Wide Area Network

Chapter 11Wide Area Network

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What is a WAN?• A wide area network (WAN) is one of the oldest kinds of data

communication networks• A WAN is a distributed network that covers a broad geographic area;

a WAN typically consists of multiple networks at geographically distributed locations that are interconnected

• Relative to LANs or MANs (see Figure 10-2) a WAN typically covers a wider geographic area (see Figure 10-1) and operates at lower speeds

• The Internet is the largest WAN that has been created– Many organizations leverage the Internet backbone to connect

geographically distributed sites– Some links in the Internet backbone have transmission speeds higher than

those found in LANs and WANs

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Figure 10-1

Figure 10-2

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Table 10-1

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Public Swathed telephone Network (PSTN)

• PSTNs were originally design exclusively for telephone but have become highly sophisticated, able to handle different kinds of data transmission, including digital data transmission.

• PSTN consists following components:– Subscriber wiring and equipment– Demarcation point– Local loops– Central offices– Switching offices– Long-distance carriers– Points of presence– Data transmission services

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Public Swathed telephone Network (PSTN)• PSTN provides a number of options for data

communications, including services that routs packets between different sites. Some available services and transmission rates are given below:Service Transmission Rate– Switched 56 56Kpbs– X.25 56Kpbs– T1 1.544Mbps– T3 44.736Mbps– Frame Relay 1.544Mbps– SMDS 1.544Mbps– ISDN 1.544Mbps– ATM 44.736Mbps

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The Internet• SLIP and PPP : Serial Line Protocol (SLIP) and Point-to-

Point Protocol (PPP) are two very common protocols used to transmit IP packets over serial line and telephone connections, most often as part of a dial-up Internet connection.

• The TCP/IP protocol suite runs over a variety of network media: IEEE802.3 (Ethernet), and 802.5 (Token Ring) LAN X.25 line satellite links and serial lines.

• PPP is a multiprotocol transport mechanism. While SLIP is design to handle one type of traffic (TCP/IP traffic) at a time, PPP can transport TCP/IP traffic as well as IPX, Apple Talk and other types of traffic simultaneously on the same connection.

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WAN Services Fundamentals

• There are two major categories of WAN connections:– Circuit-switched networks– Packet-switched networks

• Switching is fundamental to both approaches– Switching technologies establish paths across networks

from senders to receivers– Switching allows connections to be established and

maintained between senders and receivers so that they can exchange messages and information

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Circuit-Switched Networks• In circuit-switched networks, a switched dedicated circuit is created to

connect two (or more) parties– To users, it is as if a direct physical point-to-point path is established

between sender and receiver– Multiple-switches may be involved is establishing a switched connection

(see Figure 12-1)• There are three phases to circuit-switched communications:

– Creation of the temporary circuit – Information transmission– Circuit termination

• Because there is a limit to the number of switched connections that can be established at a particular point in time, circuit-switched network users may not be able to initiate communication sessions during peak usage times

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Figure 12-1

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Packet-Switched Networks• In packet-switched networks (see Figure 12-2), data is packetized

prior to transmission– Each packet is a group of bits organized in a predetermined

structure– Each packet contains data bits as well as additional overhead

information to ensure error-free transmission to intended recipients

– Packets may be called blocks, cells, datagrams, data units, or frames

• Packet assembler/disassemblers (PADs) are responsible for assembling outgoing data into packets for transmission over the packet-switching network as well as for unpacking incoming packets so that data can be delivered to intended recipients

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Figure 12-2

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Packet Formats

• Figure 12-3 illustrates the format of HDLC packets used in X.25 packet-switching networks Major overhead fields include:– Flag: used to delimit the beginning and end of a packet– Address: specifies the address of the intended packet

recipient– Control: transports packet sequence numbers and

retransmission requests– Frame check: used for error checking. CRC-16 or a 16-

bit checksum may be used with HDLC frames

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Figure 12-3

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Packet-Switching Advantages and Disadvantages

• Relative to circuit-switching, packet-switching has a number of advantages and disadvantages. Advantages include:– A single-link between packet-switching nodes can be simultaneously shared

by multiple senders and receivers; senders are not denied access to the network during peak usage periods

– Packet-priority systems can be established– Subscribers to packet-switching services are often charged on the volume of

data (number of packets) transmitted rather than connection time• Disadvantages include:

– Variable transmission delays caused by packet processing and packet queues at packet switches

– Some packet-switching networks support variable packet sizes; this contributes to longer packet processing times at packet switches

– The inclusion of overhead data in packets means that data transmission efficiency and throughput is lower than that in circuit-switched networks

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Switching Alternatives in Packet-Switched Networks

• Two fundamental approaches are used to route packets from senders to receivers:– Datagram approach: individual packets, even those associated with a single

file, are routed independently• Two packets (datagrams) from the same source can have two different

temporary circuits established to the same recipient• This type of circuit allocation is called connectionless because a

dedicated connection is not established and because the packets that make up a single file do not follow each other over the same circuit from sender to receiver

– Virtual circuit approach: this is similar to establishing a dedicated circuit in a circuit-switched network. Packets that comprise a single file (or message) follow the same route in sequence from sender to receiver.

