Department Of Electronics and Communication Engineering NOTES ON LESSON CLASS: III YEAR ECE SU!ECT: CO"#UTER NET$OR%S
To introduce the concept ,terminologies and technologies used in modern data communication and computer networking. OBJECTIVES: To introduce the students the unctions o dierent la!ers. To introduce IEEE standard emplo!ed in computer networking. To make students to get amiliari"ed with dierent 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 classiications $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-
Multi ultipoint point Line Line Confi Configu gurat ratio ion n
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 $! an! machine. n address ield short withinmessages, the packetcalled speciies the in intended recei'ing a
packet, machine checks the address ield. I packet is intended or itsel, it processes the packet i packet is not intended or itsel it is simpl! simpl! ignored. This s!stem generall! also allows possi$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.
Star Topol olog ogy
Classification -ased on Scale
lternati'e criteria or classi!ing networks are their scale. The! are di'ided into 6ocal 6oca l rea /630, 2etropolitan rea 3etwork /230 and 1ide rea 3etworks /130.
Local Area Network
Metropolitan Area Network Network
Wie Area Network
Local Area Net)or* .LAN/
63 is usuall! pri'atel! owned and links the de'ices in a single oice, $uilding or campus o up to ew kilometers in si"e. These are used to share resources /ma! $e hardware or sotware resources0 and to e#change inormation. 63s are distinguished rom other kinds o networks $! three categories: their si"e, transmission technolog! and topolog!. 63s 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 23 and 13. 8nowing this $ound makes it possi$le to use certain kinds o design that would not otherwise $e possi$le. It also simpliies network management. 63 t!picall! used transmission technolog! consisting o single ca$le to which all machines are connected. Traditional 63s run at speeds o )9 to )99 2$ps /$ut now much higher speeds can $e achie'ed0. The most common 63 topologies are $us, ring and star. 2etropolitan rea 3etworks /230 23 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 63s into a larger network so that resources ma! $e shared as shown in (ig. ).).. (or e#ample, a compan! can use a 23 to connect the 63s in all its oices in a cit!. 23 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 /230
The main reason or distinguishing 23s 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 /130 13 pro'ides long&distance transmission o data, 'oice, image and inormation o'er large geographical areas that ma! comprise a countr!, continent or e'en the whole world. In contrast to 63s, 13s ma! utili"e pu$lic, leased or pri'ate communication de'ices, usuall! in com$inations, and can thereore span an unlimited num$er o miles as shown
13 that is wholl! owned and used $! a single compan! is oten reerred to as enterprise network. The Internet Internet is a collection o networks or network o networks. Various networks such as 63 and 13 connected through suita$le hardware and sotware to work in a seamless manner. Schematic diagram o the Internet is shown in (ig. ).).<. It allows 'arious applications such as e&mail, ile transer, remote log&in, 1orld 1ide 1e$, 2ultimedia, etc run across the internet. The $asic dierence $etween 13 and Internet is that 13 is owned $! a single organi"ation while internet is not so. But with the time the line $etween 13 and Internet is shrinking, and these terms are sometimes used interchangea$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 lie. lie. 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 deense 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 dierent $ranch oices located at dierent 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 ater 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 dierent. Some o the ser'ices are access to remote inormation, 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 proessionals 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 transer, which allow user to transer mone! without going into a $ank /an automated teller machine is an e#ample o electronic und transer, automatic pa!&check is another0. 2anuacturing: Computer networks are used in man! aspects o manuacturing including manuacturing process itsel. Two o them that use network to pro'ide essential ser'ices are computer&aided design /C50 and computer&assisted manuacturing /C20, $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. Inormation ser'ices: 3etwork inormation ser'ice includes $ulletin $oards and data $anks. 1orld 1ide 1e$ site oering technical speciication speciication or a new product is an inormation ser'ice. Electronic data interchange /E5I0: E5I allows $usiness inormation, including documents such as purchase orders and in'oices, to $e transerred without using paper. Electronic mail: pro$a$l! it=s the most widel! used computer network application. Teleconerencing: Teleconerencing allows conerence to occur without the participants $eing in the same place. pplications include simple te#t conerencing /where participants communicate through their normal ke!$oards and monitor0 and 'ideo conerencing where participants can e'en see as well as talk to other ellow participants. 5ierent t!pes o e?uipments are used or 'ideo conerencing 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 $roadl! classiied into ollowing categories:
Scientiic and Technical Computing Client Ser'er 2odel, 5istri$uted %rocessing %arallel %rocessing, Communication 2edia Commercial d'ertisement, Telemarketing, Teleconerencing 1orldwide (inancial Ser'ices 3etwork or the %eople /this is the most widel! used application nowada!s0 Telemedicine, 5istance Education, ccess to >emote Inormation, %erson&to&%erson Communication, Interacti'e Entertainment
Open S,stem Interconnection Reference "odel Figure 3-1
Moe oel !S" M
An #$change %sing %sing the !S" Moel
Link Layer Data Link
Net etwork work Layer
Presentation on Layer Presentati
Applicati ication on Layer
The Open S!stem Interconnection /OSI0 reerence model descri$es how inormation rom a sotware application in one computer mo'es through a network medium to a sotware application in another computer. The OSI reerence 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 inormation $etween networked computers into se'en smaller, more managea$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 oered $! one la!er to $e updated without ad'ersel! aecting the other la!ers. The OSI >eerence 2odel includes se'en la!ers: 10 Application Layer : %ro'ides pplications with access to network ser'ices. 20 Presentation Layer : 5etermines the ormat used to e#change data among networked
computers. . Session 6a!er: llows two applications to esta$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$le or addressing messages and data so the! are sent to the correct destination, and or translating logical addresses add resses and names /like a machine name (62E0 into ph!sical addresses. This la!er is also responsi$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 inormation. t the recei'ing computer, the 5ata&6ink 6a!er reads the incoming rame, and generates its own error detection inormation $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 deines 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 reerence model can $e di'ided into two categories: upper la!ers and lower la!ers as shown in (ig. ).*.*. The upper la!ers o the OSI OS I model deal with application issues and generall! are implemented onl! in sotware. The highest la!er, the application la!er, is closest to the end user. Both users and application a pplication la!er processes interact with sotware applications that contain a communications component. The term upper la!er is sometimes used to reer to an! la!er a$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 sotware. 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 inormation 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 $! using communication protocols. In the conte#t o data networking, a protocol is a ormal set o rules and con'entions that go'erns how computers e#change inormation o'er a network medium. protocol implements the unctions o one or more o the OSI la!ers. wide 'ariet! o communication protocols e#ist. Some So me o these protocols include 63 protocols, 13 protocols, network protocols, and routing protocols. 63 protocols operate at the ph!sical and data link la!ers o the OSI model and deine communication o'er 'arious 63 media. 13 protocols operate at the lowest three la!ers o the OSI model and deine communication o'er the 'arious wide&area media. >outing protocols are network la!er protocols that are responsi$le or e#changing e#chang ing inormation $etween routers so that the routers can select the proper p roper path or network traic. (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 inormation $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 Inormation $eing transerred rom a sotware application in one computer s!stem to a sotware application in another must pass through the OSI la!ers. (or e#ample, i a sotware application in S!stem has inormation to transmit to a sotware application in S!stem B, the application program in S!stem will pass its inormation to the application la!er /6a!er 0 o S!stem . The application la!er then passes the inormation 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 inormation 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 inormation rom the ph!sical medium, and then its ph!sical la!er passes p asses the inormation 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 inormation 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 Inormation E#change The se'en OSI la!ers use 'arious orms o control co ntrol inormation to communicate with their peer la!ers in other computer s!stems. This control inormation inormation consists o speciic re?uests and instructions that are e#changed $etween peer OSI la!ers. Control inormation 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 