Wan

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WAN mreže WAN tehnologije
• Računarske mreže koje obuhvataju neograničen geografski prostor, sa propusnim opsegom reda 10 kbps do 10 Gbps

WAN Technology
• A WAN is a data communications network that operates beyond the geographic scope of a LAN. • One primary difference between a WAN and a LAN is that a company or organization must subscribe to an outside WAN service provider in order to use WAN carrier network services. • Devices on the subscriber premises are called customer premises equipment (CPE). The subscriber owns the CPE or leases the CPE from the service provider. • A copper or fiber cable connects the CPE to the service provider’s nearest exchange or central office (CO). • This cabling is often called the local loop, or "last-mile".

WAN Technology

WAN Service Providers

DTE and DCE

DTE and DCE
• In order for the local loop to carry data, a device such as a modem is needed to prepare the data for transmission. • Devices that put data on the local loop are called data circuit-terminating equipment, or data communications equipment (DCE). • The customer devices that pass the data to the DCE are called data terminal equipment (DTE). • The DCE primarily provides an interface for the DTE into the communication link on the WAN cloud. • The DTE/DCE interface uses various physical layer protocols, such as High-Speed Serial Interface (HSSI) and V.35. These protocols establish the codes and electrical parameters the devices use to communicate with each other.

WAN tehnologije
• Layer 1 (RS-232, RS-449, X.21, V.35, G.703 itd.) • Layer 2 (HDLC, PPP, Frame Relay, X.25) • Multilayer (ISDN, ATM, SDH/SONET)

Layer 1 WAN prenos
• Preko WAN linkova bitovi se prenose serijski (serijski linkovi) • Tipovi serijskih linkova
– sinhroni link (trenutno do 10 Gbps) – asinhroni link (do 115200 bps)

Layer 1 WAN standardi
• Definišu električni interfejs za prenos bitova između DTE i DCE uređaja • Dve vrste prenosa bitova:
– Sa zajedničkom masom
• RS-232 (V.24, V.28, V.10) • X.21

– Diferencijalni
• V.35 • RS-449 (RS-530, V.11)

Physical Layer Connectors

Physical Layer: WANs

WAN Line Types and Bandwidth

WAN Devices

CSU/DSU
• The communications link needs signals in an appropriate format. • For digital lines, a channel service unit (CSU) and a data service unit (DSU) are required. The two are often combined into a single piece of equipment, called the CSU/DSU. The CSU/DSU may also be built into the interface card in the router.

Modem Transmission
• A modem is needed if the local loop is analog rather than digital. • Modems transmit data over voice-grade telephone lines by modulating and demodulating the signal. • The digital signals are superimposed on an analog voice signal that is modulated for transmission. • The modulated signal can be heard as a series of whistles by turning on the internal modem speaker. • At the receiving end the analog signals are returned to their digital form, or demodulated.

Modem Transmission

Communication Server

• Communication servers concentrate dial-in user communication and remote access to a LAN. They may have a mixture of analog and digital (ISDN) interfaces and support hundreds of simultaneous users.

WAN Standards
• WANs use the OSI reference model, but focus mainly on Layer 1 and Layer 2. • WAN standards typically describe both physical layer delivery methods and data link layer requirements, including physical addressing, flow control, and encapsulation. • WAN standards are defined and managed by a number of recognized authorities.

HDLC
• Layer 2 tehnologija koja omogućuje prenos Layer 3 datagrama preko sinhrone serijske linije • Postoji HDLC ISO standard koji ne podržava više Layer 3 protokola • Proizvođači su razvili sopstvene nekompatibilne HDLC derivate

Format HDLC okvira

HDLC Fields - 1
• The frame always starts and ends with an 8-bit flag field, the bit pattern 01111110. • Because there is a likelihood that this pattern will occur in the actual data, the sending HDLC system always inserts a 0 bit after every five 1s in the data field, so in practice the flag sequence can only occur at the frame ends. The receiving system strips out the inserted bits. • When frames are transmitted consecutively the end flag of the first frame is used as the start flag of the next frame.