• This type of packet-switching is called connection-oriented• It is not identical to circuit-switched connections because the route

segments in virtual circuits are shared, not dedicated

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Virtual Circuits• Call setup packets are used to establish virtual circuits; these are used

to identify the best path to the destination across the network.• Virtual circuit details are stored in virtual circuit tables at packet

switches• The paths followed by packets in virtual circuits are called logical

channels; each packet includes a logical channel number when created by the PAD

• There are two major types of virtual circuits:– Switched virtual circuits (SVCs): which are similar to temporary circuit-

switched connections– Permanent virtual circuits (PVCs): which is similar to a leased, circuit-

switched connection• Once a PVC is allocated, no call setup or call clearing is needed; the logical

circuit is permanently stored in virtual circuit tables

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Packet Switching Protocol• Packet-switching concepts• A PDN is sometimes called an X.25 network or public data network.

– The X.25 designation stems from ITU’s recommendation X.25 which defines the interface between DTE and DCE for public data networks (see Figure 12-4)

– The term value-added network (VAN) is often used in conjunction with PDNs because network proprietors offer additional services beyond mere data transmission including virtual circuits, error recovery, network management, message priorities, and store-and-forward capabilities

• X.25 PDNs are more widely available outside the U.S.– In the U.S., frame relay services are more common than X.25

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Figure 12-4

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PDN Error Correction Processes• PDNs employ node-to-node (aka hop-to-hop or point-to-

point) error detection and correction (see Figure 12-8)• Each packet is checked for errors at each packet switch

before being forwarded to the next hop on its path• If no errors are detected, an ACK is sent to the previous

hop• If errors are detected, a NAK is sent to the previous hop

which triggers retransmission of the packet• This process means that PDNs are store-and-forward

networks; packets are stored at switching nodes until positive acknowledgements are received

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Figure 12-8

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Frame Relay Technologies• Key technologies in frame relay networks are illustrated in

Figure 12-9. These include:– Frame assembler/dissembler devices (FRADs) which like X.25

PADs are responsible for building outgoing frames and unpacking incoming frames

– Frame relay switches which are responsible for accepting frames, checking them for errors, and transmitting them to their next hops in the network

• Both switched and permanent virtual circuits are supported in frame relay networks

– Frame relay circuits. Frame relay switches are typcially connected by DS-1 (T-1) or DS-3 (T-3) circuits. The Frame Relay Form (FRF) has addressed connections up to 622 mbps (OC-12)

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Figure 12-9

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Frame Formats • Frames are formatted by FRAD devices or software• Variable length frames may be supported; some may

include up to 8,000 characters• Figure 12-11 shows a LAPD (Link access Procedure—D

channel) frame relay• The address field carries the recipient’s network address as

well as a data link connection identifier (DLCI) that serves the same purpose as a virtual circuit identifier in X.25 (see Figure 12-12)

• The BCEN, FCEN, and DE fields are used to address network congestion during peak usage periods

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Figure 12-11

Figure 12-12

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Asynchronous Transfer Mode (ATM)

• ATM is a high-bandwidth, low-delay, packet-switching and multiplexing technology that can handle many types of network traffic and WAN services

• ATM represents a step in the evolution of frame relay by using frames (called cells) that do not vary in size

– The use of small fixed-size packets translates into easier switching and faster transmission rates.

