9 2$ps. GOperates at distances up to )99 km /metropolitan area0. G -ses analog signaling. G Technolog! used in ca$le tele'ision. Thus, it is alread! a'aila$le at sites such as uni'ersities that ma! ha'e TV classes. G Total a'aila$le spectrum t!picall! di'ided into smaller channels channe ls o  27" each. That is, to get more than 27" o $andwidth, !ou ha'e to use two smaller channels and somehow com$ine the signals. G >e?uires ampliiers to $oost signal strength $ecause ampliiers are one wa!, data lows in onl! one direction. T)o t,pes of s,stems +a4e emerged:

)). 5ual ca$le s!stems use two ca$les, one or transmission in each direction: One ca$le is used or recei'ing data. Second ca$le used to communicate with headend. 1hen a node wishes to transmit data, it sends the data to a special node called the headend. The headend then resends the data on the irst ca$le. Thus, the headend acts as a root o the tree, and all data must $e sent to the root or redistri$ution to the other nodes. **. 2idsplit s!stems di'ide the raw channel into two smaller channels, with each su$ channel ha'ing the same purpose as a$o'e. 1hich is $etter, $road$and or $ase $andF There is rarel! a simple answer to such ?uestions. ? uestions. Base $and is simple to install, interaces are ine#pensi'e, $ut doesn=t ha'e the same range. Broad$and is more complicated, more e#pensi'e, and re?uires regular adAustment $! a trained technician, $ut oers more ser'ices /e.g., it carries audio and 'ideo too0.

Fiber Optics In i$er optic technolog!, the medium consists o a hair&width strand o silicon or glass, and the signal consists o pulses o light. (or instance, a pulse o light means )==, lack o  pulse means 9==. It has a c!lindrical shape and consists o three concentric sections: the core, the cladding, and the Aacket as shown in (ig. *.*.4. The core, innermost section consists o a single solid dielectric c!linder o o  diameter d) and o reracti'e inde# n). The core is surrounded $! a solid dielectric cladding o reracti'e inde# n* that is less than n). s a conse?uence, the light is propag propagated ated through multiple total internal relection. The core material is usuall! made o ultra pure used

 

silica or glass and the cladding is either made o glass or plastic. The cladding is surrounded $! a Aacket made o plastic. The Aacket is used to protect against moisture, a$rasion, crushing and other en'ironmental ha"ards. Three components are re?uired: )). (i$er medium: Current technolog! carries ca rries light pulses or tremendous distances /e.g., )99s o source: kilometers0 with a'irtuall! no signal5iode loss. /6E50 or laser diode. >unning current *.6ight t!picall! 6ight Emitting through the material generates a pulse o light. +.  photo diode light detector, which con'erts light pulses into electrical signals. d'antages: )). Ver! high data rate, low error rate. )999 2$ps /) D$ps0 o' o'er er distances o kilometers common. Error rates are so low the! are almost negligi$le. *. 5iicult to tap, which makes it hard or unauthori"ed taps as well. This is responsi$le or higher relia$ilit! o this medium. 7ow diicult is it to pre'ent coa# tapsF Ver! diicult indeed, unless one can keep the entire ca$le in a locked room )+. 2uch thinner /per logical phone line0 than e#isting copper circuits. Because o its thinness, phone companies can replace thick copper wiring with i$ers ha'ing much more capacit! 'olume.without This is the important $ecause means that aggregate capacit! or cansame $e upgraded need or indingitmore ph!sical space tophone hire the new ca$les. 4. 3ot suscepti$le to electrical intererence /lightning0 or corrosion /rust0. . Dreater repeater distance than coa#. 5isad'antages: )G 5iicult to tap. It reall! is point&to&point technolog!. In contrast, con trast, tapping into coa# is tri'ial. 3o special training or e#pensi'e tools or parts are re?uired. G One&wa! channel. Two i$ers needed to get ull duple# /$oth wa!s0 communication. Optical (i$er works in three dierent t!pes o modes /or we can sa! that we ha'e + t!pes o communication using Optical i$er0. Optical i$ers are a'aila$le in two 'arieties 2ulti&2ode (i$er /22(0 and Single&2ode (i$er /S2(0. (or multi&mode i$er the core and cladding diameter liescore in the 9&*99Mm andlie )*&499Mm, respecti'el!. in single&mode i$er, the andrange cladding diameters in the range <&)*Mm and1hereas )*Mm, respecti'el!. Single&mode i$ers are also known as 2ono&2ode (i$er. 2oreo'er, $oth single&mode and multi&mode i$ers can ha'e two t!pes step inde# and graded inde#. In the ormer case the reracti'e inde# o the core is uniorm throughout and at the core cladding $oundar! there is an a$rupt change in reracti'e inde#. In the later case, the reracti'e inde# o the core 'aries ' aries radiall! rom the centre to the core&cladding $oundar! $oundar ! rom n) to n* in a linear manner. (ig. *.*. shows the optical i$er transmission modes. (igure *.*. Schematics o three optical i$er t!pes, /a0 Single&mode step&inde#, /$0 2ulti&mode step&inde#, and /c0 2ulti&mode graded&inde# C+aracteristics:  Optical i$er acts as a dielectric wa'eguide that operates at optical re?uencies /)9)4 to )9) 7"0. Three re?uenc! $ands centered around <9,)+99 and )99 nanometers are used or $est results. 1hen light is applied at one end o the optical i$er core, it reaches the other end e nd $! means o total internal relection $ecause o the choice o reracti'e inde# o core and cladding material /n) N n*0. The light source can

 

 $e either light emitting diode /6E50 or inAection laser diode /I650. These semiconductor semiconductor de'ices emit a $eam o light when a 'oltage is applied ac across ross the de'ice. t the recei'ing end, a photodiode can $e used to detect the signal&encoded light. Either %I3 detector or %5 /'alanche photodiode0 detector can $e used as the light detector. In a multi&mode i$er, the ?ualit! o signal&encoded light deteriorates more rapidl! than single&mode i$er, $ecause o intererence o man! light ra!s. s a conse?uence, single& mode i$er allows longer distances without repeater. (or multi&mode i$er, the t!pical ma#imum length o the ca$le without a repeater is *km, whereas or single&mode i$er it is *9km. (i$er -ses: Because o greater $andwidth /*D$ps0, smaller diameter, lighter weight, low attenuation, immunit! to electromagnetic intererence and longer repeater spacing, optical i$er ca$les are inding widespread use in long&distance telecommunications. Especiall!, the single mode i$er is suita$le or this purpose. (i$er optic ca$les are also used in high&speed 63 applications. 2ulti&mode i$er is commonl! used in 63. )G 6ong&haul trunks&increasingl! common in telephone network /Sprint ads0 G 2etropolitan trunks&without repeaters /a'erage < miles in length0 G >ural e#change trunks&link towns and 'illages G 6ocal loops&direct rom central e#change to a su$scri$er /$usiness or home0 G 6ocal area networks&)992$ps ring networks.

Unguided Transmission -nguided transmission is used when running a ph!sical ca$le /either i$er or copper0  $etween two end points is not possi$le. (or e#ample, running wires $etween $uildings is  pro$a$l! not legal i the $uilding is separated $! a pu$lic street. Inrared signals t!picall! used or short distances /across the street or within same room0, 2icrowa'e signals commonl! used or longer distances /)9=s o km0. Sender and recei'er use some sort o dish antenna as shown in (ig. *.*.. 5iiculties: )). 1eather intereres with signals. (or instance, clouds, rain, lightning, etc. ma! ad'ersel! aect communication. *. >adio transmissions eas! to tap.  $ig concern or companies worried a$out competitors stealing plans. +. Signals $ouncing o o structures ma! lead to out&o&phase signals that the recei'er must ilter out. Satellite Communication Satellite communication is $ased on ideas similar to those used or line&o&sight.  communication satellite is essentiall! a $ig microwa'e repeater or rela! station in the sk!. 2icrowa'e signals rom a ground station is picked up $! a transponder, ampliies the signal and re$roadcasts it in another re?uenc!, which can $e recei'ed $! ground stations at long distances as shown in (ig. *.*.. To keep the satellite stationar! with respect to the ground $ased stations, the satellite is placed in a geostationar! or$it a$o'e the e?uator at an altitude o o  a$out +,999 km. s the spacing $etween two satellites on the e?uatorial plane should not $e closer than 49, there can $e +94  @9 communication co mmunication satellites in the sk! at a time.  satellite can $e used or point&to&point p oint&to&point communication $etween two ground&$ased stations or it

 

can $e used to $roadcast a signal recei'ed rom one station to man! ground&$ased stations as shown in (ig. *.*.<. 3um$er o geo&s!nchronous satellites limited /a$out @9 total, to minimi"e intererence0. International agreements regulate how satellites are used, and how re?uencies are allocated. 1eather aects certain re?uencies. Satellite transmission diers rom terrestrial communication in another important wa!: One&wa!  propagation dela! is roughl! *9 ms. In interacti'e terms, propagation dela! alone inserts a ) second dela! $etween t!ping a character and recei'ing its echo. Characteristics: Optimum re?uenc! range or satellite communication is ) to )9 D7". The most popular re?uenc! $and is reerred to as 4 $and, which uses +. to 4.* D7" or down link and .@* to .4* or uplink transmissions. The 99 27" $andwidth is usuall! split o'er a do"en transponders, each with + 27" $andwidth. Each + 27"  $andwidth is shared $! time di'ision multiple#ing. s this preerred preerred $and is alread! saturated, the ne#t highest $and a'aila$le a 'aila$le is reerred to as )*)4 D7". It uses )4 to )4.D7" or upward transmission and )). to )*.* D7" or downward transmissions. Communication satellites ha'e se'eral uni?ue properties. The most important is the long communication dela! or the round trip /a$out *9 ms0 $ecause o the long distance /a$out *,999 km0 the signal has to tra'el $etween two earth stations. This poses a num$er o pro$lems, which are to $e tackled or successul and relia$le communication. nother interesting propert! o satellite communication is its $roadcast capa$ilit!. ll stations under the downward $eam can recei'e the transmission. It ma! $e necessar! to send encr!pted data to protect against pirac!. -se: 3ow&a&da!s communication satellites are not onl! used to handle han dle telephone, tele# and tele'ision traic o'er long distances, $ut are used to support 'arious internet $ased ser'ices such as e&mail, (T%, 1orld 1ide 1e$ /1110, etc. 3ew t!pes o ser'ices,  $ased on communication satellites, are emerging. Comparisoncontrast with other technologies: )). %ropagation dela! 'er! high. On 63s, or e#ample, propagation time is in nanoseconds && essentiall! negligi$le. *. One o ew alternati'es to phone pho ne companies or long distances. +. -ses $roadcast technolog! o'er a wide area & e'er!one on earth could recei'e a message at the same time 4. Eas! to place unauthori"ed taps into signal. Satellites ha'e recentl! allen out o a'or relati'e to i$er. 7owe'er, i$er has one $ig disad'antage: no one has it coming into their house or  $uilding, whereas an!one can place an antenna on a roo and lease a satellite channel.

Introduction In the pre'ious module we ha'e discussed 'arious encoding and modulation techni?ues, which are used or con'erting data da ta in to signal. To send signal through the transmission media, it is necessar! to de'elop suita$le mechanism or interacing data terminal e?uipments /5TEs0, which are the sources o data, to the data circuit terminating e?uipments /5CEs0, which con'erts data to signal and interaces with the transmission media. The wa! it takes place is shown in (ig. +.).*. The link $etween the two de'ices is known as interface interface.. But, $eore we discuss a$out the interace we shall introduce 'arious modes o communication in Sec. +.).*. Various aspects o raming and s!nchroni"ation or $it&oriented raming ha'e $een presented in Sec. +.).+. Character&oriented raming

 

has discussed in Sec. +.).4. (inall!, 1e shall discuss a$out the interace in detail along with some standard interaces in Sec. +.)..

#ossi-le "odes of communication Transmission o digital data through a transmission medium can $e perormed pe rormed either in serial or in parallel mode. In the serial mode, one $it is sent per clock tick, whereas in  parallel mode multiple $its are sent per clock tick. There are two su$classes o transmission or $oth the serial and parallel modes, as shown in (ig +.).+

Different modes of transmission %arallel Transmission %arallel transmission in'ol'es grouping se'eral $its, sa! n, together and sending all the n  $its at a time. This can $e accomplishes with the help o eight wires $undled together in the orm o a ca$le with a connector at each end. dditional wires, such as re?uest /re?0 and acknowledgement /ack0 are re?uired or as!nchronous transmission. %rimar! ad'antage o parallel transmission is higher speed, which is achie'ed at the e#pense o higher cost o ca$ling. s this is e#pensi'e or longer distances, parallel transmission is easi$le onl! or short distances. (igure +.).4 %arallel mode o communication with n  < Serial Serial Transmission transmission in'ol'es sending one data $it at a time. (igure +.). shows how serial transmission occurs. It uses a pair o wire or communication o data in $it&serial orm. Since communication within de'ices is parallel, it needs parallel&to&serial and serial&to&  parallel con'ersion at $oth ends. Serial mode o communication widel! used $ecause o the ollowing ad'antages: )G >educed cost o ca$ling: ca $ling: 6esser num$er o wires is re?uired as compared to parallel connection G >educed cross talk: 6esser num$er o wires result in reduced cross talk G 'aila$ilit! o suita$le communication media G Inherent de'ice characteristics: 2an! de'ices are inherentl! serial in nature G %orta$le de'ices like %5s, etc use u se serial communication to reduce the si"e o o the connector 7owe'er, it is slower than parallel mode o communication. There are two $asic approaches or serial communication to achie'e s!nchroni"ation o data transer $etween the source&destination pair. These are reerred to as P as!nchronous and s!nchronous. In the irst case, data are transmitted in small si"es, sa! character $! character, to a'oid timing pro$lem and make data transer sel&s!nchroni"ing, as discussed later. 7owe'er, it is not 'er! eicient $ecause o large o'erhead. To o'ercome o'e rcome this pro$lem, s!nchronous mode is used. In s!nchronous mode, a $lock with large num$er o $its can $e sent at a time. 7owe'er, this re?uires tight s!nchroni"ation  $etween the transmitter and recei'er clocks. 5irection o data low: There are three possi$le modes in serial communication: simple#, ull duple# and hal duple#. In simple# mode, the communication is unidirectional, such as rom a computer to a printer, as shown in (ig. +.)./a0. In ull&duple# mode $oth the sides can communicate simultaneousl!, as shown in (ig. +.). /$0. On the other hand, in hal&

 

duple# mode o communication, each station can $oth send and recei'e data, But, when one is sending, the other one can onl! recei'e and 'ice 'ersa. 1h! (raming and S!nchroni"ationF  3ormall!, units o data transer are larger than a single analog or digital digital encoding s!m$ol. It is necessar! to reco'er clock inormation or $oth $ oth the signal /so we can reco'er the right num$er o s!m$ols and reco'er each s!m$ol as accuratel! as possi$le0, and o$tain s!nchroni"ation or larger units o data d ata /such as data words and rames0. It is necessar! to reco'er the data in words or $locks $ecause this is the onl! wa! the recei'er  process will $e a$le to interpret the data recei'ed or a gi'en $it stream. 5epending on the $!te $oundaries, there will $e se'en or eight wa!s to interpret the $it stream as SCII S CII characters, and these are likel! to $e 'er! dierent. So, it is necessar! to add other $its to the $lock that con'e! control inormation used in the data link control procedures. The data along with pream$le, postam$le, and control inormation orms a rame. This raming is necessar! or the purpose o s!nchroni"ation and other data control unctions. 7S,nc+roni8ation

5ata sent $! a sender in $it&serial orm through a medium must $e correctl! interpreted at the recei'ing end. This re?uires that the $eginning, the end and logic le'el and duration o each $it as sent at the transmitting end must $e recogni"ed at the recei'ing end. There are three s!nchroni"ation le'els: Bit, Character and (rame. 2oreo'er, to achie'e s!nchroni"ation, two approaches known as as!nchronous and s!nchronous transmissions are used. (rame s!nchroni"ation is the process $! which wh ich incoming rame alignment signals /i.e., distincti'e $it se?uences0 are identiied, i.e. distinguished rom data $its, permitting the data $its within the rame to $e e#tracted or decoding or retransmission. The usual practice is to insert, in a dedicated time slot within the rame, a non&inormation $it that is used or the actual s!nchroni"ation o the incoming data with the recei'er. In order to recei'e $its in the irst place, the recei'er must $e a$le to d determine etermine how ast $its are $eing sent and when it has recei'ed a signal s!m$ol. (urther, the recei'er needs to $e a$le to determine what the relationship o the $its in the recei'ed stream ha'e to one another, that is, what the logical units o transer are, and where each recei'ed $it its into the logical units. 1e call these logical units rames. This means that in addition to $it /or transmission s!m$ol0 s!nchroni"ation, the recei'er needs word and rame s!nchroni"ation. S,nc+ronous communication .-it9oriented/