Standardni HDLC format 0x7E 1 Address Control 1 1 Data 0-1500 FCS 2 0x7E 1

Cisco HDLC format 0x7E 1 0x0F ili 0x8F 1 0x00 1 Protocol 2 Data 0-1500 FCS 1 0x7E 1

HDLC Fields - 2
• • The address field is not needed for WAN links, which are almost always point-to-point. The address field is still present and may be one or two bytes long. The control field indicates the frame type, which may be information, supervisory, or unnumbered:
• • • Unnumbered frames carry line setup messages. Information frames carry network layer data. Supervisory frames control the flow of information frames and request data retransmission in the event of an error.

HDLC Encapsulation





The control field is normally one byte, but will be two bytes for extended sliding windows systems. Together the address and control fields are called the frame header. The encapsulated data follows the control field. Then a frame check sequence (FCS) uses the cyclic redundancy check (CRC) mechanism to establish a two or four byte field

WAN Data-Link Protocols

WAN Link Options

Circuit Switching

Packet Switching

Point-to-Point Protocol (PPP)
• Layer 2 tehnologija koja omogućuje prenos Layer 3 datagrama preko sinhrone i asinhrone serijske linije • Komponente PPP protokola:
– Enkapsulacija datagrama preko serijskog linka – Link Control Protocol (LCP) – Network Control Protocols (NCP)

PPP Layered Architecture

Format PPP okvira

LCP protokol
• Uspostavljanje linka • Određivanje kvaliteta linka (echo reply i echo request) • Uspostavljanje algoritma kompresije • Autentifikacija (PAP i CHAP protokoli) • Uspostavljanje NCP protokola • Terminacija linka • PPP Callback

Format PPP okvira 0x7E 1 0xFF 1 0x03 1 Protocol 1(2) Data 0-1500 FCS 1(2) 0x7E 1

PPP algoritmi kompresije
• STAC • Predictor • Microsoft Point-to-Point Compression (MPPC)

Metode PPP autentifikacije
• Password Authentication Protocol (PAP)
– Klijent šalje korisničko ime i lozinku u čistom tekstu, i to samo na početku uspostavljanja sesije

• Challenge Authentication Protocol (CHAP)
– Server šalje challenge – Klijent vraća vrednost izračunatu unapred poznatim algoritmom (MD5) – Server i sam računa tu vrednost, i poredi sa dobijenom

PPP Multilink
• Mehanizam kojim se više fizičkih serijskih veza spajaju u jednu logičku • Svaki datagram se deli na delove fiksne veličine koji se naizmenično šalju preko svakog od serijskih linkova

NCP protokoli
• Protokoli koji razmenjuju informacije na mrežnom nivou • Primeri
– IPCP (razmenjuje IP adrese krajeva linka, IP adrese DNS servera itd.) – IPXCP – CDPCP

PPP Frame Fields
• • • Flag – Indicates the beginning or end of a frame and consists of the binary sequence 01111110. Address – Consists of the standard broadcast address, which is the binary sequence 11111111. PPP does not assign individual station addresses. Control – 1 byte that consists of the binary sequence 00000011, which calls for transmission of user data in an unsequenced frame. A connection-less link service similar to that of Logical Link Control (LLC) Type 1 is provided. Protocol – 2 bytes that identify the protocol encapsulated in the data field of the frame. Data – 0 or more bytes that contain the datagram for the protocol specified in the protocol field. The end of the data field is found by locating the closing flag sequence and allowing 2 bytes for the frame check sequence (FCS) field. The default maximum length of the data field is 1,500 bytes. FCS – Normally 16 bits or 2 bytes that refers to the extra characters added to a frame for error control purposes.

Establishing a PPP Session
• PPP session establishment progresses through three phases.
– link establishment, – authentication, – network layer protocol phase.

• •



• LCP frames are used to accomplish the work of each of the LCP phases. The following three classes of LCP frames are used in a PPP session: • Link-establishment frames are used to establish and configure a link. • Link-termination frames are used to terminate a link. • Link-maintenance frames are used to manage and debug a link.

PPP Session - Linkestablishment phase
• • Link-establishment phase each PPP device sends LCP frames to configure and test the data link. LCP frames contain a configuration option field that allows devices to negotiate the use of options such as the
– maximum transmission unit (MTU), – compression of certain PPP fields, – link-authentication protocol.