inormation unit. t the network la!er, or e#ample, an inormation unit consists o a 6a!er + header and data. t the data link la!er, howe'er, all the inormation 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 inormation 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. Inormation E#change %rocess The inormation e#change process occurs $etween peer OSI la!ers. Each la!er in the source s!stem adds control inormation to data, and each la!er in the destination s!stem anal!"es and remo'es the control inormation rom that data. I s!stem has data rom sotware 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 inormation re?uired $! the application la!er in S!stem B $! pre&pending a header to the data. The resulting inormation unit /a header and the data0 is passed to the presentation la!er, which pre&pends its own header containing control inormation intended or the presentation la!er in S!stem B. The inormation unit grows in si"e as each la!er pre& pends its own header /and, in some cases, a trailer0 that contains control inormation inormation to $e used $! its peer la!er in S!stem S !stem B. t the ph!sical la!er, the entire inormation unit is placed onto the network medium. The ph!sical la!er in S!stem B recei'es the inormation unit and passes it to the data link la!er. The data link la!er in S!stem B then reads the control inormation 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 inormation unit is passed to the network la!er. Each la!er perorms the same actions: The la!er reads the header rom its peer la!er, strips it o, and passes the remaining inormation unit to the ne#t highest h ighest la!er. ter the application la!er
perorms these actions, the data is passed to the recipient sotware sotware 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 speciies the mechanical, electrical and procedural network interace speciications 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&[email protected]
standards de'eloped $! Electronics Industries ssociation /EI0. ).*.4.* 5ata 6ink 6a!er The goal o the data link la!er is to pro'ide relia$le, eicient communication $etween adAacent machines connected $! a single communication channel. Speciicall!: ). 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! dier, 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 Eort: The recei'er does not return acknowledgments to the sender, so the sender has no wa! o knowing i a rame has $een successull! deli'ered. 1hen would such a ser'ice $e appropriateF ). 1hen higher la!ers can reco'er rom errors with little loss in perormance. That is, when errors are so inre?uent that there is little to $e gained $! the data link la!er perorming 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! Thecknowledged 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 reers 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 $oundariesF Various techni?ues are used or this: 6ength Count, Bit Stuing, and Character stuing. e0. Error Control Error control is concerned with insuring that all rames are e'entuall! e'en tuall! deli'ered /possi$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 $uer space. 6ink 2anagement In some cases, the data link la!er ser'ice must $e opened== $eore use: The data link la!er uses open op en operations or allocating $uer 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 aect a set o $its or a short time ater the lightning strike. 5etecting and correcting errors re?uires redundanc! /i.e., sending additional inormation 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 perormed using two $asic approaches ap proaches known as connection& oriented or connectionless network&la!er ser'ices. 3our issues:
). Interace $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 dierent 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 $its that includes data plus source and destination addresses, rom node to node called 'irtual circuit and datagram methods. These are also reerred to as connection&oriented and connectionless network&la!er ser'ices. In 'irtual circuit approach, a route, which consists o logical connection, is irst esta$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.* speciication. The datagram is a sel& contained message unit, which contains suicient inormation 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 $eore message transmission, se?uentiall! transmitted messages can ollow completel! dierent paths. The datagram method is analogous to the postal s!stem and the 'irtual&circuit method is analogous to the telephone s!stem.
O4er4ie) of Ot+er Net)or* La,er Issues:
The network la!er is responsi$le or routing packets rom the source to destination. The routing algorithm is the piece o sotware 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 $eore the remaining routers recogni"e the topolog! changeF /Some ne'er do.0 (airness and optimalit!: an inherentl! intracta$le pro$lem. 5einition 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 traic 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 $e processed, dela!s increase and perormance decreases. I the situation continues, the su$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, perormance degrades $ecause the network is using /wasting0 resources processing packets that e'entuall! get discarded. Internetworking (inall!, when we consider internetworking && connecting dierent network technologies together && one inds the same pro$lems, onl! worse: %ackets ma! tra'el through man! dierent networks Each network ma! ha'e a dierent 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 inormation 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 dierent 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 dierences $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! relect what the underl!ing la!ers happen to pro'ide. /Similar to the $eautiication 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 deiciencies 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 signiicant issue. That is, now the user must deal with it $eore 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 simpliication 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. Speciicall!, the su$net ma! $uer 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 $uers is inappropriate $ecause a machine ma! ha'e hundreds o connections sharing a single ph!sical link. In addition, appropriate settings or the low control parameters depend on the communicating end points /e.g., Cra! supercomputers 's. %Cs0, not on the protocol used. 5on=t send data unless there is room. lso, the network la!erdata link la!er solution o simpl! not acknowledging rames or which the recei'er has no space is unaccepta$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 $uer 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$lishing, using, and terminating a connection. (or data gram&oriented protocols, opening a connection simpl! allocates and initiali"es data structures in the operating s!stem kernel. Connection oriented protocols oten e#changes messages that negotiate options with the remote peer at the time a connection are opened. Esta$lishing a connection ma! $e trick! $ecause o the possi$ilit! o old or duplicate packets. (inall!, although not as diicult 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 $een deli'ered to the remote application. Session La,er
This la!er allows users on dierent machines to esta$lish session $etween them. session allows ordinar! data transport $ut it also pro'ides enhanced ser'ices useul in some applications. session ma! $e used to allow a user to log into a remote time& Sharing machine or to transer a ile $etween two machines. Some o the session related ser'ices are: ). This la!er manages 5ialogue Control. Session can allow traic 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 perorm 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 transer a 4& hour ile transer with a *&hour mean time $etween crashes. ter each transer was a$orted, the whole transer 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 ater a crash, onl! the data transerred ater the last checkpoint ha'e to $e repeated. #resentation La,er
This la!er is concerned with S!nta# and Semantics o the inormation transmitted, unlike other la!ers, which are interested in mo'ing data relia$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 dierent, some applications are so useul that the! ha'e $ecome standardi"ed. The Internet has deined standards or: )G (ile transer /(T%0: Connect to a remote machine and send or etch an ar$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 deined as ph!sical ph !sical path $etween transmitter and recei'er in a data transmission s!stem. nd it ma! $e classiied into two t!pes as shown in (ig. *.*.). 5uided: Transmission capacit! depends criticall! on the medium, the length, and whether the medium is point&to&point or multipoint /e.g. 630. E#amples are co&a#ial ca$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 intererence $etween the two conductors .Twisting decreases the cross&talk intererence $etween adAacent adAace nt pairs in a ca$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 $its, so we oten think o them as carr!ing digital data. d ata. 5ata rates o se'eral 2$ps common. Spans distances o se'eral kilometers. 5ata rate determined $! wire thickness and length. In addition, shielding to eliminate intererence 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 oices && 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, ampliiers are re?uired a$out e'er! 8m 8 m and or digital signals, repeaters are needed or a$out * 8m. To reduce intererence, 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 63 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 63, 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 CTV application coa#ial ca$le o )*K diameter and L impedance is used. This ca$le oers $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 intererence 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 /CTV0 or the distri$ution o TV signals. nother importance use o co&a#ial ca$le is in 63.