Timing is reco'ered rom the signal itsel /$! the carrier i the signal is analog, or $! regular transitions in the data signal or $! a separate clock line i the signal is digital0. Scram$ling is oten used to ensure re?uent transitions needed. The data transmitted ma!  $e o an! $it length, $ut is oten constrained $! the rame transer protocol /data link or 2C protocol0. Bit&oriented raming onl! assumes that $it s!nchroni"ation has $een achie'ed $! the underl!ing hardware, and the incoming $it stream is scanned at all  possi$le $it positions or special patterns generated generated $! the sender. The sender uses a special pattern /a lag pattern0 to delimit rames /one lag at each end0, and has to pro'ide or data transparenc! $! use o o  $it stuing /see $elow0.  commonl! used lag pattern is 756C=s 9))))))9 lag as shown in (ig. +.).. The $it se?uence 9))))))9 is used or

 

 $oth pream$le and postam$le or the purpose o s!nchroni"ation.  rame ormat or $it& oriented s!nchronous rame is shown in (ig. +.).<. + .).<. part rom the lag $its there are control ields. This ield contains the commands, responses and se?uences num$ers used to maintain the data low accounta$ilit! o the link, deines the unctions o the rame and initiates the logic to control the mo'ement o traic $etween sending and recei'ing stations. Speciic pattern to represent start o rame Speciic pattern to represent end o rame Summar! o the approach: )G Initiall! ) or * s!nchroni"ation characters are sent G 5ata characters are then continuousl! co ntinuousl! sent without an! e#tra $its G t the end, some error e rror detection data is sent

d'antages: )G 2uch less o'erhead G 3o o'erhead is incurred e#cept or s!nchroni"ation characters 5isad'antages: )G 3o tolerance in clock re?uenc! is allowed G The clock re?uenc! should $e same at $oth the sending and recei'ing ends Bit stuing: I the lag pattern appears an!where in the header or data o a rame, then the recei'er ma! prematurel! detect the start or end en d o the recei'ed rame. To o'ercome this  pro$lem, the sender makes sure that the rame $od! it sends has no lags in it at an!  position /note that since there is no character s!nchroni"ation, the lag pattern can start at an! $it location within the stream0. It does this $! $it stuing, inserting an e#tra $it in an! pattern that is $eginning to look like a lag. In 756C, whene'er  consecuti'e )=s are encountered in the data, a 9 is inserted ater the th ), regardless o the ne#t $it in the data as shown in (ig. +.).@. On the recei'ing end, the $it stream is piped through a shit register as the recei'er looks or the lag pattern. I  consecuti'e )=s ollowed $! a 9 is seen, then the 9 is dropped $eore sending the data on /the recei'er destus the stream0. I  )=s and a 9 are seen, it is a lag and an d either the current rame are ended or a new rame is started, depending on the current state o the recei'er. I more than  consecuti'e )=s are seen, then the recei'er has detected an in'alid pattern, and usuall! the current rame, i an!, is discarded. a0. ))9)))))))))99))))))))999)))))))999  $0. 9))))))9 ))9)))))9))))99)))))9)))999)))))9))999 ))9)))))9))))99) ))))9)))999)))))9))999 9))))))9 9)))) ))9 9Qs stued ater e'er! i'e )Qs 1ith $it stuing, the $oundar! $etween two rames can $e unam$iguousl! recogni"ed $! the lag pattern. Thus, i recei'er loses track o where it is, all it has to do is to scan the input In oraddition lag se?uence, sincethe the! caninonl! occur at rame ne'ershould within data. to recei'ing data logical units called$oundaries rames, theand recei'er ha'e some wa! o determining i the data has $een corrupted or not. I it has $een

 

corrupted, it is desira$le not onl! to reali"e that, $ut also to make an attempt to o$tain the correct data. This process is called error detection and error correction, which will $e discussed in the ne#t lesson. As,nc+ronous communication .)ord9oriented/

In as!nchronous communication, small, i#ed&length words /usuall!  to @ $its long0 are transerred without an!asclock line or the clock is data reco'ered rom the signal itsel. word has a start $it /usuall! a 90 $eore irst $it o the word and a stop $itEach /usuall! as a )0 ater the last data $it o the word, as shown in (ig. ( ig. +.).)9. The recei'er=s local clock is started when the recei'er detects the )&9 transition o the start $it, and the line is sampled in the middle o the i#ed $it inter'als /a $it inter'al is the in'erse o the data rate0. The sender outputs the $it at the agreed&upon rate, holding the line in the appropriate state or one $it inter'al or each $it, $ut using its own local clock to determine the length o these $it inter'als. The recei'er=s clock and the sender=s clock ma! not run at the same speed, so that there is a relati'e clock drit /this ma! $e caused  $! 'ariations in the cr!stals used, temperature, 'oltage, etc.0. I the recei'er=s clock drits drits too much relati'e to the sender=s clock, then the $its ma! $e sampled while the line is in transition rom one state to another, causing the recei'er to misinterpret the recei'ed data. There can $e 'aria$le amount o gap $etween two rames as shown in (ig. +.).)). d'antages o as!nchronous character oriented mode o communication are summari"ed  $elow: )G Simple to implement G Sel s!nchroni"ation Clock signal need not $e sent G Tolerance in clock re?uenc! is possi$le G The $its are sensed in the middle hence R  $it tolerance is pro'ided This mode o data communication, howe'er, suers rom high o'erhead incurred in data transmission. 5ata must $e sent in multiples o the data length o o  the word, and the two or more $its o s!nchroni"ation o'erhead compared co mpared to the relati'el! short data length causes the eecti'e data rate to $e rather low. (or e#ample, )) $its are re?uired to transmit < $its o data. In other words, $aud rate /num$er o signal elements0 is higher than data rate. C+aracter Oriented 3raming The irst raming method uses a ield in the header to speci! the num$er o characters in the rame. 1hen the data link&la!er sees the character count, it knows how man! characters ollow, and hence where whe re the end o the rame is. The trou$le with this algorithm is that the count can $e gar$led $! a transmission error. E'en i the check checksum sum is incorrect so the destination knows that the rame is $ad, $ ad, it still had no wa! o telling where the ne#t rame starts. Sending a rame $ack to the source and asking or retransmission does not help either, since the destination doesnQt know how man! characters to skip o'er to the start o retransmission. (or this reason the character count method is rarel! used. Character&oriented raming assumes that character s!nchroni"ation has alread! $een achie'ed $! the hardware. The sender uses special characters to indicate the start and end o rames, and ma! also use them to indicate header $oundaries and to assist the recei'er gain character s!nchroni"ation. (rames must $e o an integral character

length.

 

Character stuing 1hen a 56E character occurs in the header or the data portion o a rame, the sender must somehow let the recei'er know that it is not intended to signal a con control trol character. The sender does this $! inserting an e#tra 56E character ater the one occurring inside the rame, so that when the recei'er encounters two 56Es in a row, it immediatel! deletes one and interpret the other as header or data. The main disad'antage o this method is that it is closel! tied to <&$it characters in general and the SCII character code c ode in particular. s networks grow, this disad'antage o em$edding the character code in raming mechanism $ecomes more and more o$'ious, so a new techni?ue had to $e de'eloped to allow ar$itrar! si"ed character. Bit& oriented rame s!nchroni"ation and $it stuing is used that allow data d ata rames to contain an ar$itrar! num$er o $its and allow character c haracter code with ar$itrar! num$er o $its per character. 5ata >ate 2easures )G The raw data rate /the num$er o $its that the transmitter can per second without ormatting0 is onl! the starting point. There ma! $e o'erhead or s!nchroni"ation, or raming, or error checking, or headers and trailers, or retransmissi retransmissions, ons, etc. G -tili"ation ma! mean more than one thing. 1hen dealing with network monitoring and management, it reers to the raction o the resource actuall! used /or useul data and or o'erhead, retransmissions, etc.0. In this conte#t, utili"ation reers to the raction o the channel that is a'aila$le or actual data transmission to the ne#t higher la!er. It is the ratio o data $its per protocol data unit u nit /%5-0 to the total si"e o the %5-, including s!nchroni"ation, headers, etc. In other words, it is the ratio o the time spent actuall! sending useul data to the time it takes to transer that data and its attendant o'erhead. The eecti'e data rate at a la!er is the net data rate a'aila$le to the ne#t higher la!er. Denerall! this is the utili"ation times the raw data rate. 5TE&5CE Interace s two persons intending to communicate must speak in the same language, or successul communication $etween two computer s!stems or $etween a computer and a  peripheral, a natural understanding $etween the two is essential. In case o two persons a common language known to $oth o them is used. In case o two computers or a computer and an appliance, this understanding can $e ensured with the help o a standard, which should $e ollowed $! $oth the parties. Standards are usuall! recommended $! some International $odies, such as, Electronics Industries ssociation /EI0, The Institution o Electrical and Electronic Engineers /IEEE0, etc. The EI and IT-&T ha'e  $een in'ol'ed in de'eloping standards or the 5TE&5CE interace known as EI&*+*, EI&44*, etc and IT-&T standards are known as V series or H series. The standards should normall! deine the ollowing our important attri$utes: 2echanical: The mechanical attri$ute concerns the actual ph!sical connection $etween the two sides. -suall! 'arious signal lines are $undled into a ca$le with a terminator  plug, male or emale at each end. Each o the s!stems, $etween which communication is to $e esta$lished, pro'ide a plug o opposite gender or connecting the terminator plugs o the ca$le, thus esta$lishing the ph!sical ph !sical connection. The mechanical part speciies ca$les and connectors to $e used to link two s!stems Electrical: The Electrical attri$ute relates to the 'oltage le'els and timing o 'oltage changes. The! in turn determine the data rates and distances that can $e used or

 

communication. So the electrical part o the standard speciies 'oltages, Impedances and timing re?uirements to $e satisied or relia$le communication (unctional: (unctional attri$ute pertains to the unction to $e perormed, $! associating meaning to the 'arious signal lines. (unctions (un ctions can $e t!picall! classiied into the $road categories o data control, timing and ground. This component o standard speciies the signal pin assignments and signal deinition o each o the pins used or interacing the de'ices %rocedural: The procedural attri$ute speciies the protocol or communication, i.e. the se?uence o e'ents that should $e ollowed during data transer, using the unctional characteristic o the interace.  'ariet! o standards e#ist, some o the most popular interaces are presented in this section (low Control and Error Control

Introduction s we ha'e mentioned earlier, or relia$le and eicient data communication a great deal o coordination is necessar! $etween at least two machines. Some o these are n necessar! ecessar!  $ecause o the ollowing constraints: )G Both sender and recei'er ha'e limited speed G Both sender and recei'er ha'e limited memor! It is necessar! to satis! the ollowing re?uirements: )G  ast sender should not o'erwhelm o 'erwhelm a slow recei'er, which must perorm a certain amount o processing $eore passing the data on to the higher&le'el sotware. G I error occur during transmission, it is necessar! to de'ise mechanism to correct it The most important unctions o 5ata 6ink la!er to satis! the a$o'e re?uirements are error control and flo) control. Collecti'el!, these unctions are known as data lin* control, as discussed in this lesson. 3lo) Control is a techni?ue so that transmitter and recei'er with dierent speed characteristics can communicate with each other. (low control ensures that a transmitting station, such as a ser'er with higher processing capa$ilit!, ca pa$ilit!, does not o'erwhelm a recei'ing station, such as a desktop s!stem, with lesser processing capa$ilit!. This is where there is an orderl! low o transmitted data $etween the source and the destination. Error Control in'ol'es $oth error detection and error correction. It is necessar!  $ecause errors are ine'ita$le in data communication, in spite o the use o $etter e?uipment and relia$le transmission media $ased on the current technolog!. In the  preceding lesson we ha'e alread! discussed how errors can $e detected. In this lesson we shall discuss how error control is perormed $ased on retransmission o the corrupted

data. 1hen an error is detected, the recei'er can ha'e the speciied rame retransmitted  $! the sender. This process is commonl! known as Automatic Repeat Reuest .AR;/0

 

(or e#ample, Internet=s -nrelia$le 5eli'er! 2odel allows packets to $e discarded i network resources are not a'aila$le, and demands that >; protocols make pro'isions or retransmission. (low Control 2odern data networks are designed to support a di'erse range o hosts and communication mediums. Consider a @++ 27" host transmitting data to a @9 27" <94<SH. O$'iousl!, the %entium will%entium&$ased $e a$le to drown the slower processor with data. 6ikewise, consider two hosts, each using u sing an Ethernet 63, $ut with the two Ethernets connected $! a  8$ps modem link. I one host $egins transmitting to the other at Ethernet speeds, the modem link will ?uickl! $ecome o'erwhelmed. In $oth cases, low control is needed to pace the data transer at an accepta$le speed. (low Control is a set o procedures that tells the sender how much data it can transmit  $eore it must wait or an acknowledgment rom the recei'er. The low o data should not  $e allowed to o'erwhelm the recei'er. >ecei'er should also $e a$le to inorm the transmitter $eore its limits /this limit limit ma! $e amount o memor! used to store the incoming data or the processing power at the recei'er end0 are reached and the sender must send ewer rames. 7ence, (low control reers to the set o procedures used to restrict the amount o data the transmitter can send $eore waiting or acknowledgment. There are two methods de'eloped or low control namel! Stop&and&wait and Sliding& window. Stop&and&wait is also known as >e?uestrepl! sometimes. >e?uestrepl! /Stop& and&wait0 low control re?uires each data packet to $e acknowledged $! the remote host  $eore the ne#t packet is sent. This is discussed in detail in the ollowing ollowing su$section. Sliding window algorithms, used $! TC%, permit pe rmit multiple data packets to $e in simultaneous transit, making more eicient use o network Stop&and&1ait This is the simplest orm o low control where a sender transmits a data rame. ter recei'ing the rame, the recei'er indicates its willingness to accept another rame $! sending $ack an C8 rame acknowledging the rame Aust recei'ed. The sender must wait until it recei'es the C8 rame $eore sending the ne#t data rame.This is sometimes reerred to as ping&pong $eha'ior, re?uestrepl! is simple to understand and eas! to implement, $ut not 'er! ' er! eicient. In 63 en'ironment with ast links, this isn=t much o a concern, $ut 13 links will spend most o their time idle, especiall! i se'eral hops are re?uired. The $lue arrows show the se?uence o data rames $eing sent across the link rom the sender /top to the recei'er /$ottom0. The protocol relies on two&wa! transmission /ull duple# or hal duple#0 to allow the recei'er at the remote node to return rames acknowledging the successul transmission. The acknowledgements are shown in green in the diagram, and low $ack $ ack to the original sender.  small processing dela! ma! $e introduced $etween reception o the last $!te o a 5ata %5- and generation o the corresponding C8. 2aAor draw$ack o Stop&and&1ait (low Control is that onl! one rame can $e in transmission at a time, this leads to ineicienc! i propagation dela! is much longer than the transmission dela!.

 

Stop&and 1ait  protocol

Some protocols prett! much re?uire stop&and&wait  $eha'ior. (or e#ample, Internet=s >emote %rocedure Call />%C0 %rotocol is used to implement su$routine calls rom a program on one machine to li$rar! routines on another machine. Since most programs are single threaded, the sender has little choice $ut to wait or a repl! $eore continuing the program and an d possi$l! sending another re?uest.