PPP Session - Authentication phase
• Authentication phase (optional) – After the link has been established and the authentication protocol decided on, the peer may be authenticated. Authentication, if used, takes place before the network layer protocol phase is entered. As part of this phase, LCP also allows for an optional link-quality determination test. The link is tested to determine whether the link quality is good enough to bring up network layer protocols.

• • •

If a configuration option is not included in an LCP packet, the default value for that configuration option is assumed. Before any network layer packets can be exchanged, LCP must first open the connection and negotiate the configuration parameters. This phase is complete when a configuration acknowledgment frame has been sent and received.

PAP - Password Authentication Protocol

CHAP - Challenge Handshake Authentication Protocol

PPP Session - Network layer protocol phase
• Network layer protocol phase – • PPP devices send NCP packets to choose and configure one or more network layer protocols, such as IP. • Once each of the chosen network layer protocols has been configured, packets from each network layer protocol can be sent over the link. • If LCP closes the link, it informs the network layer protocols so that they can take appropriate action.

Frame Relay
• Layer 2 tehnologija koja omogućuje prenos Layer 3 datagrama preko jedne fizičke veze na više različitih destinacija • Svaki uređaj u Frame relay mreži (DTE) se priključuje na Frame relay svič (DCE) preko jedne serijske veze, a uspostvlja više logičkih veza prema drugim DTE uređajima preko virtuelnih veza (VC)

Introducing Frame Relay
• Frame Relay is an International Telecommunication Union Telecommunications Standardization Sector (ITUT) and American National Standards Institute (ANSI) standard. • Frame Relay is a packet-switched, connection-oriented, WAN service. • It operates at the data link layer of the OSI reference model. • Frame Relay uses a subset of the high-level data link control (HDLC) protocol called Link Access Procedure for Frame Relay (LAPF).

Frame Relay Operation

Frame relay mreža

Frame Relay Terminology
• The connection through the Frame Relay network between two DTEs is called a virtual circuit (VC). • Virtual circuits may be established dynamically by sending signaling messages to the network. In this case they are called switched virtual circuits (SVCs). However, SVCs are not very common. • Generally permanent virtual circuits (PVCs) that have been preconfigured by the carrier are used. • Because it was designed to operate on high-quality digital lines, Frame Relay provides no error recovery mechanism. If there is an error in a frame it is discarded without notification.

Veza DTE-DCE Veza DCE-DCE Permanent virtual circuit (PVC)

Virtual Circuits

Local Significance of DLCIs
The DLCI is stored in the address field of every frame transmitted.

Format Frame Relay okvira

LAPF Control field

Format Frame Relay okvira Q.922 2 Protocol 1 Data 0-1500 FCS 2

Parametri Frame Relay veze
• Za svaku virtuelnu vezu mogu se definisati određeni parametri:
– DLCI (Datalink Connection Identifier) je broj koji identifikuje vezu. Za jednu vezu postoje dva različita DLCI-a na DTE stranama – CIR (commited information rate) je garantovani propusni opseg za tu vezu – EIR (excess information rate) je dodatni propusni opseg koga ta veza može koristiti (ukupni propusni opseg = CIR + EIR)

Frame Relay kontrola protoka
• Okvirima koji imaju niži prioritet, pošiljalac setuje bit DE (Discard Egligibility). Pri zagušenju, tj. kada se prevazilazi PVC CIR, prvo se odbacuju ti okviri • Kada se otkrije zagušenje u mreži, paketima prema odredištu se setuje bit FECN (ForwardExplicit Congestion Notification), a paketima prema izvorištu bit BECN (Backward-Explicit Congestion Notification)

Local Management Interface
• Local Management Interface (LMI) definiše poruke izmešu DTE i DCE uređaja • LMI stanardi
– ITU Q.933 – ANSI T1.617 – Cisco

LMI - Local Management Interface
• A mechanism for DTEs to dynamically acquire information about the status of the network. This feature was omitted in the original design. The extensions for this status transfer are called the Local Management Interface (LMI). • The LMI extensions include the following: • The heartbeat mechanism, which verifies that a VC is
• • • • operational The multicast mechanism The flow control The ability to give DLCIs global significance The VC status mechanism

Non-Data DLCIs
• The 10-bit DLCI field allows VC identifiers 0 through 1023. • The LMI extensions reserve some of these identifiers. • This reduces the number of permitted VCs. • LMI messages are exchanged between the DTE and DCE using these reserved DLCIs.