Broadband Coaxial The term $road$and reers to analog transmission o'er coa#ial ca$le. /3ote, howe'er, that the telephone olks use $road$and to reer 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$ps. GOperates at distances up to )99 km /metropolitan area0. G -ses analog signaling. G Technolog! used in ca$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 ampliiers to $oost signal strength $ecause ampliiers are one wa!, data lows in onl! one direction. T)o t,pes of s,stems +a4e emerged:
)). 5ual ca$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, interaces 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, $ut oers more ser'ices /e.g., it carries audio and 'ideo too0.
Fiber Optics In i$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 reracti'e inde# n). The core is surrounded $! a solid dielectric cladding o reracti'e inde# n* that is less than n). s a conse?uence, the light is propag propagated ated through multiple total internal relection. 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. *. 5iicult to tap, which makes it hard or unauthori"ed taps as well. This is responsi$le or higher relia$ilit! o this medium. 7ow diicult is it to pre'ent coa# tapsF Ver! diicult 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$le to electrical intererence /lightning0 or corrosion /rust0. . Dreater repeater distance than coa#. 5isad'antages: )G 5iicult to tap. It reall! is point&to&point technolog!. In contrast, con trast, tapping into coa# is tri'ial. 3o special training or e#pensi'e tools or parts are re?uired. G One&wa! channel. Two i$ers needed to get ull duple# /$oth wa!s0 communication. Optical (i$er works in three dierent 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 reracti'e inde# o the core is uniorm throughout and at the core cladding $oundar! there is an a$rupt change in reracti'e inde#. In the later case, the reracti'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 relection $ecause o the choice o reracti'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, $ecause o intererence o man! light ra!s. s a conse?uence, single& mode i$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 intererence 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 63 applications. 2ulti&mode i$er is commonl! used in 63. )G 6ong&haul trunks&increasingl! common in telephone network /Sprint ads0 G 2etropolitan trunks&without repeaters /a'erage < miles in length0 G >ural e#change trunks&link towns and 'illages G 6ocal loops&direct rom central e#change to a su$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. Inrared 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. *.*.. 5iiculties: )). 1eather intereres with signals. (or instance, clouds, rain, lightning, etc. ma! ad'ersel! aect 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 $! a transponder, ampliies the signal and re$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 +94 @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 intererence0. International agreements regulate how satellites are used, and how re?uencies are allocated. 1eather aects certain re?uencies. Satellite transmission diers 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 reerred to as 4 $and, which uses +. to 4.* D7" or down link and [email protected]
* 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 preerred preerred $and is alread! saturated, the ne#t highest $and a'aila$le a 'aila$le is reerred to as )*)4 D7". It uses )4 to )4.D7" or upward transmission and )). to )*.* D7" or downward transmissions. Communication satellites ha'e se'eral uni?ue properties. The most important is the long communication dela! or the round trip /a$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 $e tackled or successul and relia$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 traic 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, $ased on communication satellites, are emerging. Comparisoncontrast with other technologies: )). %ropagation dela! 'er! high. On 63s, or e#ample, propagation time is in nanoseconds && essentiall! negligi$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 interacing 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 interaces 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, $eore we discuss a$out the interace 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 interace in detail along with some standard interaces in Sec. +.)..
#ossi-le "odes of communication Transmission o digital data through a transmission medium can $e perormed pe rormed 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 %5s, 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 transer $etween the source&destination pair. These are reerred 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 transer sel&s!nchroni"ing, as discussed later. 7owe'er, it is not 'er! eicient $ecause o large o'erhead. To o'ercome o'e rcome this pro$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 $oth send and recei'e data, But, when one is sending, the other one can onl! recei'e and 'ice 'ersa. 1h! (raming and S!nchroni"ationF 3ormall!, units o data transer are larger than a single analog or digital digital encoding s!m$ol. It is necessar! to reco'er clock inormation 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! dierent. So, it is necessar! to add other $its to the $lock that con'e! control inormation used in the data link control procedures. The data along with pream$le, postam$le, and control inormation 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 identiied, 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&inormation $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 transer 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 oten used to ensure re?uent transitions needed. The data transmitted ma! $e o an! $it length, $ut is oten constrained $! the rame transer protocol /data link or 2C 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 stuing /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, deines the unctions o the rame and initiates the logic to control the mo'ement o traic $etween sending and recei'ing stations. Speciic pattern to represent start o rame Speciic 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 $oth the sending and recei'ing ends Bit stuing: I the lag pattern appears an!where in the header or data o a rame, then the recei'er ma! prematurel! detect the start or end en d o the recei'ed rame. To o'ercome this pro$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 stuing, 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 ater the th ), regardless o the ne#t $it in the data as shown in (ig. +.)[email protected]
On the recei'ing end, the $it stream is piped through a shit register as the recei'er looks or the lag pattern. I consecuti'e )=s ollowed $! a 9 is seen, then the 9 is dropped $eore sending the data on /the recei'er destus 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 stued ater e'er! i'e )Qs 1ith $it stuing, 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 transerred without an!asclock line or the clock is data reco'ered rom the signal itsel. word has a start $it /usuall! a 90 $eore irst $it o the word and a stop $itEach /usuall! as a )0 ater 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 drit /this ma! $e caused $! 'ariations in the cr!stals used, temperature, 'oltage, etc.0. I the recei'er=s clock drits drits 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, suers 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 eecti'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
Character stuing 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 $! inserting an e#tra 56E character ater the one occurring inside the rame, so that when the recei'er encounters two 56Es in a row, it immediatel! deletes one and interpret the other as header or data. The main disad'antage o this method is that it is closel! tied to <&$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 $it stuing is used that allow data d ata rames to contain an ar$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! $e o'erhead or s!nchroni"ation, or raming, or error checking, or headers and trailers, or retransmissi retransmissions, ons, etc. G -tili"ation ma! mean more than one thing. 1hen dealing with network monitoring and management, it reers to the raction o the resource actuall! used /or useul data and or o'erhead, retransmissions, etc.0. In this conte#t, utili"ation reers to the raction o the channel that is a'aila$le or actual data transmission to the ne#t higher la!er. It is the ratio o data $its per protocol data unit u nit /%5-0 to the total si"e o the %5-, including s!nchroni"ation, headers, etc. In other words, it is the ratio o the time spent actuall! sending useul data to the time it takes to transer that data and its attendant o'erhead. The eecti'e data rate at a la!er is the net data rate a'aila$le to the ne#t higher la!er. Denerall! this is the utili"ation times the raw data rate. 5TE&5CE Interace s two persons intending to communicate must speak in the same language, or successul 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 /EI0, The Institution o Electrical and Electronic Engineers /IEEE0, etc. The EI and IT-&T ha'e $een in'ol'ed in de'eloping standards or the 5TE&5CE interace known as EI&*+*, EI&44*, etc and IT-&T standards are known as V series or H series. The standards should normall! deine the ollowing our important attri$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 speciies 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 speciies 'oltages, Impedances and timing re?uirements to $e satisied or relia$le communication (unctional: (unctional attri$ute pertains to the unction to $e perormed, $! associating meaning to the 'arious signal lines. (unctions (un ctions can $e t!picall! classiied into the $road categories o data control, timing and ground. This component o standard speciies the signal pin assignments and signal deinition o each o the pins used or interacing the de'ices %rocedural: The procedural attri$ute speciies the protocol or communication, i.e. the se?uence o e'ents that should $e ollowed during data transer, using the unctional characteristic o the interace. 'ariet! o standards e#ist, some o the most popular interaces are presented in this section (low Control and Error Control