6ink -tili"ation in Stop&and&1ait 6et us assume the ollowing: Transmission time: The time it takes or a station to transmit a rame /normali"ed to a 'alue o )0. %ropagation dela!: The time it takes or a $it to tra'el rom sender to recei'er /e#pressed as a0. a  ) :The rame is suicientl! long such that the irst $its o the rame arri'e at the destination $eore the source has completed transmission o the rame.  P a N ): Sender completes transmission o the entire rame $eore the leading $its o the rame arri'e at the recei'er.  P The link utili"ation -  )/)U*a0, a  %ropagation time  transmission time It is e'ident rom the a$o'e e?uation e?u ation that the link utili"ation is strongl! dependent on the ratio o the propagation time to the transmission time. 1hen the propagation time is small, as in case o 63 en'ironment, the link utili"ation is good. But, in case o long  propagation dela!s, as in case o satellite communication, the utili"ation can $e 'er!  poor. To impro'e the link utili"ation, we can use the ollowing /sliding&window0 protocol instead o using stop&and&wait protocol. Sliding 1indow 1ith the use o multiple rames or a single message, the stop&and&wait protocol does not  perorm well. Onl! one rame at a time can $e in transit. In stop&and&wait low low control, i a N ), serious ineiciencies result. Eicienc! can $e greatl! impro'ed $! allowing multiple rames to $e in transit at the same time. Eicienc! can also $e impro'ed $! making use o the ull&duple# line. To keep track o the rames, sender station sends se?uentiall! num$ered rames. Since the se?uence num$er to $e used occupies a ield in the rame, it should $e o limited si"e. I the header o the rame allows k $its, the se?uence num$ers range rom 9 to *k P ). Sender maintains a list o se?uence num$ers that it is allowed to send /sender window0. The si"e o the senderQs window is at most *k  P ). The sender is pro'ided with a $uer e?ual to the window si"e. >ecei'er also maintains a window o si"e *k P ). The recei'er acknowledges a rame $! sending an C8 rame that includes the se?uence num$er o the ne#t rame e#pected. This also e#plicitl! announces that it is prepared to recei'e the ne#t 3 rames, $eginning with the

 

num$er speciied. This scheme can $e used to acknowledge multiple rames. It could recei'e rames *, +, 4 $ut withhold C8 until rame 4 has arri'ed. B! returning an C8 with se?uence num$er , it acknowledges rames *, +, 4 in one go. The recei'er needs a  $uer o si"e ). Sliding window algorithm is a method o low control or network data transers. TC%, the Internet=s stream transer protocol, uses a sliding window algorithm.  sliding window algorithm places a $uer $etween the application program and the network data low. (or TC%, the $uer is t!picall! in the operating s!stem kernel, $ut this is more o an implementation detail than a hard&and& ast re?uirement.

Buer in sliding window

5ata recei'ed rom the network is stored in the $uer, rom where the application can read at its own pace. s the application reads data, $uer space is reed up to accept more input rom the network. The window is the amount o data that can $e Kread aheadK & the si"e o the $uer, less the amount o 'alid data stored in it. 1indow announcements are used to inorm the remote host o the current window si"e. Sender sliding 1indow: t t an! instant, the sender is permitted to send rames with se?uence num$ers in a certain range /the sending window0 >ecei'er sliding 1indow: T The he recei'er alwa!s maintains a window o si"e ) as shown in It looks or a speciic rame /rame 4 as shown in the igure0 to arri'e in a speciic order. I it recei'es an! other rame /out o order0, it is discarded and it needs to $e resent. 7owe'er, the recei'er window also slides $! one as the speciic rame is recei'ed and accepted as shown in the igure. The recei'er acknowledges a rame $! sending an C8 rame that includes se?uence num$erthe one#t the ne#t rame $eginning e#pected. This e#plicitl! announces that it is the prepared to recei'e 3 rames, with also the num$er speciied. This scheme can $e used to acknowledge multiple rames. It could recei'e rames *, +, 4 $ut withhold C8 until rame 4 has arri'ed. B! returning an C8 with se?uence num$er , it acknowledges rames *, +, 4 at one time. The recei'er needs a  $uer o si"e ). >ecei'er sliding window On the other hand, i the local application can process data at the rate it=s $eing transerred sliding window still gi'es us an ad'antage. I the window si"e is larger than the packet si"e, then multiple packets can $e outstanding in the network, since the sender knows that $uer space is a'aila$le on the recei'er to hold all o them. Ideall!, a stead!& state condition can $e reached where a series o packets /in the orward direction0 and window announcements /in the re'erse direction0 are constantl! in transit. s each new window announcement is recei'ed $! the sender, more data packets are transmitted. s the application reads data rom the $uer /remem$er, we=re assuming the application can

 

keep up with the network0, more window announcements are generated. 8eeping a series o data packets in transit ensures the eicient use o network resources. The link utili"ation in case o Sliding S liding 1indow %rotocol -  ), or 3 N *a U )  3/)U*a0, or 3  *a U ) 1here 3  the window si"e, and a  %ropagation time  transmission time Error Control Techni?ues 1hen an error is detected in a message, the recei'er sends a re?uest to the transmitter to retransmit the ill&ated message or packet. The most popular retransmission scheme is known as utomatic&>epeat&>e?uest />;0. Such schemes, where recei'er asks transmitter to re&transmit i it detects an error, are known as re'erse error correction techni?ues. Stop&and&1ait >; In Stop&and&1ait >;, which is simplest among all protocols, the sender /sa! station 0 transmits a rame and then waits till it recei'es positi'e acknowledgement /C80 or negati'e acknowledgement /3C80 rom the recei'er /sa! station B0. Station B sends an C8 i the rame is recei'ed correctl!, otherwise it sends 3C8. Station  sends a new rame ater recei'ing C8 otherwise it retransmits the old rame, i it recei'es a 3C8. Stop&nd&1ait >; techni?ue To tackle the pro$lem o a lost or damaged rame, the sender is e?u e?uipped ipped with a timer. In case o a lost C8, the sender send er transmits the old rame. In the (ig. +.+., the second %5o 5ata is lost during transmission. The sender is unaware o this loss, $ut starts a timer ater sending each %5-. In this case no C8 is recei'ed, and the timer counts down to "ero and triggers retransmission o the same %5- $! the sender. The sender alwa!s starts a timer ollowing transmission, $ut in the second transmission recei'es an C8 %5- $eore the timer e#pires, inall! indicating that the data has now $een recei'ed $! the remote node. >etransmission due to lost rame The recei'er now can identi! that it has recei'ed a dup duplicate licate rame rom the la$el o the rame and it is discarded To tackle the pro$lem o damaged rames, sa! a rame that has $een corrupted during the transmission due to noise, there is a concept o 3C8 rames, i.e. 3egati'e cknowledge rames. >ecei'er transmits a 3C8 rame to the sender i it ounds the recei'ed rame to $e corrupted. 1hen a 3C8 is recei'ed $! a transmitter $eore the time&out, the old rame is sent again >etransmission due to damaged rame The main ad'antage o o  stop&and&wait >; is its simplicit!. simplicit!. It also re?uires minimum  $uer si"e. 7owe'er, it makes highl! ineicient use o communication links, particularl! particularl! when WaQ is large. Do&$ack&3 >;

 

The most popular >; protocol is the go&$ack&3 >;, where the sender sends the rames continuousl! without waiting or acknowledgement. That is wh! it is also called as continuous >;. s the recei'er recei'es the rames, it keeps on sending C8s or a  3C8, in case a rame is incorrectl! recei'ed. 1hen the sender recei'es a 3C8, it retransmits the rame in error plus all the succeeding rames as shown in (ig.+.+.@. 7ence, the name o the protocol is go&$ack&3 >;. I a rame is lost, the recei'er sends  38 ater recei'ing the ne#t rame as shown in (ig. +.+.)9. In case there is long dela!  $eore sending the 38, the sender will resend the lost rame ater its timer times out. I the C8 rame sent $! the recei'er is lost, the sender resends the rames ater its timer times out ssuming ull&duple# transmission, the recei'ing end sends pigg!$acked acknowledgement $! using some num$er in the C8 ield o its data rame. 6et us assume that a +&$it se?uence num$er is used and suppose that a station sends rame 9 and gets $ack an >>), and then sends rames ), *, +, 4, , , , 9 and gets another >>).This might either mean that >>) is a cumulati'e C8 or all < rames were damaged. This am$iguit! can $e o'ercome i the ma#imum window si"e is limited to , i.e. or a k&$it se?uence num$er ield it is limited to *k&). The num$er 3 /*k&)0 speciies how man! rames can $e sent without recei'ing acknowledgement. I no acknowledgement is recei'ed ater sending 3 rames, the sender takes the help o a timer. ter theotime&out, resumesand retransmission. TheThis go&$ack&3 also takes care damagedit rames damaged C8s. schemeprotocol is little more comple# than the pre'ious one $ut gi'es much higher throughput. ssuming ull&duple# transmission, the recei'ing end sends pigg!$acked acknowledgement $! using some num$er in the C8 ield o its data rame. 6et us assume that a +&$it se?uence num$er is used and suppose that a station sends rame 9 and gets $ack an >>), and then sends rames ), *, +, 4, , , , 9 and gets another >>).This might either mean that >>) is a cumulati'e C8 or all < rames were damaged. This am$iguit! can $e o'ercome i the ma#imum window si"e is limited to , i.e. or a k&$it se?uence num$er ield it is limited to *k&). The num$er 3 /*k&)0 speciies how man! rames can $e sent without recei'ing acknowledgement. I no acknowledgement is recei'ed ater sending 3 rames, the sender takes the help o a timer. ter the time&out, it resumes retransmission. The go&$ack&3 protocol also takes care o damaged rames and damaged C8s. This scheme is little more comple# than the pre'ious one $ut gi'es much higher throughput. Selecti4e9Repeat AR;

The selecti'e&repetiti'e >; scheme retransmits onl! those or which 38s are recei'ed or or which timer has e#pired, e#p ired, This is the most eicient among the >; schemes, $ut the sender must $e more comple# so that it can send out&o&order rames. The recei'er also must ha'e storage space to store the post&38 rames and processing  power to reinsert rames in proper se?uence. <DLC

 Introduction 756C is a $it&oriented protocol. It was de'eloped $! the International Organi"ation or Standardi"ation /ISO0. It alls under the ISO standards ISO ++9@ and ISO 4++. It speciies a packiti"ation standard or serial links. It has ound itsel $eing used throughout the world. It has $een so widel! implemented $ecause it supports $oth hal&duple# and

 

ull&duple# communication lines, point&to&point /peer to peer0 and multi&point networks, and switched or non&switched channels. 756C supports se'eral modes o operation, including a simple sliding&window mode or relia$le deli'er!. Since Internet pro'ides retransmission at higher le'els /i.e., TC%0, most Internet applications use 756C=s unrelia$le deli'er! mode, -nnum$ered Inormation. Other $eneits o 756C are that the control inormation is alwa!s in the same  position, and speciic $it patterns used or control dier dramaticall! dramaticall! rom those in representing data, which reduces the chance o errors. It has also led to man! su$sets. Two su$sets widel! in use are S!nchronous 5ata 6ink Control /S56C0 and 6ink ccess %rocedure&Balanced /6%&B0. Figure 11-14-continued

&DLC Confi Configurat guratiion

In this lesson we shall consider the ollowing aspects o 756C: )G Stations and Conigurations G Operational 2odes G 3on&Operational 2odes G (rame Structure G Commands and >esponses G 756C Su$sets /S56C and 6%B0

756C Stations and Conigurations 756C speciies the ollowing three t!pes o stations or data link control: )G %rimar! Station G Secondar! Station G Com$ined Station %rimar! Station 1ithin a network using 756C data link protocol, i a coniguration used in the which there is a primar! station,asitits is used as the controlling station on the is link. It has responsi$ilit! o controlling all other stations on the link /usuall! secondar! stations0. 

 

 primar! issues commands and secondar! issues responses. 5espite this this important aspect o $eing on the link, the primar! station is also responsi$le or the organi"ation o data low on the link. It also takes care o error reco'er! at the data link le'el /la!er * o the OSI model0. Secondar! Station I the data link protocol $eing used is 756C, and a primar! station is present, a secondar! station must also $e present on the data link. The secondar! station is under the control o the primar! station. It has no a$ilit!, a $ilit!, or direct responsi$ilit! or controlling the link. It is onl! acti'ated when re?uested $! the primar! station. It onl! responds to the  primar! station. The secondar! station=s rames are called responses. It can onl! send response rames when re?uested $! the primar! station.  primar! station maintains a separate logical link with each secondar! station. Com$ined Station  com$ined station is a com$ination o a primar! and secondar! station. On the link, all com$ined stations are a$le to send and recei'e commands and responses without an!  permission rom an! other stations on the link. Each com$ined station is in ull control control o itsel, and does not rel! on an! other stations on the link. 3o other stations can control an! com$ined station. 756C also deines three t!pes o conigurations or the three t!pes o stations. The word coniguration reers to the relationship $etween the hardware de'ices d e'ices on a link. (ollowing are the three conigurations deined $! 756C: )G -n$alanced Coniguration G Balanced Coniguration G S!mmetrical Coniguration -n$alanced Coniguration The un$alanced coniguration in an 756C link consists o a primar! station and one or more secondar! stations. The un$alanced condition arises $ecause one station controls the other stations. In an un$alanced coniguration, an! o the ollowing can $e used: )G (ull&5uple# or 7al&5uple# operation G %oint to %oint or 2ulti&point networks Balanced Coniguration The $alanced coniguration in an 756C link consists o two or more com$ined stations. Each o the stations has e?ual and complimentar! responsi$ilit! compared to each other. Balanced conigurations can use onl! the ollowing: )G (ull & 5uple# or 7al & 5uple# operation G %oint to %oint networks S!mmetrical Coniguration This third t!pe o coniguration is not widel! in use toda!. It consists o two independent  point&to&point, un$alanced station conigurations. In this coniguration, each station has a  primar! and secondar! status. Each station is logicall! considered as two stations. stations. 756C Operational 2odes  mode in 756C is the relationship $etween two de'ices in'ol'ed in an e#change the mode descri$es who controls the link. E#changes o'er un$alanced conigurations are alwa!s conducted in normal response mode. E#changes o'er s!mmetric or $alanced

 

conigurations can $e set to speciic mode using a rame design to deli'er the command. 756C oers three dierent modes o operation. These three modes o operations are: )G 3ormal >esponse 2ode /3>20 G s!nchronous >esponse 2ode />20 G s!nchronous Balanced 2ode /B20 Normal Response "ode This is the mode in which the primar! station initiates transers to the secondar! station. The secondar! station can onl! o nl! transmit a response when, and onl! when, it is instructed to do so $! the primar! station. In other words, the secondar! station must recei'e e#plicit permission rom the primar! station to transer a response. ter recei'ing  permission rom the primar! station, station, the secondar! station initiates its transmi transmission. ssion. This transmission rom the secondar! station to the primar! station ma! $e much more than  Aust an acknowledgment o a rame. It ma! in act $e more than one inormation rame. Once the last rame is transmitted $! the secondar! station, it must wait once again rom e#plicit permission to transer an!thing, rom the primar! station. 3ormal >esponse 2ode is onl! used within an un$alanced coniguration. As,nc+ronous Response "ode In this mode, the primar! station doesn=t initiate transers to the secondar! station. In act, the secondar! station does not ha'e to wait to recei'e e#plicit permission rom the  primar! station to transer an! rames. The rames ma! ma! $e more than Aust acknowledgment rames. The! ma! contain data, or control inormation regarding the status o the secondar! station. This mode can reduce o'erhead on the link, as no rames need to $e transerred in order to gi'e the secondar! station permission to initiate a transer. 7owe'er, some limitations do e#ist. 5ue to the act that this mode is as!nchronous, the secondar! station must wait until it detects and an d idle channel $eore it can transer an! rames. This is when the >2 link is operating at hal&duple#. I the >2 link is operating at ull duple#, the secondar! station can transmit at an! time. In this mode, the primar! station still retains responsi$ilit! or error reco'er!, link setup, and link disconnection. S,nc+ronous alanced "ode This mode is used in case o com$ined stations. There is no need or permission on the part o an! station in this mode. This is $ecause com$ined stations do not re?uire an! sort o instructions to perorm an! task on the link.