Frame Relay u OSI
• Dva pogleda sa mrežnog nivoa:
– Frame Relay interfejs se smatra kao NBMA (non-broadcast multiaccess network) mreža. Statički ARP definiše mapiranje između IP adresa DTE uređaja povezanih PVC-ovima i njihovih DLCI – Na Frame Relay interfejsu se definiše pointto-point podinterfejs za svaki PVC

ISDN
• ISDN (Integrated Services Digital Network) je mreža zasnovana na tehnologiji komutacije kola (circuit switching) koja omogućava prenos više medija (glas, podaci, pokretna slika) • Po prirodi povezana sa telefonskom mrežom (POTS)

ISDN Standards
• Work on standards for ISDN began in the late 1960s. • A comprehensive set of ISDN recommendations was published in 1984 and is continuously updated by the International Telecommunication Union Telecommunication Standardization Sector (ITU-T), formerly known as the Consultative Committee for International Telegraph and Telephone (CCITT). • The ISDN standards are a set of protocols that encompass digital telephony and data communications.

ISDN uređaji
• Terminalni uređaji
– digitalni (ISDN) terminalni uređaji (TE1) – analogni (ne-ISDN) terminalni uređaji (TE2)

ISDN Reference Points

• Mrežni završetak (NT1) • Komutacioni uređaji (ISDN svičevi)

ISDN Devices

BRI Reference Points

Povezivanje ISDN uređaja

Kanali u ISDN vezi
• B (Bearer) kanal služi za prenos korisničkih podataka • D kanal služi za prenos kontrolnih informacija • B kanal je uvek propusnog opsega od 64 kbps, dok za D kanal varira od servisa

ISDN servisi
• BRI servis
– Dva B kanala – D kanal (16 kbps)
• • •

ISDN Access Options
The B channels can be used for digitized speech transmission. In this case, specialized methods are used for the voice encoding. Also, the B channels can be used for relatively high-speed data transport. In this mode, the information is carried in frame format, using either high-level data link control (HDLC) or PPP as the Layer 2 protocol. PPP is more robust than HDLC because it provides a mechanism for authentication and negotiation of compatible link and protocol configuration. ISDN is considered a circuit-switched connection. The B channel is the elemental circuit-switching unit. The D channel carries signaling messages, such as call setup and teardown, to control calls on B channels. Traffic over the D channel employs the Link Access Procedure on the D Channel (LAPD) protocol. LAPD is a data link layer protocol based on HDLC.

• PRI servis
– 30 (E1) / 23 (T1) B kanala – D kanal (64 kbps)
• •

ISDN 3-Layer Model
• ISDN utilizes a suite of ITU-T standards spanning the physical, data link, and network layers of the OSI reference model:
• The ISDN BRI and PRI physical layer specifications are defined in ITUT I.430 and I.431, respectively. • The ISDN data link specification is based on LAPD and is formally specified in the following:
• • • • ITU-T Q.920 ITU-T Q.921 ITU-T Q.922 ITU-T Q.923

ISDN 3-Layer Model



The ISDN network layer is defined in ITU-T Q.930, also known as I.450 and ITU-T Q.931, also known as I.451. These standards specify userto-user, circuit-switched, and packet-switched connections.

ISDN Physical Layer (BRI)
• BRI service is provided over a local copper loop that traditionally carries analog phone service. While there is only one physical path for a BRI, there are three separate information paths, 2B+D. Information from the three channels is multiplexed into the one physical path. • ISDN physical layer, or Layer 1, frame formats differ depending on whether the frame is outbound or inbound. If the frame is outbound, it is sent from the terminal to the network. Outbound frames use the TE frame format. If the frame is inbound, it is sent from the network to the terminal. Inbound frames use the NT frame format.

ISDN Physical Layer (BRI)

ISDN Layer 2 nivo
• Definisan u ITU-T standardima Q.920 i Q.921 • Odgovoran za prenos okvira

ISDN Layer 3 nivo
• Definisan u ITU-T standardima Q.930 i Q.931 • Odgovoran za uspostavljanje i raskidanje konekcije • Tipovi Q.931 poruka
– – – – – – – SETUP CONNECT RELEASE USER INFORMATION CANCEL STATUS DISCONNECT

Uspostavljanje ISDN veze

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