Introduction s we ha'e mentioned earlier, or relia$le and eicient 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 perorm a certain amount o processing $eore passing the data on to the higher&le'el sotware. G I error occur during transmission, it is necessar! to de'ise mechanism to correct it The most important unctions o 5ata 6ink la!er to satis! the a$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 dierent 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 perormed $ased on retransmission o the corrupted
data. 1hen an error is detected, the recei'er can ha'e the speciied rame retransmitted $! the sender. This process is commonl! known as Automatic Repeat Reuest .AR;/0
(or e#ample, Internet=s -nrelia$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 63, $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 $oth cases, low control is needed to pace the data transer at an accepta$le speed. (low Control is a set o procedures that tells the sender how much data it can transmit $eore 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 inorm the transmitter $eore 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 reers to the set o procedures used to restrict the amount o data the transmitter can send $eore 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?uestrepl! sometimes. >e?uestrepl! /Stop& and&wait0 low control re?uires each data packet to $e acknowledged $! the remote host $eore the ne#t packet is sent. This is discussed in detail in the ollowing ollowing su$section. Sliding window algorithms, used $! TC%, permit pe rmit multiple data packets to $e in simultaneous transit, making more eicient 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 $eore sending the ne#t data rame.This is sometimes reerred to as ping&pong $eha'ior, re?uestrepl! is simple to understand and eas! to implement, $ut not 'er! ' er! eicient. In 63 en'ironment with ast links, this isn=t much o a concern, $ut 13 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 successul 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 ineicienc! 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! $eore continuing the program and an d possi$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 suicientl! long such that the irst $its o the rame arri'e at the destination $eore the source has completed transmission o the rame. P a N ): Sender completes transmission o the entire rame $eore 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 63 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 perorm well. Onl! one rame at a time can $e in transit. In stop&and&wait low low control, i a N ), serious ineiciencies result. Eicienc! can $e greatl! impro'ed $! allowing multiple rames to $e in transit at the same time. Eicienc! 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 $uer 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 speciied. 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 $uer o si"e ). Sliding window algorithm is a method o low control or network data transers. TC%, the Internet=s stream transer protocol, uses a sliding window algorithm. sliding window algorithm places a $uer $etween the application program and the network data low. (or TC%, the $uer is t!picall! in the operating s!stem kernel, $ut this is more o an implementation detail than a hard&and& ast re?uirement.
Buer in sliding window
5ata recei'ed rom the network is stored in the $uer, rom where the application can read at its own pace. s the application reads data, $uer 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 $uer, less the amount o 'alid data stored in it. 1indow announcements are used to inorm the remote host o the current window si"e. Sender sliding 1indow: t t an! instant, the sender is permitted to send rames with se?uence num$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 speciic rame /rame 4 as shown in the igure0 to arri'e in a speciic 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 speciic 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 one#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 speciied. 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 $uer o si"e ). >ecei'er sliding window On the other hand, i the local application can process data at the rate it=s $eing transerred 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 $uer space is a'aila$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 $uer /remem$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 eicient 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 /3C80 rom the recei'er /sa! station B0. Station B sends an C8 i the rame is recei'ed correctl!, otherwise it sends 3C8. Station sends a new rame ater recei'ing C8 otherwise it retransmits the old rame, i it recei'es a 3C8. 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 ater 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- $eore 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 3C8 rames, i.e. 3egati'e cknowledge rames. >ecei'er transmits a 3C8 rame to the sender i it ounds the recei'ed rame to $e corrupted. 1hen a 3C8 is recei'ed $! a transmitter $eore 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 $uer si"e. 7owe'er, it makes highl! ineicient use o communication links, particularl! particularl! when WaQ is large. Do&$ack&3 >;
The most popular >; protocol is the go&$ack&3 >;, where the sender sends the rames continuousl! without waiting or acknowledgement. That is wh! it is also called as continuous >;. s the recei'er recei'es the rames, it keeps on sending C8s or a 3C8, in case a rame is incorrectl! recei'ed. 1hen the sender recei'es a 3C8, it retransmits the rame in error plus all the succeeding rames as shown in ([email protected]
7ence, the name o the protocol is go&$ack&3 >;. I a rame is lost, the recei'er sends 38 ater recei'ing the ne#t rame as shown in (ig. +.+.)9. In case there is long dela! $eore sending the 38, the sender will resend the lost rame ater its timer times out. I the C8 rame sent $! the recei'er is lost, the sender resends the rames ater its timer times out ssuming ull&duple# transmission, the recei'ing end sends pigg!$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 speciies how man! rames can $e sent without recei'ing acknowledgement. I no acknowledgement is recei'ed ater sending 3 rames, the sender takes the help o a timer. ter theotime&out, resumesand retransmission. TheThis go&$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 speciies how man! rames can $e sent without recei'ing acknowledgement. I no acknowledgement is recei'ed ater sending 3 rames, the sender takes the help o a timer. ter the time&out, it resumes retransmission. The go&$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 38s are recei'ed or or which timer has e#pired, e#p ired, This is the most eicient 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&38 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 [email protected]
and ISO 4++. It speciies 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 Inormation. Other $eneits o 756C are that the control inormation is alwa!s in the same position, and speciic $it patterns used or control dier dramaticall! dramaticall! rom those in representing data, which reduces the chance o errors. It has also led to man! su$sets. Two su$sets widel! in use are S!nchronous 5ata 6ink Control /S56C0 and 6ink ccess %rocedure&Balanced /6%&B0. Figure 11-14-continued
&DLC Confi Configurat guratiion
In this lesson we shall consider the ollowing aspects o 756C: )G Stations and Conigurations G Operational 2odes G 3on&Operational 2odes G (rame Structure G Commands and >esponses G 756C Su$sets /S56C and 6%B0
756C Stations and Conigurations 756C speciies the ollowing three t!pes o stations or data link control: )G %rimar! Station G Secondar! Station G Com$ined Station %rimar! Station 1ithin a network using 756C data link protocol, i a coniguration used in the which there is a primar! station,asitits is used as the controlling station on the is link. It has responsi$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 deines three t!pes o conigurations or the three t!pes o stations. The word coniguration reers to the relationship $etween the hardware de'ices d e'ices on a link. (ollowing are the three conigurations deined $! 756C: )G -n$alanced Coniguration G Balanced Coniguration G S!mmetrical Coniguration -n$alanced Coniguration The un$alanced coniguration 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 coniguration, an! o the ollowing can $e used: )G (ull&5uple# or 7al&5uple# operation G %oint to %oint or 2ulti&point networks Balanced Coniguration The $alanced coniguration in an 756C link consists o two or more com$ined stations. Each o the stations has e?ual and complimentar! responsi$ilit! compared to each other. Balanced conigurations can use onl! the ollowing: )G (ull & 5uple# or 7al & 5uple# operation G %oint to %oint networks S!mmetrical Coniguration This third t!pe o coniguration is not widel! in use toda!. It consists o two independent point&to&point, un$alanced station conigurations. In this coniguration, 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 conigurations are alwa!s conducted in normal response mode. E#changes o'er s!mmetric or $alanced