 3ormal >esponse 2ode is used most re?uentl! in multi&point multi&point lines, where the  primar! station controls the link. s!nchronous >esponse 2ode is $etter or point&to& point&to&  point links, as it reduces o'erhead. s!nchronous Balanced 2ode is not used widel! toda!. The Kas!nchronousK in $oth >2 and B2 does not reer to the ormat o the data on the link. It reers to the act that an! gi'en station can transer rames without e#plicit permission or instruction rom an! other station. 756C 3on&Operational 2odes 756C also deines three non&operational modes. These three non&operational modes are: )G 3ormal 5isconnected 2ode /3520 G s!nchronous 5isconnected 2ode /520

 

G Initiali"ation 2ode /I20 The two disconnected modes /352 and 520 dier rom the operational modes in that the secondar! station is logicall! disconnected rom the link /note the secondar! station is not ph!sicall! disconnected rom the link0. The I2 mode is dierent rom the operations modes in that the secondar! station=s data link control program is in need o regeneration or it is in need o an e#change o parameters to $e used in an operational mode. 756C (rame Structure There are three dierent t!pes o rames as shown ields are shown Ta$le +.4.). Ta$le +.4.) Si"e o dierent ields (ield 3ame (lag (ield/ ( 0 ddress (ield/  0 Control (ield/ C 0 Inormation (ield/ I 0 O> 5ata 5ata (rame Check Se?uence/ (CS (CS 0 Closing (lag (ield/ ( 0

in (ig. +.4.4 and the si"e o dierent

Si"e/in $its0 < $its < $its < or ) $its Varia$le 3ot 3ot used used in some rames ) or +* $its < $its

The (lag ield E'er! rame on the link must $egin and end with a lag se?uence ield /(0. Stations attached to the data link must continuall! listen or a lag se?uence. The lag se?uence is an octet looking like 9))))))9. (lags are continuousl! transmitted on the link $etween rames to keep the link acti'e. Two other $it se?uences are used in 756C as signals or the stations on the link. These two $it se?uences are: )G Se'en )=s, $ut less than ) signal an a$ort signal. The stations on the link know there is a pro$lem on the link. G ) or more )=s indicate that the channel is in an idle state. The time $etween the transmissions o actual rames is called the interrame time ill. The interrame time ill is accomplished $! transmitting continuous lags $etween rames. The lags ma! $e in < $it multiples. 756C is a code&transparent protocol. It does not rel! on a speciic code or interpretation o line control. This means that i a $it at position 3 in an octet has a speciic meaning, regardless o the other $its in the same octet. I an octet has a $it se?uence o 9))))))9, $ut is not a lag ield, 765C uses a techni?ue called $it& stuing to dierentiate this $it se?uence rom a lag ield as we ha'e h a'e discussed in the  pre'ious lesson. t the recei'ing end, the recei'ing station inspects the incoming rame. I it detects  consecuti'e )=s it looks at the ne#t n e#t $it. I it is a 9, it pulls it out. I it is a ), it looks at the <th $it. I the <th $it is a 9, it knows an a$ort or idle signal has $een sent. It then proceeds to inspect the ollowing $its to determine appropriate action. This is the manner in which

 

756C achie'es code&transparenc!. 756C is not concerned with an! speciic $it code inside the data stream. It is onl! concerned with keeping lags uni?ue. The ddress ield The address ield /0 identiies the primar! or secondar! stations in'ol'ement in the rame transmission or reception. Each station on the link has ha s a uni?ue address. In an un$alanced coniguration, the  ield in $oth commands and responses reer to the secondar! station. In a $alanced coniguration, the command rame contains the destination station address and the response rame has the sending station=s address. The Control ield 756C uses the control ield /C0 to determine how to control the communications  process. This ield contains the commands, responses and se?uences num$ers used to maintain the data low accounta$ilit! o the link, deines the unctions o the rame and initiates the logic to control the mo'ement o traic $etween sending and recei'ing stations. There three control ield ormats: )G Inormation Transer (ormat: The rame is used to transmit end&user data $etween two de'ices. G Super'isor! (ormat: The control ield perorms control unctions such as acknowledgment o rames, re?uests or re&transmission, and re?uests or temporar! suspension o rames $eing transmitted. Its use depends on the operational mode $eing used. )G -nnum$ered (ormat: This control ield ormat is also used or control purposes. It is used to perorm link initiali"ation, link disconnection and other link control unctions. The %oll(inal Bit /%(0 The th $it position in the control ield is called the pollinal $it, or %( $it. It can onl!  $e recogni"ed when it is set to ). I it is set to 9, it is ignored. The pollinal pollinal $it is used to  pro'ide dialogue $etween the primar! station and secondar! station. The primar! station uses %) to ac?uire a status response rom the secondar! station. The % $it signiies a  poll. The secondar! station responds to the % $it $! transmitting a data or status rame rame to the primar! station with the %( $it set to (). The ( $it can also a lso $e used to signal the end o a transmission rom the secondar! station under 3ormal >esponse 2ode. The Inormation ield or 5ata ield This ield is not alwa!s present in a 756C rame. It is onl! present when the Inormation Transer (ormat is $eing used in the control ield. The inormation ield contains the actuall! data the sender is transmitting to the recei'er in an I&(rame and network management inormation in -&(rame. The (rame check Se?uence ield This ield contains a )&$it, or +*&$it c!clic redundanc! redund anc! check $its. It is used or error detection. <DLC Commands and Responses

The set o commands and responses in 756C is summari"ed in Ta$le +.4.*. Inormation transer ormat command and response /I&(rame0 The unction o the inormation command and response is to transer se?uentiall! num$ered rames, each containing an inormation ield, across the data link. Super'isor! ormat command and responses /S&(rame0

 

Super'isor! /S0 commands and responses are used to perorm num$ered super'isor! unctions such as acknowledgment, polling, temporar! suspension o inormation transer, or error reco'er!. (rames with the S ormat control ield cannot canno t contain an inormation ield.  primar! station ma! use the S ormat command rame with the % $it set to ) to re?uest a response rom a secondar! station regarding its status. Super'isor! (ormat commands and responses are as ollows: )G >ecei'e >ead! />>0 is used $! the primar! or secondar! station to indicate that it is read! to recei'e an inormation rame andor acknowledge pre'iousl! recei'ed rames. G >ecei'e 3ot >ead! />3>0 is used to indicate that the primar! or secondar secondar! ! station is not read! to recei'e an! inormation rames or acknowledgments. G >eAect />EJ0 is used to re?uest the retransmission o rames. G Selecti'e >eAect /S>EJ0 is used $! a station to re?uest retransmission o speciic rames. n S>EJ must $e transmitted or each erroneous rame each rame is treated as a separate error. Onl! one S>EJ can remain outstanding on the link at an! one time. TB6E +.4.* 756C Commands and >esponses Inormation Transer (ormat Commands I & Inormation Super'isor! (ormat Commands >> & >ecei'e read! >3> & >ecei'e not read! >EJ & >eAect S>EJ & Selecti'e reAect -nnum$ered (ormat Commands S3>2 & Set 3ormal >esponse 2ode S>2 & Set s!nchronous >esponse 2ode SB2 & Set s!nchronous Balanced 2ode 5ISC & 5isconnect S3>2E & Set 3ormal >esponse 2ode E#tended S>2E & Set s!nchronous >esponse 2ode E#tended SB2E & Set s!nchronous Balanced 2ode E#tended SI2 & Set Initiali"ation 2ode -% & -nnum$ered %oll -I & -nnum$ered Inormation HI5 & E#change identiication >SET & >eset

Inormation Transer (ormat >esponses I & Inormation Super'isor! (ormat >esponses >> & >ecei'e read! >3> & >ecei'e not read! >EJ & >eAect S>EJ & Selecti'e reAect -nnum$ered (ormat Commands - & -nnum$ered cknowledgment 52 & 5isconnected 2ode >I2 & >e?uest Initiali"ation 2ode >5 & >e?uest 5isconnect -I & -nnum$ered Inormation HI5 & E#change Identiication (>2> & (rame >eAect TEST & Test

 

TEST & Test -nnum$ered (ormat Commands and responses /-&(rame0 The unnum$ered ormat commands and responses are used to e#tend the num$er o data link control unctions. The unnum$ered ormat rames ha'e  modiier $its, which allow or up to +* additional commands and +* additional response unctions. Below, )+ command unctions, and < response unctions are descri$ed. )G Set 3ormal >esponse 2ode /S3>20 places the secondar! station into 3>2. 3>2 does not allow the secondar! station to send an! unsolicited rames. 7ence the primar! station has control o the link. G Set s!nchronous >esponse 2ode /S>20 allows a secondar! station to transmit rames without a poll rom the primar! station. G Set s!nchronous Balanced 2ode /SB20 sets the operational mode o the link to B2. G 5isconnect /5ISC0 places the secondar! station in to a disconnected mode. G Set 3ormal >esponse 2ode E#tended /S3>2E0 increases the si"e o the control ield to * octets instead o one in 3>2. This is used or e#tended se?uencing. The same applies or S>2E and SB2E. G Set Initiali"ation 2ode /SI20 is used to cause the secondar! station to initiate a station& speciic procedure/s0 to initiali"e its data link le'el control unctions. G -nnum$ered %oll /-%0 polls a station without regard to se?uencing or acknowledgment. G -nnum$ered Inormation /-I0 is used to send inormation to a secondar! station. G E#change Identiication /HI50 is used to cause the secondar! station to identi! itsel and pro'ide the primar! station identiications characteristics o itsel. G >eset />SET0 is used to reset the recei'e state 'aria$le in the addressed station. G Test /TEST0 is used to cause the addressed secondar! station to respond with a TEST response at the irst response opportunit!. It perorms a $asic test o the data d ata link control. G -nnum$ered cknowledgment /-0 is used $! the secondar! station to acknowledge the receipt and acceptance o an S3>2, S>2, SB2, S3>2E, S>2E, SB2E, >SET, SI2, or 5ISC commands. G 5isconnected 2ode /520 is transmitted rom a secondar! station to indicate it is in disconnected mode/non&operational mode.0 G >e?uest Initiali"ation 2ode />I20 is a re?uest rom a secondar! station or initiali"ation to a primar! station. Once the secondar! station sends >I2, it can onl! respond to SI2, 5SIC, TEST or HI5 commands. G >e?uest 5isconnect />50 is sent $! the secondar! station to inorm the primar! station that it wishes to disconnect rom the link and go into a non&operational mode/352 or 52. G (rame >eAect /(>2>0 is used $! the secondar! station in an operation mode to report that a condition has occurred occu rred in transmission o a rame and retransmission o the rame will not correct the condition. 756C Su$sets 2an! other data link protocols ha'e $een deri'ed rom 756C. 7owe'er, some o them reach $e!ond the scope o 756C. Two other popular osets o 756C are S!nchronous 5ata 6ink Control /S56C0, and 6ink ccess %rotocol, Balanced /6%&B0. S56C is used and de'eloped $! IB2. It is used in a 'ariet! o terminal to computer applications. It is

 

also a part o IB2=s S3 communication architecture. 6%&B was de'eloped $! the IT-&T. It is deri'ed mainl! rom the as!nchronous response mode />20 o 756C. It is commonl! used or attaching de'ices to packet&switched networks. )G Com$ined Station:  com$ined station is a com$ination o a primar! and secondar! station. On the link, all com$ined stations are a$le to send and recei'e commands and responses without an! permission rom an! other stations on the link.

S)itc+ed Communicat Communication ion Net)or*s Lesson 7 S)itc+ing Tec+niues: Circuit S)itc+ing Speciic Instructional O$Aecti'es t the end o this lesson the student will $e a$le to: )G -nderstand the need or circuit switching G Speci! the components o a switched communication network G E#plain how circuit switching takes place G E#plain how switching takes place using space&di'ision and time&di'ision switching G E#plain how routing is perormed G E#plain how signalling in perormed Introduction 1hen there are man! de'ices, it is necessar! to de'elop suita$le mechanism or communication $etween an! two de'ices. One alternati'e is to esta$lish point&to&point communication $etween each pair o de'ices using mesh topolog!. 7owe'er, mesh topolog! is impractical or large num$er o de'ices, $ecause the num$er o links increases e#ponentiall! /n/n&)0*, where n is the num$er o de'ices0 with the num$er o de'ices.  $etter alternati'e is to use switching techni?ues leading to switched communication network. In the switched network methodolog!, the network consists o a set o interconnected nodes, among which inormation is transmitted rom source to destination 'ia dierent routes, which is controlled $! the switching mechanism.  $asic model o a switched communication is shown in (ig. 4.).). The end de'ices that wish to communicate with each other are called stations. The switching de'ices are called n nodes. odes. Some nodes connect to other nodes and some are to connected to some stations. 8e! eatures o a switched communication network are gi'en $elow: )G 3etwork Topolog! is not regular. G -ses (52 or T52 or node&to&node communication. co mmunication. G There e#ist multiple paths $etween a source&destination pair or $etter network relia$ilit!. G The switching nodes are not concerned with the contents o data.

 

G Their purpose is to pro'ide a switching acilit! that will mo'e data rom node to node until the! reach the destination. The switching perormed $! dierent nodes can $e categori"ed into the ollowing three t!pes: )G Circuit Switching G %acket Switching G 2essage Switching Figure 14-1

Swit Switche che Network

Figure 14-3 14-3

Circuit-Switche Network Circuit

 

Figure 14-14

Datagram Approach

Figure 14-17 14-17

Message Swi witching tching

Circuit switching Techni?ue Communication 'ia circuit switching implies that there is a dedicated communication  path $etween the two stations. The path is a connected through a se?uence o links  $etween network nodes. On each ph!sical link, logical channel is dedicated tothe thecaller connection. Circuit switching is commonl! useda techni?ue in telephon!, where sends a special message with the address add ress o the callee /i.e. $! dialling a num$er0 to state its destination. It in'ol'ed the ollowing three distinct steps, Circuit Esta$lishment: To esta$lish an end&to&end connection $eore an! transer o data. Some segments o the circuit ma! $e a dedicated link, while some other segments ma! $e shared. 5ata transer: )G Transer data is rom the source to the destination. G The data ma! $e analog or digital, depending on the nature o the network. G The connection is generall! ull&duple#. Circuit disconnect: )G Terminate connection at the end o data transer. G Signals must $e propagated to deallocate the dedicated resources.

 

Thus the actual ph!sical electrical path or circuit $etween the source and de destination stination host must $e esta$lished $eore the message is transmitted. This connection, once esta$lished, remains e#clusi'e and continuous or the complete duration o inormation e#change and the circuit $ecomes disconnected onl! when the source wants to do so. Switching 3ode 6et us consider the operation o a single circuit switched node comprising a collection o stations attached to a central switching unit, which esta$lishes a dedicated path $etween an! two de'ices that wish to communicate. 2aAor elements o a single&node network are summari"ed $elow: )G 5igital switch: That pro'ides a transparent /ull&duple#0 signal path $etween an! pair o attached de'ices. G 3etwork interace: That represents the unctions and hardware h ardware needed to connect digital d igital de'ices to the network /like telephones0. G Control unit: That esta$lishes, maintains, and tears down a connection. The simpliied schematic diagram o a switching node is shown in (ig. 4.).+. n important characteristic o a circuit&switch node is whether it is $locking or non&$locking.  $locking network is one, which ma! $e una$le to connect two stations $ecause all  possi$le paths $etween them are alread! in use.  non&$locking network permits all stations to $e connected /in pairs0 at once o nce and grants all possi$le connection re?uests as long as the called part! p art! is ree. (or a network that supports onl! 'oice traic, a $locking coniguration ma! $e accepta$le, since most phone calls are o short duration. (or data applications, where a connection ma! remain acti'e or hours, non&$locking coniguration is desira$le. Circuit switching uses an! o the three technologies: Space&di'ision switches, Time& di'ision switches or a com$ination o $oth. In Space&di'ision switching, the paths in the circuit are separated with each other spatiall!, i.e. dierent ongoing connections, at a same instant o time, uses dierent switching paths, which are separated spatiall!. This was originall! de'eloped or the analog en'ironment, and has $een carried o'er to the digital domain. Some o the space switches are cross$ar switches, 2ulti&stage switches /e.g. Omega Switches0.  cross$ar switch is shown in (ig. 4.).4. Basic $uilding $lock o the switch is a metallic crosspoint or semiconductor gate that can $e ena$led or disa$led  $! a control unit. )G The num$er o crosspoints grows with the s?uare o the num$er o attached stations. G Costl! or a large switch. G The ailure o a crosspoint pre'ents connection co nnection $etween the two de'ices wh whose ose lines intersect at that crosspoint. G The crosspoints are ineicientl! utili"ed. G Onl! a small raction o crosspoints are engaged e'en i all o the attached de'ices are acti'e. Some o the a$o'e pro$lems can $e o'ercome with the help o multistage space di'ision switches. B! splitting the cross$ar switch into smaller units and interconnecting them, it is possi$le to $uild multistage switches with ewer crosspoints. T+ree9stage space di4ision s)itc+: In this case the num$er o crosspoints needed goes down rom 4 to 49. There is more than one path through the network to connect two