conigurations can $e set to speciic mode using a rame design to deli'er the command. 756C oers three dierent 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 transers 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 transer 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 inormation rame. Once the last rame is transmitted $! the secondar! station, it must wait once again rom e#plicit permission to transer an!thing, rom the primar! station. 3ormal >esponse 2ode is onl! used within an un$alanced coniguration. As,nc+ronous Response "ode In this mode, the primar! station doesn=t initiate transers 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 transer an! rames. The rames ma! ma! $e more than Aust acknowledgment rames. The! ma! contain data, or control inormation regarding the status o the secondar! station. This mode can reduce o'erhead on the link, as no rames need to $e transerred in order to gi'e the secondar! station permission to initiate a transer. 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 $eore it can transer an! rames. This is when the >2 link is operating at hal&duple#. I the >2 link is operating at ull duple#, the secondar! station can transmit at an! time. In this mode, the primar! station still retains responsi$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 perorm 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 reer to the ormat o the data on the link. It reers to the act that an! gi'en station can transer rames without e#plicit permission or instruction rom an! other station. 756C 3on&Operational 2odes 756C also deines 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 dier 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 dierent 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 $e used in an operational mode. 756C (rame Structure There are three dierent t!pes o rames as shown ields are shown Ta$le +.4.). Ta$le +.4.) Si"e o dierent ields (ield 3ame (lag (ield/ ( 0 ddress (ield/ 0 Control (ield/ C 0 Inormation (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 dierent
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 interrame time ill. The interrame 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 speciic code or interpretation o line control. This means that i a $it at position 3 in an octet has a speciic 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& stuing to dierentiate 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! speciic $it code inside the data stream. It is onl! concerned with keeping lags uni?ue. The ddress ield The address ield /0 identiies the primar! or secondar! stations in'ol'ement in the rame transmission or reception. Each station on the link has ha s a uni?ue address. In an un$alanced coniguration, the ield in $oth commands and responses reer to the secondar! station. In a $alanced coniguration, the command rame contains the destination station address and the response rame has the sending station=s address. The Control ield 756C uses the control ield /C0 to determine how to control the communications process. This ield contains the commands, responses and se?uences num$ers used to maintain the data low accounta$ilit! o the link, deines the unctions o the rame and initiates the logic to control the mo'ement o traic $etween sending and recei'ing stations. There three control ield ormats: )G Inormation Transer (ormat: The rame is used to transmit end&user data $etween two de'ices. G Super'isor! (ormat: The control ield perorms 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 perorm 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 pollinal $it, or %( $it. It can onl! $e recogni"ed when it is set to ). I it is set to 9, it is ignored. The pollinal pollinal $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 signiies 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 Inormation ield or 5ata ield This ield is not alwa!s present in a 756C rame. It is onl! present when the Inormation Transer (ormat is $eing used in the control ield. The inormation ield contains the actuall! data the sender is transmitting to the recei'er in an I&(rame and network management inormation in -&(rame. The (rame check Se?uence ield This ield contains a )&$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.*. Inormation transer ormat command and response /I&(rame0 The unction o the inormation command and response is to transer se?uentiall! num$ered rames, each containing an inormation ield, across the data link. Super'isor! ormat command and responses /S&(rame0
Super'isor! /S0 commands and responses are used to perorm num$ered super'isor! unctions such as acknowledgment, polling, temporar! suspension o inormation transer, or error reco'er!. (rames with the S ormat control ield cannot canno t contain an inormation 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 inormation rame andor 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! inormation 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 speciic rames. n S>EJ must $e transmitted or each erroneous rame each rame is treated as a separate error. Onl! one S>EJ can remain outstanding on the link at an! one time. TB6E +.4.* 756C Commands and >esponses Inormation Transer (ormat Commands I & Inormation Super'isor! (ormat Commands >> & >ecei'e read! >3> & >ecei'e not read! >EJ & >eAect S>EJ & Selecti'e reAect -nnum$ered (ormat Commands S3>2 & Set 3ormal >esponse 2ode S>2 & Set s!nchronous >esponse 2ode SB2 & Set s!nchronous Balanced 2ode 5ISC & 5isconnect S3>2E & Set 3ormal >esponse 2ode E#tended S>2E & Set s!nchronous >esponse 2ode E#tended SB2E & Set s!nchronous Balanced 2ode E#tended SI2 & Set Initiali"ation 2ode -% & -nnum$ered %oll -I & -nnum$ered Inormation HI5 & E#change identiication >SET & >eset
Inormation Transer (ormat >esponses I & Inormation 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$ered Inormation HI5 & E#change Identiication (>2> & (rame >eAect TEST & Test
TEST & Test -nnum$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 modiier $its, which allow or up to +* additional commands and +* additional response unctions. Below, )+ command unctions, and < response unctions are descri$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 /SB20 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 SB2E. G Set Initiali"ation 2ode /SI20 is used to cause the secondar! station to initiate a station& speciic 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 Inormation /-I0 is used to send inormation to a secondar! station. G E#change Identiication /HI50 is used to cause the secondar! station to identi! itsel and pro'ide the primar! station identiications 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 perorms 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, SB2, S3>2E, S>2E, SB2E, >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 inorm 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 osets 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+niues: Circuit S)itc+ing Speciic 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 perormed G E#plain how signalling in perormed 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 inormation is transmitted rom source to destination 'ia dierent 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 perormed $! dierent 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 14-17 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 $eore an! transer o data. Some segments o the circuit ma! $e a dedicated link, while some other segments ma! $e shared. 5ata transer: )G Transer 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 transer. 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 $eore the message is transmitted. This connection, once esta$lished, remains e#clusi'e and continuous or the complete duration o inormation 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 $etween an! pair o attached de'ices. G 3etwork interace: That represents the unctions and hardware h ardware needed to connect digital d igital de'ices to the network /like telephones0. G Control unit: That esta$lishes, maintains, and tears down a connection. The simpliied 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 traic, a $locking coniguration 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&$locking coniguration is desira$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. dierent ongoing connections, at a same instant o time, uses dierent 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 $etween the two de'ices wh whose ose lines intersect at that crosspoint. G The crosspoints are ineicientl! utili"ed. G Onl! a small raction o crosspoints are engaged e'en i all o the attached de'ices are acti'e. Some o the a$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 $locking eature is illustrated in (ig. 4.).. ater setting up connections or )&to&+ and *&to&4, the switch cannot esta$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, dierent IO 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 $e high enough so that the slots recur suicientl! re?uentl!. (or )99 ull&duple# lines at )@.*99 8$ps, the data rate on the $us must $e greater than )[email protected]
* 2$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. dierent ongoing connections can use same switching path $ut at dierent 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.)[email protected]
s shown in the igure, writing can $e perormed 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$lic switched telephone network /%ST30 is an e#ample o circuit&switched network. ItQs also known %lain OldinTelephone Ser'icenamel!: /%OTS0.>egional The switching used or the switching areasorganised dierent le'els, o icescentres oices /class )0, Section oices /class *0, primar! oices /class +0, Toll oices /class 40 and inall! End oices /class 0. 6e'el ) is at the highest le'el and 6e'el is the lowest le'el. Su$scri$ers or the customers are directl! connected to these end oices. nd each oice is connected directl! to a num$er o oices at a le'el $elow and mostl! a single oice at higher le'el. Su$scri$er Telephones are connected, through 6ocal 6oops to end oices /or central oices0. small town ma! ha'e onl! one end oice, $ut large cities ha'e se'eral end oices. 2an! end oices are connected to one Toll oice, which are connected to primar! oices. Se'eral primar! oices are connected to a section oice, which normall! ser'es more than one state. ll regional oices o ices are connected using mesh topolog!. ccessing the switching station at the end oices o ices is accomplished through dialling. In the past, telephone eatured rotar! or pulse dialling, in which digital signals were sent to the end oice 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 oice, which is one le'el up. (or e#ample, 5ierent connections will $e multiple#ed when the! are to $e orwarded rom an end&oice to Toll oice.
'igure ()-( ()-(*
'rame +elay 'ram 'rame
ATM Multi ltiple$i ple$ing ng
Introduction 3rame Rela, is a high&perormance 13 protocol that operates at the ph!sical and data link la!ers o the OSI reerence model. (rame >ela! originall! was designed or use across Integrated Ser'ices 5igital 3etwork /IS530 interaces. Toda!, it is used o'er a 'ariet! o other network interaces as well. (rame >ela! is a simpliied orm o %acket
Switching, similar in principle toon H.*, in which s!nchronous ramesdierence o data are$etween routed to dierent destinations depending header inormation. 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! traic consists o TC%I% or other protocols that pro'ide their own low control and error correction mechanisms. 2uch o this traic 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 traic out onto a new line as soon as it has read the irst two $!tes o addressing inormation 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! dierent rom direct leased line connections. s a result, the perormance 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$le&length packets are used or more eicient and le#i$le data transers. 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 eicient use o $andwidth. 2ost o toda!=s popular 63s, such as Ethernet and Token >ing, are packet&switched networks.
Frame Relay Devices 5e'ices attached to a (rame ( rame >ela! 13 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 speciic 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 13. 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 deines the mechanical, electrical, unctional, and procedural speciications or the connection $etween the de'ices. One o the most commonl! used ph!sical la!er interace speciications is the recommended standard />S0&*+* speciication. The link la!er component deines the protocol that esta$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 deine the 5TEs connected to the network. (rame >ela! pro'ides connection&oriented data link la!er communication. This means that a deined communication e#ists $etween each pair o de'ices and that these connections are associated with a connection identiier. 7owe'er, 'irtual circuit identiiers 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! identiied $! a data&link connection identiier /56CI0. num$er o 'irtual circuits can $e multiple#ed into a single ph!sical circuit or transmission across the network. This capa$ilit! oten 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. Beore 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 transer $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 transerX5ata 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 transerred. I an SVC remains in an idle state or a deined 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 transers $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 transer: 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 transerred. -nlike SVCs, %VCs will not $e terminated under an! circumstances when in an idle state. 5TE de'ices can $egin transerring data whene'er the! are read! $ecause the circuit is permanentl! esta$lished. 5ata&6ink Connection Identiier /56CI0 (rame >ela! 'irtual circuits are identiied data&link identiiers /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 signiicance, which means that their 'alues are uni?ue in the 63, $ut not necessaril! in the (rame >ela! 13. The local 5TEs use this 56CI to send rames to the remote 5TE. 56CIs are not onl! used to deine d eine the 'irtual circuit $etween a 5TE and a 5CE, $ut also to deine the 'irtual circuit $etween two 5CEs /switches0 inside the network. switch assigns a 56CI to each 'irtual connection in an interace. This means that two dierent connections $elonging to two dierent interaces ma! ha'e the same 56CIs /as shown in the a$o'e igure0. In other words, 56CIs are uni?ue or a particular interace.
connection $etween 5TE and 5TE 5 has $een shown in this igure, 56CI assigned inside the (rame >ela! network is also shown in the network. 5CEs inside the network use u se incoming interace P 56CI com$ination to decide the outgoing interace 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$le matches the incoming interace& 56CI com$ination with an outgoing interace&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 speciic protocol is deined 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 helpul 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 identiier and $its o ields related to congestion management. This identiier commonl! is reerred to as the data& link connection identiier /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 inormation: 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 $! a uni?ue 56CI. The 56CI 'alues ha'e local signiicance onl!, which means that the! are uni?ue onl! to the ph!sical channel on which the! reside. Thereore, de'ices at opposite ends o a connection can use u se dierent 56CI 'alues to reer to the same 'irtual connection. con nection. The irst &$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 /E0XThe 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 signiicant 56CI $!te in the ddress ield. The C> $it is not currentl! deined. Congestion ControlXThis consists o the + $its that control the (rame >ela! congestion& notiication mechanisms. These are the (EC3, BEC3, and 5E $its, which are the last + $its in the ddress ield.
(orward&e#plicit congestion notiication /(EC30 is a single&$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! $eneit o the use o the (EC3 and BEC3 ields is the capa$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$ilities. Backward&e#plicit congestion notiication /BEC30 is a single&$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 $eore other rames in a congested network. This allows or a $asic prioriti"ation mechanism in (rame >ela! networks. Backward&e#plicit congestion notiication G5ataXContains encapsulated upper&la!er data. Each rame in this 'aria$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 'eriied $! 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! traic. 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 13s. 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 dierent users. 5ue to 'aria$le dela! it is not no t suita$le or real&time communication
s!nchronous Transer 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 inormation 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 transer 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! traic /in contrast to circuit switching0, and allows communications $etween de'ices that operate at dierent speeds. -nlike packet switching, T2 is designed or high&perormance 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 deined end points and routes $ut does not ha'e $andwidth dedicated to it. Bandwidth is allocated on demand $! the network as users ha'e traic to transmit. T2 also deines '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 $eneits 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 eicient use o $andwidth. G Can +andle "i6ed net)or* traffic 4er, efficientl,: Variet! o packet si"es makes traic unpredicta$le. ll network e?uipments should incorporate ela$orate sotware s!stems to manage the 'arious si"es o packets. T2 handles these pro$lems eicientl! 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 uniormit!. )G Class&o&ser'ice support or multimedia traic 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 6313 architecture allowing T2 to $e used consistentl! rom one desktop to another traditionall!, 63 and 13 technologies ha'e $een 'er! dierent, with implications or perormance and interopera$ilit!. But T2 technolog! can $e used either as a 63 technolog! or a 13 technolog!. G International standards compliance in central&oice and customer&premises en'ironments allowing or multi'endor operation. T2 5e'ices and the 3etwork En'ironment T2 is a cell&switching and multiple#ing technolog! that com$ines co m$ines the $eneits o circuit switching /guaranteed capacit! and constant c onstant transmission dela!0 with those o packet switching /le#i$ilit! and eicienc! or intermittent intermittent traic0. It pro'ides scala$le $andwidth rom a ew mega$its per second /2$ps0 to man! giga$its per second /D$ps0. Because o its as!nchronous nature, T2 is more eicient than s!nchronous technologies, such as time&di'ision multiple#ing /T520. 1ith T52, each user is assigned to a time slot, and no other station can send in that time slot as shown in (ig. 4..). I a station has much d data ata to send, it can send onl! when its time slot comes up, e'en i all other time slots are empt!. 7owe'er, i a station has nothing to transmit when its time slot comes up, the time slot is sent empt! and is wasted.