 

endpoints, there$! increasing relia$ilit!. 2ultistage switches ma! lead to $locking. The  pro$lem ma! $e tackled $! increasing the num$er or si"e o the intermediate switches, which also increases the cost. The $locking eature is illustrated in (ig. 4.).. ater setting up connections or )&to&+ and *&to&4, the switch cannot esta$lish connections or +&to& and 4&to&. Time Di4ision S)itc+ing Both 'oice and data can $e transmitted using digital signals through the same switches. ll modern circuit switches use digital time&di'ision multiple#ing /T520 techni?ue or esta$lishing and maintaining circuits. S!nchronous T52 allows multiple low&speed $it streams to share a high&speed line.  set o inputs is sampled in a round ro$in manner. The samples are organi"ed seriall! into slots /channels0 to orm a recurring rame o slots. 5uring successi'e time slots, dierent IO pairings are ena$led, allowing a num$er o connections to $e carried o'er the shared $us. To keep up with the input lines, the data rate on the $us must $e high enough so that the slots recur suicientl! re?uentl!. (or )99 ull&duple# lines at )@.*99 8$ps, the data rate on the $us must $e greater than ).@* 2$ps. The source&destination pairs corresponding to all acti'e connections are stored in the control memor!. Thus the slots need not speci! the source and destination addresses. Schematic diagram o time di'ision switching. Time&di'ision switching uses time&di'ision multiple#ing to achie'e switching, i.e. dierent ongoing connections can use same switching path $ut at dierent interlea'ed time inter'als. There are two popular methods o time&di'ision switching namel!, Time& Slot Interchange /TSI0 and the T52 $us. TSI changes the ordering o the slots $ased on desired connection and it has a random&access memor! to store data and lip the time slots as shown in (ig. 4.).<. The operation o a TSI is depicted in (ig. 4.).@. s shown in the igure, writing can $e perormed in the memor! se?uentiall!, $ut data is read selecti'el!. In T52 $us there are se'eral input and outputs connected to a high&speed  $us. 5uring a time slot onl! one particular output switch is closed, so onl! one connection at a particular instant o time %u$lic Switched Telephone 3etworks %u$lic switched telephone network /%ST30 is an e#ample o circuit&switched network. ItQs also known %lain OldinTelephone Ser'icenamel!: /%OTS0.>egional The switching used or the switching areasorganised dierent le'els, o icescentres oices /class )0, Section oices /class *0, primar! oices /class +0, Toll oices /class 40 and inall! End oices /class 0. 6e'el ) is at the highest le'el and 6e'el  is the lowest le'el. Su$scri$ers or the customers are directl! connected to these end oices. nd each oice is connected directl! to a num$er o oices at a le'el $elow and mostl! a single oice at higher le'el. Su$scri$er Telephones are connected, through 6ocal 6oops to end oices /or central oices0.  small town ma! ha'e onl! one end oice, $ut large cities ha'e se'eral end oices. 2an! end oices are connected to one Toll oice, which are connected to  primar! oices. Se'eral primar! oices are connected to a section oice, which normall! ser'es more than one state. ll regional oices o ices are connected using mesh topolog!. ccessing the switching station at the end oices o ices is accomplished through dialling. In the  past, telephone eatured rotar! or pulse dialling, in which digital signals were sent to the end oice or each dialled digit. This t!pe o dialling was prone to errors due to inconsistenc! in humans during dialling. %resentl!, dialling is accomplished $! Touch&

 

Tone techni?ue. In this method the user sends a small $urst o re?uenc! called dual tone,  $ecause it is a com$ination o two re?uencies. This com$ination o re?uencies sent depends on the row and column o the pressed pad. The connections are multiple#ed when ha'e to send to a switching oice, which is one le'el up. (or e#ample, 5ierent connections will $e multiple#ed when the! are to $e orwarded rom an end&oice to Toll oice.

(rame >ela!

 

'igure ()-( ()-(*

'rame +elay 'ram 'rame

'igure (,-

ATM Multi ltiple$i ple$ing ng

 

Introduction 3rame Rela, is a high&perormance 13 protocol that operates at the ph!sical and data link la!ers o the OSI reerence model. (rame >ela! originall! was designed or use across Integrated Ser'ices 5igital 3etwork /IS530 interaces. Toda!, it is used o'er a 'ariet! o other network interaces as well. (rame >ela! is a simpliied orm o %acket

Switching, similar in principle toon H.*, in which s!nchronous ramesdierence o data are$etween routed to dierent destinations depending header inormation. The $iggest (rame >ela! and H.* is that H.* guarantees data integrit! and network managed low control at the cost o some network dela!s. (rame >ela! switches packets end to end much aster, $ut there is no guarantee g uarantee o data integrit! at all. s line speeds ha'e increased rom speeds $elow 4k$ps to T)E) and $e!ond, the dela!s inherent in the store&and&orward mechanisms o H.* $ecome intolera$le. t the same time, impro'ements in digital transmission techni?ues ha'e reduced line errors to the e#tent that node&to&node error correction throughout the network is no longer necessar!. The 'ast maAorit! o (rame >ela! traic consists o TC%I% or other protocols that pro'ide their own low control and error correction mechanisms. 2uch o this traic is ed into the Internet, another packet packe t switched network without an! $uilt&in error control. Because (rame >ela! does not =care= whether the rame it is switching is error&ree or not, a (rame >ela! node can start switching traic out onto a new line as soon as it has read the irst two $!tes o addressing inormation at the $eginning o the rame. Thus a rame o data can tra'el end&to&end, end &to&end, passing through se'eral switches, and still arri'e at its destination with onl! a ew $!tes= dela!. These dela!s are small enough that network latenc! under (rame >ela! is not n ot noticea$l! dierent rom direct leased line connections. s a result, the perormance o a (rame >ela! network is 'irtuall! identical to that o a leased line, $ut $ecause most o the network is shared, costs are lower. 3rame Rela, is an e#ample o a packet&switched technolog!. %acket&switched networks ena$le end stations to d!namicall! share the network medium and the a'aila$le  $andwidth. The ollowing two techni?ues are used in packet&switching technolog!: )G Varia$le&length packets G Statistical multiple#ing Varia$le&length packets are used or more eicient and le#i$le data transers. These  packets are switched $etween the 'arious segments in the network until the destination is reached. Statistical multiple#ing techni?ues control network access in a packet&switched network. The ad'antage o this techni?ue is that it accommodates more le#i$ilit! and more eicient use o $andwidth. 2ost o toda!=s popular 63s, such as Ethernet and Token >ing, are packet&switched networks.

Frame Relay Devices 5e'ices attached to a (rame ( rame >ela! 13 all into the ollowing two general categories: )G 5ata terminal e?uipment /5TE0 G 5ata circuit&terminating e?uipment /5CE0

 

5TEs generall! are considered to $e terminating e?uipment or a speciic network and t!picall! are located on the premises o a customer. In act, the! ma! $e owned $! the customer. E#amples o 5TE de'ices are terminals, personal computers, routers, and  $ridges. 5CEs are carrier&owned internetworking de'ices. The purpose o 5CE e?uipment is to pro'ide clocking and switching ser'ices in a network, which are the de'ices that actuall! transmit data through the 13. In most cases, these are packet switches. The connection $etween a 5TE de'ice and a 5CE de'ice consists o $oth a ph!sical la!er component and a link la!er component. The ph!sical component deines the mechanical, electrical, unctional, and procedural speciications or the connection  $etween the de'ices. One o the most commonl! used ph!sical la!er interace speciications is the recommended standard />S0&*+* speciication. The link la!er component deines the protocol that esta$lishes the connection $etween the 5TE de'ice, such as a router, and the 5CE de'ice, such as a switch. Virtual Circuits (rame >ela! is a 'irtual circuit network, so it doesnQt use ph!sical addresses to deine the 5TEs connected to the network. (rame >ela! pro'ides connection&oriented data link la!er communication. This means that a deined communication e#ists $etween each pair o de'ices and that these connections are associated with a connection identiier. 7owe'er, 'irtual circuit identiiers in (rame rela! operate at the data link la!er, in contrast with H.*, where the! operate at the network la!er. This ser'ice is implemented  $! using a (rame >ela! 'irtual circuit, which is a logical connection created $etween two data terminal e?uipment /5TE0 de'ices across a (rame >ela! packet&switched network /%S30. Virtual circuits pro'ide a $idirectional communication path rom one 5TE de'ice to another and are uni?uel! identiied $! a data&link connection identiier /56CI0.  num$er o 'irtual circuits can $e multiple#ed into a single ph!sical circuit or transmission across the network. This capa$ilit! oten can reduce the e?uipment and network comple#it! re?uired to connect multiple 5TE de'ices.  'irtual circuit can pass through an! an ! num$er o intermediate 5CE de'ices /switches0 located within the (rame >ela! %S3. %S 3. Beore going into the details o 56CI let us irst ha'e a look at the two t!pes o (rame >ela! Circuits, namel!: switched 'irtual circuits /SVCs0 and permanent 'irtual circuits /%VCs0. Switched 'irtual circuits /SVCs0 are temporar! connections used in situations re?uiring onl! sporadic data transer $etween 5TE de'ices across the (rame >ela! network.  communication session across an SVC consists o the ollowing our operational states: )G Call setupXThe 'irtual circuit $etween two (rame >ela! 5TE de'ices is esta$lished. G 5ata transerX5ata is transmitted $etween the 5TE de'ices o'er the 'irtual circuit. G IdleXThe connection $etween 5TE de'ices is still acti'e, $ut no data is transerred. I an SVC remains in an idle state or a deined period o time, the call can $e terminated. G Call terminationXThe 'irtual circuit $etween 5TE de'ices is terminated.

 

ter the 'irtual circuit is terminated, the 5TE de'ices must esta$lish a new n ew SVC i there is additional data to $e e#changed. e# changed. It is e#pected that SVCs will $e esta$lished, maintained, and terminated using the same signaling protocols used in IS53. %ermanent Virtual Circuits %ermanent 'irtual circuits /%VCs0 are permanentl! esta$lished connections that are used or re?uent and consistent data transers $etween 5TE de'ices across the (rame >ela! network. Communication across %VC does not re?uire the call setup and termination states that are used with SVCs. %VCs alwa!s operate in one o the ollowing two operational states: )Y 5ata transer: 5ata is transmitted $etween the 5TE de'ices o'er the 'irtual circuit. YIdle: The connection $etween 5TE de'ices is acti'e, $ut no data is transerred. -nlike SVCs, %VCs will not $e terminated under an! circumstances when in an idle state. 5TE de'ices can $egin transerring data whene'er the! are read! $ecause the circuit is permanentl! esta$lished. 5ata&6ink Connection Identiier /56CI0 (rame >ela! 'irtual circuits are identiied data&link identiiers /56CIs0. the 56CI 'alues t!picall! are assigned $! the $! (rame >ela!connection ser'ice pro'ider /or e#ample, telephone compan!0. (rame >ela! 56CIs ha'e local signiicance, which means that their 'alues are uni?ue in the 63, $ut not necessaril! in the (rame >ela! 13. The local 5TEs use this 56CI to send rames to the remote 5TE. 56CIs are not onl! used to deine d eine the 'irtual circuit $etween a 5TE and a 5CE, $ut also to deine the 'irtual circuit $etween two 5CEs /switches0 inside the network.  switch assigns a 56CI to each 'irtual connection in an interace. This means that two dierent connections $elonging to two dierent interaces ma! ha'e the same 56CIs /as shown in the a$o'e igure0. In other words, 56CIs are uni?ue or a particular interace.

 connection $etween 5TE  and 5TE 5 has $een shown in this igure, 56CI assigned inside the (rame >ela! network is also shown in the network. 5CEs inside the network use u se incoming interace P 56CI com$ination to decide the outgoing interace P 56CI com$ination to switch out the rame, rom that 5CE.

Each switch in a (rame rela! network has a ta$le to route rames The ta$le matches the incoming interace& 56CI com$ination with an outgoing interace&56CI com$ination. ) (rame >ela! 6a!ers

 

(rame >ela! has onl! * la!ers, namel! %h!sical la!er and 5ata 6ink la!er. nd as compared to other la!er o packet pac ket switching network such as H.*, rame rela! has onl! ). la!ers whereas H.* has * la!ers. (rame >ela! eliminates all network la!er unctions and a portion o con'entional data&link la!er unctions. %h!sical 6a!er  3o speciic protocol is deined or ph!sical la!er in rame rela!. rela!. (rame rela! supports an! one o the protocols recogni"ed $! 3SI, and thus the choice o ph!sical la!er  protocol is up to the implementer. 5ata 6ink 6a!er t 5ata&link 6a!er (rame emplo!s a simpler 'ersion o 756C. Simpler 'ersion is used  $ecause 756C pro'ides e#tensi'e error and low control ields that are not needed in rame rela!. To understand much o the unctionalit! o (rame >ela!, it is helpul to understand the structure o the (rame >ela! rame. (igure 4..4 4. .4 depicts the $asic ormat o the (rame >ela! rame. (lags indicate the $eginning and end o the rame. Three primar! components make up the (rame >ela! rame: the header and address area, the user&data  portion, and the rame check se?uence /(CS0. The address area, which is * $!tes in length, is comprised o )9 $its representing the actual circuit identiier and  $its o ields related to congestion management. This identiier commonl! is reerred to as the data& link connection identiier /56CI0. )G (lagsX5elimits the $eginning and end o the rame. The 'alue o this ield is alwa!s the same and is represented either as the he#adecimal num$er E or as the $inar! num$er 9))))))9. G ddressXContains the ollowing inormation: 56CIXThe )9&$it 56CI is the essence o the (rame >ela! header. This 'alue represents the 'irtual connection $etween the 5TE de'ice and the switch. Each 'irtual connection that is multiple#ed onto the ph!sical channel will $e represented $! a uni?ue 56CI. The 56CI 'alues ha'e local signiicance onl!, which means that the! are uni?ue onl! to the ph!sical channel on which the! reside. Thereore, de'ices at opposite ends o a connection can use u se dierent 56CI 'alues to reer to the same 'irtual connection. con nection. The irst &$its o the irst $!te make up part ) o the 56CI, and second part o 56CI uses the irst 4&$its o second $!te. E#tended ddress /E0XThe E is used to indicate whether the $!te in which the E 'alue is ) is the last addressing ield. I the 'alue is ), then the cu current rrent $!te is determined to $e the last 56CI octet. lthough current (rame >ela! implementations all use a two& octet 56CI, this capa$ilit! does allow longer 56CIs to $e $ e used in the uture. The eighth  $it o each $!te o the ddress ield is used to indicate the E. C>XThe C> is the $it that ollows the most signiicant 56CI $!te in the ddress ield. The C> $it is not currentl! deined. Congestion ControlXThis consists o the + $its that control the (rame >ela! congestion& notiication mechanisms. These are the (EC3, BEC3, and 5E $its, which are the last +  $its in the ddress ield.

 

(orward&e#plicit congestion notiication /(EC30 is a single&$it ield that can $e set to a 'alue o ) $! a switch to indicate to an end 5TE de'ice, such as a router, that congestion was e#perienced in the direction o the rame transmission rom source to destination as shown in (ig. 4... The primar! $eneit o the use o the (EC3 and BEC3 ields is the capa$ilit! o higher&la!er protocols to react intelligentl! to these congestion indicators. Toda!, 5ECnet and OSI are the onl! higher&la!er protocols that implement these capa$ilities. Backward&e#plicit congestion notiication /BEC30 is a single&$it ield that, when set to a 'alue o ) $! a switch, indicates that congestion was e#perienced in the network in the direction opposite o the rame transmission rom source to destination. 5iscard eligi$ilit! /5E0 is set $! the 5TE de'ice, such as a router, to indicate that the marked rame is o lesser importance relati'e to other rames $eing transmitted. (rames that are marked as Kdiscard eligi$leK should $e discarded $eore other rames in a congested network. This allows or a $asic prioriti"ation mechanism in (rame >ela! networks. Backward&e#plicit congestion notiication G5ataXContains encapsulated upper&la!er data. Each rame in this 'aria$le& length ield includes a user data or pa!load ield that will 'ar! in length up to ),999 octets. This ield ser'es to transport the higher&la!er protocol packet /%5-0 through a (rame >ela! network. G(rame Check Se?uenceXEnsures the integrit! o transmitted data. This 'alue is computed $! the source de'ice and 'eriied $! the recei'er to ensure integrit! o transmission. Summar! )G (rame rela! operates onl! in data link and ph!sical la!er. G (rame >ela! allows $urst! traic. G It allows rame si"e o @999 $!tes, $ !tes, which can accommodate all local area network rames. G (rame rela! is less e#pensi'e then other traditional 13s. G (rame rela! pro'ides $oth %ermanent and switched connections. G (rame rela! allow 'aria$le&length rames, this ma! create 'ar!ing dela!s or dierent users. 5ue to 'aria$le dela! it is not no t suita$le or real&time communication

s!nchronous Transer 2ode Switching /T20

 

'igure (,-. (,-..