Because T2 is as!nchronous, time slots are a'aila$le on demand with inormation 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 deined. It accepts the incoming cell rom an T2 endpoint or another T2 switch. It then reads and updates the cell header inormation and ?uickl! switches the cell to an output interace towards its destination. n T2 endpoint /or end s!stem0 contains an T2 network interace adapter. E#amples o T2 endpoints are workstations, routers, digital ser'ice units /5S-s0, 63 switches, and 'ideo coder&decoders /CodecQs0. T2 3etwork Interaces n T2 network consists o a set o T2 switches interconnected $! point&to&point T2 links or interaces. T2 switches support two primar! t!pes o interaces: -3I and 33I as shown in (ig. 4..+. The -3I /-ser&3etwork Interace0 connects T2 end s!stems /such as hosts and routers0 to an T2 switch. The 33I /3etwork&3etwork Interace0 connects two T2 switches. 5epending on whether the switch is owned and located at the customer=s premises or is pu$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 transers inormation 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 inormation, and the remaining 4< contain the pa!load /user inormation0. Small, i#ed&length cells are well suited rom to transer andor 'ideo traic $eca $ecause use such traic is among intolerant to dela!s result ha'ing'oice to wait a large d ata data packet to download, other things.that
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 Identiier /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
%a!load /4< $!tes0 T2 Cell 7eader (ields The ollowing descriptions summari"e the T2 cell header ields shown in (ig. 4... )G Deneric (low Control /D(C0X%ro'ides local unctions, such as identi!ing multiple stations that share a single T2 interace. This ield is t!picall! not used and a nd is set to its deault 'alue o 9 /$inar! 99990. G Virtual %ath Identiier /V%I0XIn conAunction with the VCI, identiies 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 Identiier /VCI0XIn conAunction with the V%I, identiies 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 preerence 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 deines 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 coniguration 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! dierent 'irtual channel connections $etween them, $elonging to dierent 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 $asic operation o an T2 switch is straightorward: The cell is recei'ed across a link with a known V%IVCI 'alue. The switch looks up the connection 'alue in a local translation ta$le to determine the outgoing port /or ports0 o the connection and the n new ew V%IVCI 'alue o the connection on that link. The switch then retransmits the cell on that outgoing link with the appropriate connection identiier. Incoming
V%I VCI V%I VCI Interace )9 )*)
V%VC T2 switch ta$le Because all VCIs and V%Is ha'e onl! local signiicance across a particular link, these 'alues are remapped, as necessar!, at each switch. (igure 4.. and (ig. 4..< shows a V%&VC switch and an onl! V% switch, respecti'el!. -suall! the intermediate switches are onl! V%I switches while switches connected to the users are V%IVCI switches. Incoming Outgoing V%I
V% T2 switch ta$le To make the switching more eicient, 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 identiier, 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$e the unctionalit! that it supports. T2 unctionalit! corresponds to the ph!sical la!er and part o the data link la!er o the OSI reerence model. The T2 reerence model, as shown in (ig. 4.[email protected]
, consists o the ollowing planes, which span all la!ers: )G ControlXThis plane is responsi$le or generating and managing signaling re?uests. G -serXThis plane is responsi$le or managing the transer o data. G 2anagementXThis plane contains two components: 6a!er management protocol pro$lems. manages la!er&speciic unctions, such as the detection o ailures and %lane management manages and coordinates unctions related to the complete s!stem.
The T2 reerence model consists o the ollowing T2 la!ers: )G %h!sical la!erXnalogous to the ph!sical la!er o o the OSI reerence model, the T2 ph!sical la!er manages the medium&dependent transmission. )G T2 la!erXCom$ined with the T2 adaptation la!er, the T2 la!er is roughl! analogous toor thethe data link la!er osharing the OSI model.o'er TheaT2 la!er is /cell responsi$le simultaneous oreerence '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 inormation 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 $! sending and recei'ing a continuous low o with associated timing inormation. G It$its speciies 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, traic management, switching and multiple#ing ser'ices. It processes outgoing traic $! accepting 4<&$!te segment rom the 6 su$& la!ers and transorming them into +&$!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 acceptsT2 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$led into their original ormats and passed to the recei'ing ser'ices. Instead o one protocol or all t!pes o data, the T2 standard di'ides the 6 la!er into categories, each supporting the re?uirements o dierent t!pes o applications. There are our t!pes o data streams that are identiied: Constant&$it rate, 'aria$le $it&rate, connection oriented packet data transer, connectionless packet data transer. 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! 'ideoconerencing. 6) 'ariation. E#amples o re?uires timing s!nchroni"ation applications that can use CB> $etween the source and the are telephone traic /i.e., n#4 destination. (or this reason, k$ps0, 'ideoconerencing, 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 inormation 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 $it ratePreal time /VB>P>T0
This class allows users to send traic at a rate that 'aries with time depending on the a'aila$ilit! o user inormation. 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 /S50 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> traic is characteri"ed as either real&time /VB>&>T0 or as non&real&time /VB>& 3>T0. 6* supports $oth t!pes o VB> traic.
Connection oriented packet transer or a'aila$le $it rate /B>0
Connectionless data transer or unspeciied $it rate /-B>0
This class o T2 ser'ices 6+4: 6+4 supports pro'ides rate&$ased low $oth connection&oriented and control and is aimed at data connectionless data. 6+4 traic such as ile transer prepares a cell or and e&mail. lthough the transmission in our steps. standard does not re?uire the cell transer dela! and (irst, the con'ergence su$ la!er /CS0 creates a protocol cell&loss ratio to $e data unit /%5-0 $! guaranteed or minimi"ed, it prepending a $eginningend is desira$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 transer most non&S25S data, such as classical I% o'er T2 and6 63 Emulation /63E0. also is known as the simple and eicient adaptation la!er /SE60
T2 pplications T2 is used in $oth 63s and 13s letQs ha'e a look at ew o the possi$le applications. T2 13s: T2 is $asicall! a 13 technolog! that deli'ers cell o'er o' er long distances. 7ere T2 is mainl! used to connect conne ct 63s or other 13s 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 $elonging to other protocol. T2 63s: 7igh data rate /) and ** 2$ps0 o T2 technolog! attracted a ttracted designers to think o implementing T2 technolog! in 63s too. t the surace le'el, to implement an T2 63 T2 switch will replace the traditional Ethernet switch, in a
switched 63. But ew things ha'e to $e kept in mind and sotware modules would $e needed to map the ollowing dierences $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 identiier: In the Traditional 63 packets are routed $ased on the source and destination addresses, while in T2 cells are routed $ased on the 'irtual circuit identiiers /V%I&VCI pair0. 2ultimedia 'irtual pri'ate networks and managed ser'ices: Ser'ice pro'iders are $uilding on their T2 networks to oer a $road range o ser'ices. E#amples include managed T2, 63, 'oice and 'ideo ser'ices /these $eing pro'ided on a per& application $asis, t!picall! including customer&located e?uipment and oered 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 inrastructure 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 inrastructure or a range o data ser'ices, and to ena$le Internet class&o&ser'ice oerings and 'irtual pri'ate intranet ser'ices. >esidential $road$and networks: T2 is the networking inrastructure o choice or carriers esta$lishing residential $road$and ser'ices, dri'en $! the need or highl! scala$le solutions. Carrier inrastructures or the telephone and pri'ate&line networks: Some carriers ha'e identiied opportunities to make more&eecti'e use o their SO3ETS57 i$er inrastructures $! $uilding an T2 inrastructure to carr! their telephon! and pri'ate& line traic.
3etwork Topolog! Introduction Topolog! reers to the wa! in which the network o computers is connected. Each topolog! is suited to speciic tasks and has its own ad'antages and disad'antages. The choice o topolog! is dependent upon t!pe and num$er o e?uipment $eing used, planned applications and rate o data transer re?uired, response time, and cost. Topolog! can also $e deined as the geometrically the geometrically interconnection pattern $! pattern $! which the stations /nodescomputers0 are connected using suita$le transmission media /which can $e point& to&point and $roadcast0. Various commonl! used topologies are discussed in the ollowing sections.
In this topolog! each node or station is connected to e'er! other station The ke! characteristics o this topolog! are as ollows: 8e! Characteristics: )o (ull! connected *o >o$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 inormation, 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 interacing 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 relection o signal rom rom the endpoints. I the terminator is not present, the endpoint acts like a mirror and relects the signal $ack causing intererence and other pro$lems. (igure .).+ Bus Topolog! 8e! Characteristics o this topolog! are: )o (le#i$le o E#panda$le o 2oderate >elia$ilit! o 2oderate perormance shared used $etween d ierent dierent stations. 7ence it is 'er! cost eecti'e. One can easil! addlink an!isnew node or delete an! node without aecting other nodes this makes this topolog! easil! e#panda$le.
some e#tra inormation 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 transerring 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 $roadcast mode. 7ence, onl! one station can transer the data at an! gi'en time. Suita$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, reerred 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! successull! 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 $uered 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 perorms 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 dierent connection.
Ver! 7igh speeds o data transer 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 preerred 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 $uering 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 $uer 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. .)[email protected]
, >) reers 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 $e accomplished easil! without aecting the alread! esta$lished network. -nconstrained Topolog! ll the topologies discussed so ar are s!mmetric and constrained $! well&deined interconnection pattern. 7owe'er, sometimes no deinite pattern is ollowed and nodes are interconnected in an ar$itrar! manner using point&to&point links -nconstrained topolog! allows a lot o coniguration le#i$ilit! $ut suers rom the comple# routing pro$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 perormance depend on $oth the topolog! and the transmission media used in the network. s a conse?uence, these two aspects are interrelated. 6et us ha'e a look at the 'arious transmission media, which are used or dierent topologies.
Internetworking 5e'ices Introduction 7I6I su$committee /IEEE<9*.)0 o the IEEE identiied the ollowing possi$le internetworking scenarios. )G single 63 G Two 63s connected together /63&630 G 63 connected to a 13 /63&130 G Two 63s connected through a 13 /63&13&630 Various internetworking de'ices such as hu$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 $e considered as two transcei'ers Aoined together and connected to two dierent segments o coa#ial ca$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 ampliied 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. Thereore, the two segments orm a single 63 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 reerence o the ISO model, a repeater is considered as a level-1 relay .It .It simpl! repeats, retimes and ampliies the $its it recei'es. The repeater is merel! used to e#tend the span o a single 63. Important eatures o a repeater are as ollows: )G repeater connects dierent segments o a 63 G repeater orwards e'er! rame it recei'es
G repeater is a regenerator, not an ampliier G It can $e used to create a single e#tended 63 <u-s 7u$ is a generic term, $ut commonl! reers 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 63 o $igger si"e with a large num$er o nodes. ridges The de'ice that can $e used to interconnect two separate 63s is known as a $ridge. It is commonl! used to connect two similar or dissimilar 63s. The $ridge operates in la!er *, that is data&link la!er and that is wh! it is called le'el&* rela! with reerence to the OSI model. It links similar or dissimilar 63s, designed to store and orward rames, it is protocol independent and transparent to the end stations. The low o inormation through a $ridge. -se o $ridges oer a num$er o ad'antages, such as higher relia$ilit!, perormance, securit!, con'enience and larger geographic co'erage. But, it is desira$le
that ?ualit! o ser'ice oered $! aa'aila$ilit!, $ridge should match that otransit a single 63. The the parameters that deine/;OS0 the ;OS include rame mishaps, dela!, rame lietime, 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 ilteringrouting G $ridge does not change the ph!sical /2C0 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 $ridged 63 en'ironment. The irst, produced as an e#tension o IEEE <9*.) and applica$le to all IEEE <9* 63s, 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 63 including token ring, token $us and CS2C5 $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 perorms two unctions: o (orwar o 6earning to create th
Bridge (orwarding Bridge orwarding operation is e#plaunctions 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 ater 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 $ridges orward the rame to 63 ] and update the ta$le with the source address o . Step *. 3ow there are two copies o the rame on 63&]. The cop! sent $! Bridge&a is recei'ed $! Bridge&$ and 'ice 'ersa. s $oth the $ridges ha'e no inormation a$out Station B, $oth will orward the rames to 63&H. Step +. gain $oth the $ridges will orward the rames to 63&] $ecause o the lack o inormation o the Station B in their data$ase and again Step&* will $e repeated, and so on. So, the rame will continue to loop around the two 63s indeinitel!.
Spanning Tree s redundanc! creates loop pro$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 63s $etween an! two segments in the entire $ridged 63. The IEEE speciication 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 perormed $! the source host and the rame speciies 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 inormation ield to the rame. Source routing approach pro'ides a shortest path at the cost o the prolieration 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 $uer GG Switch maintains a director!: & port^ Each rame is orwarded ater ^address e#amining the ^address and orwarded to the proper port^
G Three possi$le orwarding approaches: Cut&through, Collision&ree and (ull!&$uered as $riel! e#plained $elow. Cut&through: switch orwards a rame immediatel! ater 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 ater 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! $uered: In this case, the switch orwards the rame onl! ater recei'ing the entire rame. So, the switch can detect $oth collision and error ree rames are orwarded. Comparison -et)een a s)itc+ and a +ulthough a hu$ and a switch apparentl! look similar, the! ha'e signiicant dierences. $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. Thereore, 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 63s. router isolates 63s in to su$nets to manage and control network traic. 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 $riel! mentioned $elow. G Input port perorms ph!sical and data&link la!er unctions o the router. s shown in )G Output ports, as shown in (ig. .).)4/$0, perorm the same unctions as the input ports, $ut in the re'erse order. G The routing processor perorms the unction o the network la!er. The process in'ol'es ta$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 $eore $e ore orwarding it to the other side. This propert! has made it suita$le or use in (irewalls discussed discussed in the ne#t module. Simple Internet simple internet comprising se'eral 63s and 13s linked with the help h elp o routers
3etwork Securit! 'igure .-/
ion an Decryption Concept of #ncryptio
Speciic 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 reers 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 $eore 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 $e used or decr!ption. S!mmetric ke! cr!ptograph! algorithms are simple re?uiring lesser e#ecution time. s a conse?uence, these are commonl! used or long messages. 7owe'er, these algorithms suer rom the ollowing limitations: >e?uirement o large num$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 diicult
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 shited $! three places, e.g. is su$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 $! dierent characters in the cipherte#t depending on its position in the plainte#t. 7ere the top row shows dierent characters in the plainte#t and the characters in dierent $ottom rows show the characters $! which a particular character is to $e replaced depending upon its position in dierent 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 successull!. 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! deines 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 perormed 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 perormed in the re'erse order $ased on theinsame ke! to get $ack the original $lock o $its. is perormed $! a Transormations 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 $! a hardware or a sotware, the hardware implementation is aster. %ermutation operation used in Block Ciphers Su-stitution: the su$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 perorms the ollowing steps: Step9&: Su$stitute each <&$it $ased on Step9': %ermute the $its $ased on the ke!
'igure . .-(
'igure .-. .-.(
0lic c 1ey #ncryption Pu0li
Pu0li 0lic c key encrypti cryptio on
Signature Authenticat ation
'igure .*-. .*-.
TCP3"P an Moel 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 perormed with the help o the main 5ES algorithm Caining .CC/ 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 perormed 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 eicient or long messages 'igure ./-2
Signing ocument Signing the whole ocu
Signing the igest
'igure .2-( .2-((
Access authori ri5ation 5ation with secret key key encrypt
). 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\, ith edition, Thomson India 6earning, *99.
. 1illiam Stallings, [5ata and Computer Communication\, Eighth Edition, %earson Education, *999.