AAL Types AAL

Introduction  Asynchronous Transfer Mode (ATM) is an International Telecommunication -nion& Telecommunications Standards Section /IT-&T0 standard or cell rela! wherein inormation or multiple ser'ice t!pes, such as 'oice, 'ideo, or data, is con'e!ed in small, i#ed&si"e cells. T2 networks are connection&oriented. s!nchronous transer mode /T20 is a technolog! that has its histor! in the de'elopment o $road$and IS53 in the )@9s and )@<9s. Technicall!, it can $e 'iewed as an e'olution o packet switching. 6ike  packet switching protocols or data /e.g., H.*, rame rela!, Transmission Control %rotocol and Internet protocol /TC% I%Z0, T2 integrates the multiple#ing and switching unctions, is well suited or $urst! traic /in contrast to circuit switching0, and allows communications $etween de'ices that operate at dierent speeds. -nlike packet switching, T2 is designed or high&perormance multimedia networking. T2 technolog! has $een implemented in a 'er! $road range o networking de'ices. The most  $asic ser'ice $uilding $lock is the T2 'irtual circuit, which is an end&to&end connection that has deined end points and routes $ut does not ha'e $andwidth dedicated to it. Bandwidth is allocated on demand $! the network as users ha'e traic to transmit. T2 also deines 'arious classes o ser'ice to meet a $road range o application needs. This lesson pro'ides an o'er'iew o T2 protocols, ser'ices, and an d operation.

enefits of AT" The high&le'el $eneits deli'ered through T2 ser'ices deplo!ed on T2 technolog! using international T2 standards can $e summari"ed as ollows: )G D,namic -and)idt+ for -urst, traffic meeting application needs and deli'ering high utili"ation o networking resources most applications are or can $e 'iewed as inherentl!

 

 $urst!, or e#ample 'oice is $urst!, as $oth parties are neither speaking at once nor all the time 'ideo is $urst!, as the amount o motion and re?uired resolution 'aries o'er time. G Smaller +eader with respect to the data to make the eicient use o $andwidth. G Can +andle "i6ed net)or* traffic 4er, efficientl,: Variet! o packet si"es makes traic unpredicta$le. ll network e?uipments should incorporate ela$orate sotware s!stems to manage the 'arious si"es o packets. T2 handles these pro$lems eicientl! with the i#ed si"e cell. G Cell net)or*: ll data is loaded into identical cells that can $e transmitted with complete predicta$ilit! and uniormit!. )G Class&o&ser'ice support or multimedia traic allowing applications with 'ar!ing throughput and latenc! re?uirements to $e met on a single network. G Scala$ilit! in speed and network si"e supporting link speeds o T)E) to OCP)* /** 2$ps0. G Common 6313 architecture allowing T2 to $e used consistentl! rom one desktop to another traditionall!, 63 and 13 technologies ha'e $een 'er! dierent, with implications or perormance and interopera$ilit!. But T2 technolog! can $e used either as a 63 technolog! or a 13 technolog!. G International standards compliance in central&oice and customer&premises en'ironments allowing or multi'endor operation. T2 5e'ices and the 3etwork En'ironment T2 is a cell&switching and multiple#ing technolog! that com$ines co m$ines the $eneits o circuit switching /guaranteed capacit! and constant c onstant transmission dela!0 with those o  packet switching /le#i$ilit! and eicienc! or intermittent intermittent traic0. It pro'ides scala$le  $andwidth rom a ew mega$its per second /2$ps0 to man! giga$its per second /D$ps0. Because o its as!nchronous nature, T2 is more eicient than s!nchronous technologies, such as time&di'ision multiple#ing /T520. 1ith T52, each user is assigned to a time slot, and no other station can send in that time slot as shown in (ig. 4..). I a station has much d data ata to send, it can send onl! when its time slot comes up, e'en i all other time slots are empt!. 7owe'er, i a station has nothing to transmit when its time slot comes up, the time slot is sent empt! and is wasted.

 

Because T2 is as!nchronous, time slots are a'aila$le on demand with inormation identi!ing the source o the transmission contained in the header o each T2 cell. (igure ( igure 4..* shows how cells rom + inputs ha'e $een multiple#ed. t the irst clock tick input * has no data to send, so multiple#er ills the slot with the cell rom third input. 1hen all cells rom input channel are multiple#ed then output slot are empt!.

T2 5e'ices n T2 network is made up o an T2 switch and T2 end endpoints. points. n T2 switch is responsi$le or cell transit through an T2 network. The Ao$ o an T2 switch is well deined. It accepts the incoming cell rom an T2 endpoint or another T2 switch. It then reads and updates the cell header inormation and ?uickl! switches the cell to an output interace towards its destination. n T2 endpoint /or end s!stem0 contains an T2 network interace adapter. E#amples o T2 endpoints are workstations, routers, digital ser'ice units /5S-s0, 63 switches, and 'ideo coder&decoders /CodecQs0. T2 3etwork Interaces n T2 network consists o a set o T2 switches interconnected $! point&to&point T2 links or interaces. T2 switches support two primar! t!pes o interaces: -3I and 33I as shown in (ig. 4..+. The -3I /-ser&3etwork Interace0 connects T2 end s!stems /such as hosts and routers0 to an T2 switch. The 33I /3etwork&3etwork Interace0 connects two T2 switches. 5epending on whether the switch is owned and located at the customer=s premises or is pu$licl! owned and operated $! the telephone compan!, -3I and 33I can $e urther su$di'ided into pu$lic and pri'ate -3Is and  33Is.  pri'ate -3I connects an T2 endpoint and a pri'ate T2 switch. Its pu$lic counterpart connects an T2 endpoint endpo int or pri'ate switch to a pu$lic switch.  p pri'ate ri'ate  33I connects two T2 switches within the same pri'ate organi"ation. organi"ation.  pu$lic one connects two T2 switches within the same pu$lic organi"ation.

T2 transers inormation in i#ed&si"e units called cells. Each cell consists o + octets, or $!tes as shown in (ig. 4..4. 4. .4. The irst  $!tes contain cell&header inormation, and the remaining 4< contain the pa!load /user inormation0. Small, i#ed&length cells are well suited rom to transer andor 'ideo traic $eca $ecause use such traic is among intolerant to dela!s result ha'ing'oice to wait a large d ata data packet to download, other things.that

 

7eader %a!load

n T2 cell header can $e one o two ormats: -3I or 33I. The -3I header is used or communication $etween T2 endpoints end points and T2 switches in pri'ate T2 networks. The 33I header is used or communication $etween T2 switches. (igure 4.. depicts the T2 -3I cell header ormat, and the T2 33I cell heade headerr ormat. -nlike the -3I, the 33I header does d oes not include the Deneric (low Control /D(C0 ield. dditionall!, the 33I header has a Virtual %ath Identiier /V%I0 ield that occupies the irst )* $its, allowing or larger trunks $etween pu$lic T2 switches.  $!tes 4< $!tes (igure 4..4 T2 cell (ormat Version * CSE IIT, 8haragpur D(C

V%I

V%I

VCI

V%I

%T

VCI

C6%

%T

7EC

C6%

%a!load

7EC

/4< $!tes0

%a!load /4< $!tes0 T2 Cell 7eader (ields The ollowing descriptions summari"e the T2 cell header ields shown in (ig. 4... )G Deneric (low Control /D(C0X%ro'ides local unctions, such as identi!ing multiple stations that share a single T2 interace. This ield is t!picall! not used and a nd is set to its deault 'alue o 9 /$inar! 99990. G Virtual %ath Identiier /V%I0XIn conAunction with the VCI, identiies the ne#t destination o a cell as it passes through a series o T2 switches on the wa! to its destination. G Virtual Channel Identiier /VCI0XIn conAunction with the V%I, identiies the ne#t destination o a cell as it passes through a series o T2 switches on the wa! to its destination. G %a!load T!pe /%T0XIndicates in the irst $it whether the cell contains user data or control data. I the cell contains user data, the $it is set to 9. I it con contains tains control data, it is set to ). The second $it indicates congestion /9  no congestion, )  congestion0, and

 

the third $it indicates whether the cell is the last in a series o cells that represent a single 6 rame /)  last cell or the rame0. G Cell 6oss %riorit! /C6%0XIndicates whether the cell should $e discarded i it encounters e#treme congestion as it mo'es through the network. I the C6% $it e? e?uals uals ), the cell should $e discarded in preerence to cells with the C6% $it e?u e?ual al to 9. G 7eader Error Control /7EC0XCalculates checksum onl! on the irst 4 $!tes o the header. 7EC can correct a single $it error in these $!tes, there$! preser'ing the cell rather than discarding it. T2 Virtual Connections T2 standard deines two t!pes o T2 connections: 'irtual path connections /V%Cs0, which contain 'irtual channel connections /VCCs0 as shown in (ig. 4...  'irtual channel connection /or 'irtual circuit0 is the $asic $ asic unit, which carries a single stream o cells, in order, rom user to user.  collection o 'irtual circuits can $e $undled together into a 'irtual path connection.  'irtual path connection can $e created rom end&to&end across an T2 network. In this case, the T2 network does not route cells $elonging to a particular 'irtual circuit. ll cells $elonging to a particular 'irtual path are routed the same wa! through the T2 network, thus resulting in aster reco'er! in case o maAor ailures. In this case, all the switches within the T2 network are onl! V% switches, i.e. the! switch the cells onl! on the $asis o V%Is. Onl! the switches, which are conn connected ected to the su$scri$ers are V%VC switches, i.e. the! use $oth V%Is and VCIs to switch the cell. This coniguration is usuall! ollowed so that the intermediate switches can do switching much aster. Virtual channel connections o T2 n T2 network also uses 'irtual paths internall! or the purpose o $undling 'irtual circuits together $etween switches. Two T2 switches ma! ha'e man! dierent 'irtual channel connections $etween them, $elonging to dierent users. These can $e $undled $! two T2 switches into a 'irtual path connection. This can ser'e the purpose o a 'irtual trunk $etween the two switches. This 'irtual trunk can then $e handled as a single entit!  $! perhaps, multiple intermediate 'irtual paths cross connects $etween the two 'irtual circuit switches.

 ATM Switching Operations The $asic operation o an T2 switch is straightorward: The cell is recei'ed across a link with a known V%IVCI 'alue. The switch looks up the connection 'alue in a local translation ta$le to determine the outgoing port /or ports0 o the connection and the n new ew V%IVCI 'alue o the connection on that link. The switch then retransmits the cell on that outgoing link with the appropriate connection identiier. Incoming

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 V%VC T2 switch ta$le Because all VCIs and V%Is ha'e onl! local signiicance across a particular link, these 'alues are remapped, as necessar!, at each switch. (igure 4.. and (ig. 4..< shows a V%&VC switch and an onl! V% switch, respecti'el!. -suall! the intermediate switches are onl! V%I switches while switches connected to the users are V%IVCI switches. Incoming Outgoing V%I

V%I Interace

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V% T2 switch ta$le To make the switching more eicient, T2 uses two t!pes o switches namel!, V% switch and V%&VC switch.  V% switch route cells onl! on the $asis o V%I, here V%Is change $ut VCIs remain same during switching. On the other hand, V%&VC switch uses the complete identiier, i.e. $oth V%I and VCI to route the cell. 1e can think o a V%&VC switch as a com$ination o Onl! V% and Onl! VC switch.

!"!" ATM #eference Model The T2 architecture uses a logical model to descri$e the unctionalit! that it supports. T2 unctionalit! corresponds to the ph!sical la!er and part o the data link la!er o the OSI reerence model. The T2 reerence model, as shown in (ig. 4..@, consists o the ollowing planes, which span all la!ers: )G ControlXThis plane is responsi$le or generating and managing signaling re?uests. G -serXThis plane is responsi$le or managing the transer o data. G 2anagementXThis plane contains two components: 6a!er management  protocol pro$lems. manages la!er&speciic unctions, such as the detection o ailures and %lane management manages and coordinates unctions related to the complete s!stem.

 

The T2 reerence model consists o the ollowing T2 la!ers: )G %h!sical la!erXnalogous to the ph!sical la!er o o  the OSI reerence model, the T2  ph!sical la!er manages the medium&dependent transmission. )G T2 la!erXCom$ined with the T2 adaptation la!er, the T2 la!er is roughl! analogous toor thethe data link la!er osharing the OSI model.o'er TheaT2 la!er is /cell responsi$le simultaneous oreerence 'irtual circuits ph!sical link multiple#ing0 and passing cells through the T2 network /cell rela!0. To do this, it uses the V%I and VCI inormation in the header o each T2 cell. G T2 adaptation la!er /60XCom$ined with the T2 la!er, the 6 is roughl! analogous to the data link la!er o the OSI model. The 6 is responsi$le or isolating higher&la!er protocols rom the details o the T2 processes. The adaptation la!er  prepares user data or con'ersion into cells and segments the data into 4<&$!te cell  pa!loads. (inall!, the higher la!ers residing a$o'e the 6 accept user data, arrange it into  packets, and hand it to the 6. The T2 %h!sical 6a!er The main unctions o the T2 ph!sical ph !sical la!er are as ollows: )G Cells are con'erted into a $it stream, G The transmission and receipt o $its on the ph!sical medium are controlled, G T2 cell $oundaries are tracked, G Cells are packaged into the appropriate t!pes o rames or the ph!sical medium. The T2 ph!sical la!er is di'ided into two parts: the ph!sical medium&dependent /%250 su$ la!er and the transmission con'ergence /TC0 su$ la!er. The %25 su$ la!er pro'ides two ke! unctions. )G It s!nchroni"es transmission and reception $! sending and recei'ing a continuous low o with associated timing inormation. G It$its speciies the ph!sical media or the ph!sical p h!sical medium used, including connector t!pes and ca$le. The TC su$ la!er has our unctions: )G Cell delineation, it maintains T2 cell $oundaries, allowing de'ices to locate cells within a stream o $its. G Denerates and checks the header error control code to ensure 'alid data. G Cell&rate decoupling, maintains s!nchroni"ation and inserts or suppresses idle /unassigned0 T2 cells to adapt the rate o 'alid T2 cells to the pa!load cap capacit! acit! o the transmission s!stem. G Transmission rame adaptation packages T2 cells into rames accepta$le to the  particular ph!sical la!er implementation. T2 6a!er

 

The T2 la!er pro'ides routing, traic management, switching and multiple#ing ser'ices. It processes outgoing traic $! accepting 4<&$!te segment rom the 6 su$& la!ers and transorming them into +&$!te cell $! addition o a &$!te header. d daptation aptation 6a!ers T2 adaptation la!ers allow e#isting packet networks to connect to T2 acilities. 6 %rotocol acceptsT2 transmission romtransmissions upper la!er ser'ices /e.g.: data0 and them into i#ed&si"ed cells. These can $e o an!packet t!pe, 'aria$le ormap i#ed data rate. t the recei'er, this process is re'ersed and segments are reassem$led into their original ormats and passed to the recei'ing ser'ices. Instead o one protocol or all t!pes o data, the T2 standard di'ides the 6 la!er into categories, each supporting the re?uirements o dierent t!pes o applications. There are our t!pes o data streams that are identiied: Constant&$it rate, 'aria$le $it&rate, connection oriented  packet data transer, connectionless packet data transer. In addition to di'iding 6 $! categor! /6), 6* and so on0, IT-&T also di'ides it on the $asis o unctionalit!. Each 6 la!er is actuall! di'ided into two la!ers: the con'ergence su$&la!er and Segmentation and reassem$l! /S>0 su$&la!er. Ta$le $elow gi'es a $rie description o these data streams and 'arious T2 adaptation la!ers which are used or each o them. Ta$le 2apping o 'arious data t!pes and T2 adaptation la!ers Ser'ice Class

;ualit! o Ser'ice %arameter

Constant Bit rate /CB>0

This class is used or 6): 6), a connection& emulating circuit switching. oriented ser'ice, is suita$le or The cell rate is constant with handling constant $it rate sources time. CB> applications are /CB>0, such as 'oice and ?uite sensiti'e to cell&dela! 'ideoconerencing. 6) 'ariation. E#amples o re?uires timing s!nchroni"ation applications that can use CB>  $etween the source and the are telephone traic /i.e., n#4 destination. (or this reason, k$ps0, 'ideoconerencing, and 6) depends on a medium, tele'ision.

T2 daptation la!ers

such as SO3ET, that supports clocking. The 6) process  prepares a cell or transmission in three steps. (irst, s!nchronous samples /or e#ample, ) $!te o data at a sampling rate o *99 microseconds0 are inserted into the %a!load ield. Second, Se?uence 3um$er /S30 and Se?uence 3um$er %rotection /S3%0 ields are added to pro'ide inormation that the recei'ing 6) uses to 'eri! that it has recei'ed cells in the correct order. Third, the remainder o the

 

%a!load ield is illed with enough single $!tes to e?ual 4<  $!tes.

Varia$le Bit >ate & non&real time /VB>P   3>T0

Varia$le $it ratePreal time /VB>P>T0

This class allows users to send traic at a rate that 'aries with time depending on the a'aila$ilit! o user inormation. Statistical multiple#ing is pro'ided to make optimum use o network resources. 2ultimedia e&mail is an e#ample o VB>P3>T. This class is similar to VB>P3>T $ut is designed or applications that are sensiti'e to cell&dela! 'ariation. E#amples or real&time VB> are 'oice with speech acti'it! detection /S50 and interacti'e compressed 'ideo.

6 *: The 6* process uses 44 $!tes o the cell  pa!load or user data and reser'es 4 $!tes o the  pa!load to support the 6*  processes. VB> traic is characteri"ed as either real&time /VB>&>T0 or as non&real&time /VB>&  3>T0. 6* supports $oth t!pes o VB> traic.

 

Connection oriented packet transer or a'aila$le $it rate /B>0

Connectionless data transer or unspeciied $it rate /-B>0

This class o T2 ser'ices 6+4: 6+4 supports  pro'ides rate&$ased low  $oth connection&oriented and control and is aimed at data connectionless data. 6+4 traic such as ile transer  prepares a cell or and e&mail. lthough the transmission in our steps. standard does not re?uire the cell transer dela! and (irst, the con'ergence su$ la!er /CS0 creates a protocol cell&loss ratio to $e data unit /%5-0 $! guaranteed or minimi"ed, it  prepending a $eginningend is desira$le or switches to tag header to the rame and minimi"e dela! and loss as appending a length ield as a much as possi$le. trailer. Second, the 5epending upon the state segmentation and reassem$l! o congestion in the /S>0 su$ la!er ragments network, the source is the %5- and prepends a re?uired to control its rate. header to it. Then the S> The users are allowed to su$ la!er appends a C>C&)9 declare a minimum cell trailer to each %5- ragment rate, which is guaranteed to or error control. (inall!, the the connection $! the completed S> %5network.  $ecomes the %a!load ield o an T2 cell to which the This class is the catch&all, T2 la!er prepends the other class and is widel! standard T2 header. used toda! or TC%I%. 6 : 6 is the primar! 6 or data and supports  $oth connection&oriented and connectionless data. It is used to transer most non&S25S data, such as classical I% o'er T2 and6 63 Emulation /63E0. also is known as the simple and eicient adaptation la!er /SE60

T2 pplications T2 is used in $oth 63s and 13s letQs ha'e a look at ew o the possi$le applications. T2 13s: T2 is $asicall! a 13 technolog! that deli'ers cell o'er o' er long distances. 7ere T2 is mainl! used to connect conne ct 63s or other 13s together.  router $etween T2 network and the other network ser'es as an end point. This router has two stacks o  protocols: one $elonging to T2 and other $elonging to other protocol. T2 63s: 7igh data rate /) and ** 2$ps0 o T2 technolog! attracted a ttracted designers to think o implementing T2 technolog! in 63s too. t the surace le'el, to implement an T2 63 T2 switch will replace the traditional Ethernet switch, in a

 

switched 63. But ew things ha'e to $e kept in mind and sotware modules would $e needed to map the ollowing dierences $etween the two technologies: )G Connectionless 'ersus connection&oriented: T2 is a 'irtual connection oriented technolog!, while traditional Ethernet uses connectionless protocols. G %h!sical address 'ersus 'irtual circuit identiier: In the Traditional 63 packets are routed $ased on the source and destination addresses, while in T2 cells are routed  $ased on the 'irtual circuit identiiers /V%I&VCI pair0. 2ultimedia 'irtual pri'ate networks and managed ser'ices: Ser'ice pro'iders are  $uilding on their T2 networks to oer a $road range o ser'ices. E#amples include managed T2, 63, 'oice and 'ideo ser'ices /these $eing pro'ided on a per& application $asis, t!picall! including customer&located e?uipment and oered on an end& to&end $asis0, and ull&ser'ice 'irtual pri'ate&networking capa$ilities /these including integrated multimedia access and network management0. (rame&rela! $ack$ones: (rame&rela! ser'ice pro'iders are deplo!ing T2 $ack$ones to meet the rapid growth o their rame&rela! ser'ices to use as a networking inrastructure or a range o data ser'ices and to ena$le rame rela! to T2 ser'ice internetworking ser'ices. Internet $ack$ones: Internet ser'ice pro'iders are likewise deplo!ing T2 $ack$ones $a ck$ones to meet the rapid growth o their rame&rela! ser'ices, to use as a networking inrastructure or a range o data ser'ices, and to ena$le Internet class&o&ser'ice oerings and 'irtual  pri'ate intranet ser'ices. >esidential $road$and networks: T2 is the networking inrastructure o choice or carriers esta$lishing residential $road$and ser'ices, dri'en $! the need or highl! scala$le solutions. Carrier inrastructures or the telephone and pri'ate&line networks: Some carriers ha'e identiied opportunities to make more&eecti'e use o their SO3ETS57 i$er inrastructures $! $uilding an T2 inrastructure to carr! their telephon! and pri'ate& line traic.

 3etwork Topolog! Introduction Topolog! reers to the wa! in which the network o computers is connected. Each topolog! is suited to speciic tasks and has its own ad'antages and disad'antages. The choice o topolog! is dependent upon t!pe and num$er o e?uipment $eing used, planned applications and rate o data transer re?uired, response time, and cost. Topolog! can also  $e deined as the geometrically the geometrically interconnection pattern $! pattern $! which the stations /nodescomputers0 are connected using suita$le transmission media /which can $e point& to&point and $roadcast0. Various commonl! used topologies are discussed in the ollowing sections.

2esh Topolog!

 

In this topolog! each node or station is connected to e'er! other station The ke! characteristics o this topolog! are as ollows: 8e! Characteristics: )o (ull! connected *o >o$ust P 7ighl! relia$le +o  3ot le#i$le 4o %oor e#panda$ilit!

Two nodes are connected $! dedicated point&point links $etween them. So the total num$er o links to connect n nodes  n/n&)0* which is proportional to n * . 2edia used or the connection /links0 can $e twisted pair, co&a#ial ca$le or optical i$er. 1ith this topolog! there is no need to pro'ide an! additional inormation, that is rom where the  packet is coming, along with the packet $ecause two nodes ha'e a point&point dedicated link $etween them. nd each node knows which link is connected to which node on the other end. 2esh Topolog! is not le#i$le and has a poor e#panda$ilit! as to add a new node n links ha'e to $e laid $ecause that new node has to $e connected to each o the e#isting nodes 'ia dedicated link as shown in (ig. .).*. (or the same reason the cost o ca$ling will $e 'er! high or a larger area. nd due to these reasons this topolog! is rarel! used in  practice.

Bus Topolog! In Bus Topolog!, all stations attach through appropriate hardware interacing known as a tap, directl! to a linear transmission medium, or $us as shown in (ig. .).+. (ull&duple# operation $etween the station and the tap allows data to $e transmitted onto the $us and recei'ed rom the $us.  transmission rom an! station propagates the length o the medium in $oth directions and can $e recei'ed $! all other stations. t each end o the  $us there is a terminator, which a$sor$s an! signal, pre'enting relection o signal rom rom the endpoints. I the terminator is not present, the endpoint acts like a mirror and relects the signal $ack causing intererence and other pro$lems. (igure .).+ Bus Topolog! 8e! Characteristics o this topolog! are: )o (le#i$le o E#panda$le o 2oderate >elia$ilit! o 2oderate perormance  shared used $etween d ierent dierent stations. 7ence it is 'er! cost eecti'e. One can easil! addlink an!isnew node or delete an! node without aecting other nodes this makes this topolog! easil! e#panda$le.

 

some e#tra inormation a$out the desired destination, i.e. to e#plicitl! speci! the destination in the packet, as compared to mesh topolog!. This is $ecause the same medium is shared among man! nodes. s each station has a uni?ue address in the network, a station copies a packet onl! when the destination address o the packet matches with the sel&address. This is how data communications take place among the stations on the $us. s there are dedicated links in the mess topolog!, there is a possi$ilit! o transerring data in parallel. But in $us topolog!, onl! o nl! one station is allowed to send data at a time and all other o ther stations listen to it, as it works in a $roadcast mode. 7ence, onl! one station can transer the data at an! gi'en time. Suita$le medium access ac cess control techni?ue should $e used so as to pro'ide some wa! to decide [who\ will go ne#t to send dataF -suall! a distri$uted medium access control techni?ue, as discussed in the ne#t lesson, is used or this purpose. s the distance through which signal tra'erses increases, the attenuation increases. I the sender sends data /signal0 with a small strength signal, the arthest station will not $e a$le to recei'e the signal properl!. 1hile on the other han hand d i the transmitter sends the signal with a larger strength /more power0 then the arthest station will get the signal properl! $ut the station near to it ma! ace o'er&dri'e. 7ence, dela! which can $e and usedsignal in $usun$alancing topolog!. will orce a ma#imum length o shared medium, ST> Topolog! In the star topolog!, each station is directl! connected to a common central node T!picall!, each station attaches to a central node, reerred to as the star coupler, 'ia two  point&to&point links, one or transmission transmission and one or reception. 8e! eatures: )o 7igh Speed *o Ver! (le#i$le +o 7igh >elia$ilit! 4o 7igh 2aintaina$ilit!

In general, there are two alternati'es a lternati'es or the operation o the central node. )o One approach is or the central node to operate in a $roadcast ashion.  transmission o a rame rom one station to the node is retransmitted on all o the

outgoing links. In this case, although the arrangement is ph!sicall! a star, it is logicall! a  $us a transmission rom an! station is recei'ed recei'ed $! all other stations, and onl! one station at a time ma! successull! transmit. In this case the central node n ode acts as a repeater. o nother approach is or the central node to act as a rame&switching de'ice. n incoming rame is $uered in the node and then retransmitted on an outgoing link to the destination station. In this approach, the central node n ode acts as a switch and perorms the switching or routing unction. operation can $eto compared with the working o a telephone e#change, whereThis the mode caller o part! is connected a single called part! and each pair o su$scri$er who needs to talk ha'e a dierent connection.

 

Ver! 7igh speeds o data transer can $e achie'ed $! using star topolog!, particularl! when the star coupler is used in the switch mode. This topolog! is the easiest to maintain, among the other topologies. s the num$er o links is proportional to n, this topolog! is 'er! le#i$le and is the most preerred topolog!. >ing topolog! In the ring topolog!, the network n etwork consists o a set o repeaters Aoined $! point&to&point po int&to&point links in a closed loop. The repeater is a comparati'el! simple de'ice, capa$le o recei'ing data on one link and transmitting them, $it $! $it, on the other link as ast as the! are recei'ed, with no $uering at the repeater. The links are unidirectional that is data are transmitted in one direction onl! and all are oriented in the same wa!. Thus, data circulate around the ring in one on e direction /clockwise or counterclockwise0. >ing Topolog! Each station attaches to the network at a repeater and can transmit data onto the network through that repeater. s with the $us and tree, data are transmitted in rames. s a rame circulates past all the other stations, the destination station recogni"es its address and copies the rame into a local $uer as it goes $!. The rame continues to circulate until it returns to the source station, where it is remo'ed. Because multiple stations share the ring, medium access control is needed to determine at what time each station ma! insert rames. 7ow the source knows whether it has to transmit a new packet and whether the pre'ious  packet has $een recei'ed properl! $! the destination or not. (or this, the destination change a particular $it /$its0 in the packet and when the recei'er sees that packet with the changed $it, it comes to know that the recei'er has recei'ed the packet. This topolog! is not 'er! relia$le, $ecause $e cause when a link ails the entire ring connection is  $roken. But relia$ilit! can $e impro'ed $! using wiring concentrator, which helps in  $!passing a ault! node and somewhat is similar to star topolog!. topolog!. >epeater works in the ollowing three modes: )G 6isten mode: In this mode, the station listens to the communication going o'er the shared medium Transmit mode: In this mode the station transmit the data o'er the network )G B!&%ass mode: 1hen the node is ault! then it can $e $!passed using the repeater in  $!pass mode, i.e. the station doesnQt care a$out what data is transmitted through the network, as shown in (ig. .).<. In this mode there is no dela! introduced $ecause o this repeater.

Tree Topolog! This topolog! can $e considered as an e#tension to $us topolog!. It is commonl! used in cascading e?uipments. (or e#ample, !ou ha'e a repeater $o# with <&port, as ar as !ou ha'e eight stations, this can $e used in a normal ashion. But i !ou need to add more stations then !ou can connect connec t two or more repeaters in a hierarchical ormat /tree ormat0 and can add more stations. In the (ig. .).@, >) reers to repeater one and so on and each repeater is considered to ha'e <&ports. Tree Topolog! This tree topolog! is 'er! good in an organi"ation as incremental e#pansion can  $e done in this wa!. 2ain eatures o this topolog! are scala$ilit! and le#i$ilit!. le#i$ilit!. This is

 

 $ecause, when the need arises or more stations that can $e accomplished easil! without aecting the alread! esta$lished network. -nconstrained Topolog! ll the topologies discussed so ar are s!mmetric and constrained $! well&deined interconnection pattern. 7owe'er, sometimes no deinite pattern is ollowed and nodes are interconnected in an ar$itrar! manner using point&to&point links -nconstrained topolog! allows a lot o coniguration le#i$ilit! $ut suers rom the comple# routing  pro$lem. Comple# routing in'ol'es unwanted o'erhead and dela!. Com$ination o topolog! and transmission media Topolog! and transmission media are interrelated. (or e#ample, e#a mple, all the important criteria o a network such as relia$ilit!, e#panda$ilit! and perormance depend on $oth the topolog! and the transmission media used in the network. s a conse?uence, these two aspects are interrelated. 6et us ha'e a look at the 'arious transmission media, which are used or dierent topologies.

Internetworking 5e'ices Introduction 7I6I su$committee /IEEE<9*.)0 o the IEEE identiied the ollowing possi$le internetworking scenarios. )G  single 63 G Two 63s connected together /63&630 G  63 connected to a 13 /63&130 G Two 63s connected through a 13 /63&13&630 Various internetworking de'ices such as hu$s, $ridges, switches, routers and gatewa!s are re?uired to link them together. These internetworking de'ices are introduced in this lesson.

>epeaters  single Ethernet segment can ha'e a ma#imum length o 99 meters with a ma#imum o )99 stations /in a cheapernet segment it is )<m0. To e#tend the length o the network, a repeater ma! $e used. (unctionall!, a repeater can $e considered as two transcei'ers  Aoined together and connected to two dierent segments o coa#ial ca$le. The repeater  passes the digital signal $it&$!&$it in $oth directions directions $etween the two segments. s the signal passes through a repeater, it is ampliied and regenerated at the other end. The repeater does not isolate one segment rom the other, i there is a collision on one segment, it is regenerated on the other segment. Thereore, the two segments orm a single 63 and it is transparent to rest o the s!stem. Ethernet allows i'e segments to $e used in cascade to ha'e a ma#imum network span o *. km. 1ith reerence o the ISO model, a repeater is considered as a level-1 relay .It .It simpl! repeats, retimes and ampliies the $its it recei'es. The repeater is merel! used to e#tend the span o a single 63. Important eatures o a repeater are as ollows: )G  repeater connects dierent segments o a 63 G  repeater orwards e'er! rame it recei'es

 

G  repeater is a regenerator, not an ampliier G It can $e used to create a single e#tended 63 <u-s 7u$ is a generic term, $ut commonl! reers to a multiport repeater. It can $e used to create multiple le'els o hierarch! o stations. The stations connect to the hu$ with >J&4 connector ha'ing ma#imum segment length is )99 meters. This t!pe o interconnected set o stations is eas! to maintain and diagnose. (igure .).+ shows how se'eral hu$s can $e connected in a hierarchical h ierarchical manner to reali"e a single 63 o $igger si"e with a large num$er o nodes. ridges The de'ice that can $e used to interconnect two separate 63s is known as a $ridge. It is commonl! used to connect two similar or dissimilar 63s. The $ridge operates in la!er *, that is data&link la!er and that is wh! it is called le'el&* rela! with reerence to the OSI model. It links similar or dissimilar 63s, designed to store and orward rames, it is  protocol independent and transparent to the end stations. The low o inormation through a $ridge. -se o $ridges oer a num$er o ad'antages, such as higher relia$ilit!,  perormance, securit!, con'enience and larger geographic co'erage. But, it is desira$le

that ?ualit! o ser'ice oered $! aa'aila$ilit!, $ridge should match that otransit a single 63. The the parameters that deine/;OS0 the ;OS include rame mishaps, dela!, rame lietime, undetected $it errors, rame si"e and priorit!. 8e! eatures o a $ridge are mentioned $elow: )G  $ridge operates $oth in ph!sical and data&link la!er G  $ridge uses a ta$le or ilteringrouting G  $ridge does not change the ph!sical /2C0 addresses in a rame G T!pes o $ridges: 9 o Transparent Bridges o Source routing $ridges  $ridge must contain addressing and routing capa$ilit!. Two routing algorithms ha'e  $een proposed or a $ridged 63 en'ironment. The irst, produced as an e#tension o IEEE <9*.) and applica$le to all IEEE <9* 63s, is known as transparent $ridge. nd the other, de'eloped or the IEEE <9*. token rings, is $ased on source routing approach. It applies to man! t!pes o 63 including token ring, token $us and CS2C5 $us. Transparent Bridges The transparent $ridge uses two processes known as $ridge orwarding and $ridge learning. I the destination address is present in the orwarding data$ase alread! created, the packet is orwarded to the port num$er to which the destination host is attached. I it is not present, orwarding is done on all parts /looding0. This process is known as $ridge orwarding. 2oreo'er, as each rame arri'es, its source address indicates where a  particular host is situated, so that the $ridge learns which wa! to orward rames rames to that address. This process is known as $ridge learning. 8e! eatures o a transparent $ridge are: )G The stations are unaware o the presence o a transparent $ridge )G It perorms two unctions: o (orwar  o 6earning to create th

 

 Bridge (orwarding Bridge orwarding operation is e#plaunctions o the $ridge orwarding ar  G 5iscard the rame i source and destination addresses are same G (orward the rame i the source and destination

Loop #ro-lem (orwarding and learning processes work without an! pro$lem as long as there is no redundant $ridge in the s!stem. On the other hand, redundanc! is desira$le rom the 'iewpoint o relia$ilit!, so that the unction o a ailed $ridge is taken o'er $! a redundant $ridge. The e#istence o redundant $ridges creates the so&called loop pro$lem. ssuming that ater initiali"ation ta$les in $oth the $ridges are empt! let us consider the ollowing steps: Step ). Station& sends a rame to Station&B. Both the $ridges orward the rame to 63 ] and update the ta$le with the source address o . Step *. 3ow there are two copies o the rame on 63&]. The cop! sent $! Bridge&a is recei'ed $! Bridge&$ and 'ice 'ersa. s $oth the $ridges ha'e no inormation a$out Station B, $oth will orward the rames to 63&H. Step +. gain $oth the $ridges will orward the rames to 63&] $ecause o the lack o inormation o the Station B in their data$ase and again Step&* will $e repeated, and so on. So, the rame will continue to loop around the two 63s indeinitel!.

Spanning Tree s redundanc! creates loop pro$lem in the s!stem, it is 'er! undesira$le. To pre'ent loop  pro$lem and proper working o the orwarding and learning processes, there must $e onl! one path $etween an! pair o $ridges and 63s $etween an! two segments in the entire  $ridged 63. The IEEE speciication re?uires that the $ridges use a special topolog!. Such a topolog! is known as spanning tree /a graph where there is no loop0 topolog!. Source >outing Bridges The second approach, known as source routing, where the routing operation is perormed  $! the source host and the rame speciies which route the rame is to ollow.  host can disco'er a route $! sending a disco'er! rame, which spreads through the entire network using all possi$le paths to the destination. Each rame graduall! gathers addresses as it goes. The destination responds to each rame and the source host chooses an appropriate route rom these responses. (or e#ample, a route with minimum hop&count can $e chosen. 1hereas transparent $ridges do not modi! a rame, a source routing $ridge add addss a routing inormation ield to the rame. Source routing approach pro'ides a shortest path at the cost o the prolieration o disco'er! rames, which can put a serious e#tra $urden on the network. (igure .).)) .).) ) shows the rame ormat o a source routing $ridge. Switches  switch is essentiall! a ast $ridge ha'ing additional sophistication that allows aster  processing o rames. Some o important unctionalities unctionalities are: )G %orts are pro'ided with $uer GG Switch maintains a director!: & port^ Each rame is orwarded ater ^address e#amining the ^address and orwarded to the proper  port^

 

G Three possi$le orwarding approaches: Cut&through, Collision&ree and (ull!&$uered as $riel! e#plained $elow. Cut&through:  switch orwards a rame immediatel! ater recei'ing the destination address. s a conse?uence, the switch orwards the rame without collision and error detection. Collision&ree: In this case, the switch orwards the rame ater recei'ing 4 $!tes, which allows detection o collision. 7owe'er, error detection is not possi$le $ecause switch is !et to recei'e the entire rame. (ull! $uered: In this case, the switch orwards the rame onl! ater recei'ing the entire rame. So, the switch can detect $oth collision and error ree rames are orwarded. Comparison -et)een a s)itc+ and a +ulthough a hu$ and a switch apparentl! look similar, the! ha'e signiicant dierences.  $oth can $e used to reali"e ph!sical star topolog!, the hu$s works like a logical $us,  $ecause the same signal is repeated on all the ports. On the other hand, a switch unctions like a logical star with the possi$ilit! o the communication o separate signals $etween an! pair o port lines. s a conse?uence, all the ports o a hu$ $elong to the same collision domain, and in case o a switch each port operates on separate collision domain. 2oreo'er, in case o a hu$, hhand, u$, the is shared $!port all the connected to all the ports. On the other in $andwidth case o a switch, each has stations dedicated $andwidth. Thereore, switches can $e used to increase the $andwidth o a hu$&$ased network $! replacing the hu$s $! switches. Routers  router is considered as a la!er&+ rela! that operates in the network la!er, that is it acts on network la!er rames. It can $e $ e used to link two dissimilar 63s.  router isolates 63s in to su$nets to manage and control network traic. 7owe'er, unlike $ridges it is not transparent to end stations.  router has our $asic components: Input ports, output  ports, the routing processor and the switching a$ric. The unctions o the our components are $riel! mentioned $elow. G Input port perorms ph!sical and data&link la!er unctions o the router. s shown in )G Output ports, as shown in (ig. .).)4/$0, perorm the same unctions as the input ports,  $ut in the re'erse order. G The routing processor perorms the unction o the network la!er. The process in'ol'es ta$le lookup. G The switching a$ric, shown in (ig. .).), mo'es the packet rom the input ?ueue to the output ?ueue $! $ ! using speciali"ed mechanisms. The switching a$ric is reali"ed with the help o multistage interconnection networks. G Communication o a rame through a router is shown in (ig. .).). Datewa!s  gatewa! works a$o'e the network la!er, such as application la!er s a conse?uence, it is known as a 6a!er& rela!. The application le'el gatewa!s can look into the content application la!er packets such as email $eore $e ore orwarding it to the other side. This  propert! has made it suita$le or use in (irewalls discussed discussed in the ne#t module.  Simple Internet  simple internet comprising se'eral 63s and 13s linked with the help h elp o routers

 

 3etwork Securit! 'igure .-/

ion an Decryption Concept of #ncryptio

Speciic Instructional O$Aecti'es On completion, the students will $e a$le to:  State

the need or secured communication  E#plain the re?uirements or secured communication  E#plain the ollowing cr!ptographic algorithms:  S!mmetric&ke! Cr!ptograph! • Traditional ciphers • 2onoalpha$etic Su$stitution • %ol!alpha$etic Su$stitution • Transpositional Cipher • Block ciphers  %u$lic&ke! Cr!ptograph! • The >S lgorithm Introduction The word cr,ptograp+, has come rom a Dreek word, which means secret means secret writing . In the present da! conte#t it reers to the tools and techni?ues used to make messages secure or communication $etween the participants and make messages immune to attacks $! hackers. (or pri'ate communication through  pu$lic network, cr!ptograph! pla!s a 'er! crucial role. The role o cr!ptograph! can $e illustrated with the help a simple model o cr!ptograph! as shown in (ig. <.).). The message to $e sent through an unrelia$le medium is known as plainte6t , which is encr!pted $eore sending o'er the medium. The encr!pted message is known as cip+erte6t , which is recei'ed at the other end o the medium and decr!pted to get $ack the original plainte#t message. In this lesson we shall discuss 'arious cr!ptograph! algorithms, which can $e di'ided into two $road categori"e & S,mmetric *e, cr,ptograp+, and #u-lic *e, cr,ptograp+,0

S!mmetric 8e! Cr!ptograph! The cipher, an algorithm that is used or con'erting the plainte#t to cipherte#, operates on a *e,, which is essentiall! a speciall! generated num$er /'alue0. To decr!pt a secret message /cipherte#t0 to get $ack the original message /plainte#t0, a decr!pt algorithm uses a decr!pt ke!. In s!mmetric ke! cr!ptograph!, same ke! is shared, i.e. the same ke! is used in $oth encr!ption and decr!ption. The algorithm used to decr!pt dec r!pt is Aust the in'erse o the algorithm used or encr!ption. (or e#ample, i addition and di'ision is used or encr!ption, multiplication and su$traction are to $e used or decr!ption. S!mmetric ke! cr!ptograph! algorithms are simple re?uiring lesser e#ecution time. s a conse?uence, these are commonl! used or long messages. 7owe'er, these algorithms suer rom the ollowing limitations:   >e?uirement o large num$er o uni?ue ke!s. (or e#ample or n users the num$er o ke!s re?uired is n/n&)0*.  5istri$ution o ke!s among the users in a secured manner is diicult

 

2onoalpha$etic Su$stitution One simple e#ample o s!mmetric ke! cr!ptograph! is the Monoalphabetic substitution. the Monoalphabetic substitution. In this case, the relationship $etween a character in the  plainte#t and a character in the cipherte#t is alwa!s one&to&one. n e#ample 2onoalpha$etic su$stitution is the Caesar cipher. In this approach a character in the cipherte#t is su$stituted $! another character shited $! three places, e.g.  is su$stituted  $! 5. 8e! eature o this approach is that it is 'er! simple $ut the code can $e attacked 'er! easil!. %ol!alpha$etic Su$stitution This is an impro'ement o'er the Caesar cipher. 7ere the relationship $etween a character in the plainte#t and a character in the cipherte#t is alwa!s one&to&man!E#ample o pol!alpha$etic su$stitution is the Vigenere cipher. In this case, a particular character is su$stituted $! dierent characters in the cipherte#t depending on its position in the plainte#t. 7ere the top row shows dierent characters in the plainte#t and the characters in dierent $ottom rows show the characters  $! which a particular character is to $e replaced depending upon its position in dierent rows rom row&9 to row&*. • 8e! eature o this approach is that it is more comple# and the code is harder to attack a ttack successull!. Transpositional Cipher The transpositional cipher, the characters remain unchanged $ut their positions are changed to create the cipherte#t. The characters are arranged in two&dimensional matri# and columns are interchanged according to a ke! is shown in the middle portion o the diagram. The ke! deines which columns are to $e swapped. s per the ke! shown in the igure, character o column is to $e swapped to column +, character o column * is to $e swapped to column , and so on. 5ecr!ption can $e done $! swapping in the re'erse order using the same ke!. Transpositional cipher is also not a 'er! secure approach. The attacker can ind the plainte#t $! trial and error utili"ing the idea o the re?uenc! o occu occurrence rrence o characters. Block Ciphers Block ciphers use a $lock o $its as the unit o encr!ption and decr!ption. To encr!pt a 4&$it $lock, one has to take each o the *4 input 'alues and map it to one o the *4 output 'alues. The mapping should $e one&to&one. Encr!ption and decr!ption operations o a $lock cipher are shown in (ig. <.).. Some operations, such as permutation and su$stitution, are perormed on the $lock o $its $ased on a ke! /a secret num$er0 to produce another $lock o $its. The permutation and su$stitution operations. In the decr!ption process, operations are perormed in the re'erse order $ased on theinsame ke! to get $ack the original $lock o $its. is perormed $! a  Transormations Block Ciphers #ermutation: , the permutation  permutation $o# at the $it&le'el, which keeps the num$er o 9s and )s same at the input and output. lthough it can $e implemented either $! a hardware or a sotware, the hardware implementation is aster. %ermutation operation used in Block Ciphers Su-stitution: the su$stitution is implemented with the help o three $uilding $locks P a decoder, one p&$o# and an encoder. (or an n&$it input, the decoder produces an * n $it output ha'ing onl! one ), which is applied to the %&$o#. The %&$o# permutes the output o the decoder and it is applied to the encoder. The encoder, in turn, produces an n&$it output. (or e#ample, i the input to the decoder is 9)), the output o the decoder is 9999)999. 6et the permuted output is 9)999999, the output o the encoder is 9)). It  perorms the ollowing steps: Step9&: Su$stitute each <&$it $ased on Step9': %ermute the  $its $ased on the ke! 

 

'igure . .-(

Permutation

'igure .-(*

Su0stitution

'igure .-. .-.(

0lic c 1ey #ncryption Pu0li

 

'igure ./-

Pu0li 0lic c key encrypti cryptio on

'igure .-.2

Signature Authenticat ation

'igure .*-. .*-.

TCP3"P an Moel an the !S" Mo

Encr!ption Standard /5ES0 One e#ample o the $lock cipher is the 5ata Enco the 5ES algorithm are gi'en $elow: •  monoalpha$etic su$stitution c• It has )@ distinct stages • lthough the input ke! onl!  $its in length. • The decr!ption can $e carried out in re'erse order. • 5ES has ) rounds, meaning the cipherte#t. • s the num$er

 

oe#ponentiall!. • Once the ke! scactual encr!ption or decr!ption is perormed with the help o the main 5ES algorithm Caining .CC/ In this mode o operation, encr!pted cipherne#t plainte#t $lock to $e encr!pted, thus making all the $locks dependent on all the pre'ious $locksCipher (eed$ack 2ode /C(B0 encr!ption techni?ue Output 3eed-ac* "ode .O3/ The encr!ption techni?ue o Output (eed$ack 2ode /O(B0 is •

shown (ig. •<.).)4. 8e! eatures o this$!mode are mentioned $elow: O(B is alsopad a • stream in cipher Encr!ption is perormed HO>ing the message with the one&time One&time pad can $e generated in ad'ance • I some $its o the cipherte#t get gar$led, onl! those $its o plainte#t get gar$led • The message can $e o an! ar$itrar! si"e • 6ess secure than other modes  Triple 5ES Tincreasing the ke! length. Its operation is e#plained $elow: • Each $lock o plainte#t is su$Aected to enencr!ption $! 8) in a se?uen• CBC is used to turn the $lock encr!ption scheme into a stream encr!ption

%u$lic ke! Cr!ptograph! In pu$lic ke! cr!ptograph!, there are two ke!s: a pri'ate ke! and a pu$lic ke!. The pu$lic ke! is announced to the pu$lic, where as the pri'ate ke! is kept $! the recei'er. The sender uses the pu$lic ke! o the recei'er or encr!ption and the recei'er uses his pri'ate ke! or decr!ption The pair o ke!s can $e used with an! other entit! The num$er o ke!s • d'antages: o re?uired is small o• 5isad'antages: It is not eicient or long messages 'igure ./-2

Signing ocument Signing the whole ocu

 

'igure ./-4

Signing the igest

'igure .2-( .2-((

Access authori ri5ation 5ation with secret key key encrypt

TE=T OO%S

). Behr Behrou ou"" . (o (oru ru"a "an, n, [5at [5ataa co comm mmun unic icat atio ion n an and d 3etw 3etwor orki king ng\, \, Tata Tata 2cDraw&7ill, *99: -nit I&IV *. ndrew S. Tannen$aum Tannen$aum,, [Computer [Computer 3etwor 3etworks\, ks\, %earson %earson Education, Education, (ourth Edition, *99+: -nit V RE3ERENCES ). 1a!ne Tomasi, [Introduction to 5ata Communication and 3etworking\, )e, %earson Education.  *. James .(. 8urouse _ 1. >ouse, [Computer 3etworking:  Topdown pproach (eaturing\,+e, %earson Education  +. C.Si'aram 2urth!, B.S.2anoA, [d hoc 1ireless 3etworks P rchitecture and %rotocols\, Second Edition, %earson Education.   4. Dreg Tomshon, Ed Tittel, 5a'id Johnson. [Duide to 3etworking Essentials\, ith edition, Thomson India 6earning, *99.  

. 1illiam Stallings, [5ata and Computer Communication\, Eighth Edition, %earson Education, *999.