– By 2002, ATM transfer rates of 38.813 gbps had been achieved over OC-768 circuits

• Virtual channels are used in ATM to establish logical connections between senders and receivers (see Figure 12-13)

– Once setup up, full-duplex variable-rate transmission is possible over the connections

• Virtual paths are also supported. These are bundles of virtual channels with the same end-points that are switched as a set. Each channel can carry a different type of data

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Figure 12-16

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ATM Cell Formats

• Two cell formats have been specified for ATM (see Figure 12-14):– User-network interface (UNI): UNI cells carry

data between the user and the ATM network– Network-network interface (NNI): NNI cells

carry network control information between ATM switches

• NNI also enables network control information to be exchanged between different ATM networks

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Figure 12-14

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T- Carrier

T-carrier Bandwidth No. Of T1 Channels used• T1 1.544Mbps 1 • T2 6.312Mbps 4• T3 44.736Mbps 28• T4 274.176Mbps 168

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T-1 Service Access Technologies

• Businesses use a variety of services to access T-1 services (see Figure 12-18). These include:– T-1 CSU/DSUs– T-1 multiplexers– T-1 channel banks– T-1 switches

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Figure 12-18

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SONET Services• Synchronous optical network (SONET) is an optical transmission

interface/specification for high-speed digital transmission over optical fiber• SONET specifications define a hierarchy of standardized data transfer rates

over optical media. An abbreviated set is provided in Table 12-2– Each level is capable of carrying multiple lower-speed signals. An STS-1

channel, for example, is capable of carrying multiple DS-1 (T-1) signals• STS-1 frames are the fundamental data transmission format in SONET (see

Figure 12-19)– Each consists of 810 octets that can logically be depicted as a matrix of 9 rows

with 90 octets in each row– 87 octets in each row carry data and can be flexibly allocated to lower

bandwidth channels such as DS-0, DS-1, and DS-2• SONET service access technologies include add-drop multiplexors, cross-

connect switches, and broadband bandwidth managers

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Table 12-2

Figure 12-19

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ISDN• Integrated Services Digital Network (ISDN) is widely used by business

to provide digital WAN services among geographically dispersed operating locations

• ISDN switches are the core of the ISDN network (see Figure 12-20)• Two major categories of ISDN are:

– Narrowband ISDN. This is essentially a circuit-switched digital network service that allows temporary connections to be dynamically created and terminated among ISDN subscribers. Two narrowband service levels exist:

• Basic rate interface (BRI) that supports two 64 kbps bearer channels and one 16 kbps data channel (2B+D)

• Primary rate interface (PRI) with 23 64 bps bearer channels and a 64 bps data channel (23B+D)

– Broadband ISDN (B-ISDN) which may be described as ATM over SONET

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Figure 12-20

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Wireless WAN Services

• Increasingly, organizations are turning to wireless WAN services to satisfy their data communication needs including:– Circuit-switched cellular systems (see Figure 12-21)– Cellular digital packet data (CDPD)– ARDIS (Advanced Radio Data Information Service)– Mobitex – Metricom (see Figure 12-22)– Personal communication services (PCS)– Broadband wireless services (such as wireless T-1 service)

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Figure 12-21

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Figure 12-22

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Fiber Distributed Data Interface (FDDI)

• FDDI (standardized as ANSI X3T9.5) backbone protocol was developed in the 1980s and popular during the 80s and 90s.

• FDDI operates at 100 Mbps over a fiber optic cable.

• Copper Distributed Data Interface (CDDI) is a related protocol using cat 5 twisted wire pairs.

• FDDI’s future looks limited, as it is now losing market share to Gigabit Ethernet and ATM.

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FDDI Topology (Figure 7-15)• FDDI uses both a physical and logical ring

topology capable of attaching a maximum of 1000 stations over a maximum path of 200 km. A repeater is need every 2 km.

• FDDI uses dual counter-rotating rings (called the primary and secondary). Data normally travels on the primary ring.

• Stations can be attached to the primary ring as single attachment stations (SAS) or both rings as dual attachment stations (DAS).

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Figure 7-15 FDDI Topology

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FDDI’s Self Healing Rings

• One important feature of FDDI is its ability to handle a break in the ring to form a temporary ring out of the pieces of the two rings.

• Figure 7-16, show an example of a cable break between two dual-attachment stations.

• After the cable break is detected, a single ring is formed out of the primary and secondary rings until the cable break can be repaired.

12 - 44Figure 7-16 FDDI’s Self-healing Rings

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FDDI Media Access Control• FDDI uses a token passing system. Computers wanting to

send packets wait to receive a token before transmitting.• Multiple packets can be attached to the token as it moves

around the network. • When a station receives the token, it looks for attached

packets addressed to it and removes them from the incoming packet.

• If the station wants to send a packet it attaches it to the token and sends the token with its attached packets to the next station.

• This controlled access technique provides a higher performance level at high traffic levels compared to a contention-based technique like Ethernet.

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Advantages and Disadvantages of FDDI: Advantages

• High bandwidth• Security• Physical durability• Resistance to EMI• Cable distance• Weight• Use of multiple token• Ability to prioritize workstation• System fault tolerance

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Advantages and Disadvantages of FDDI: Disadvantages

• Complex technology• Installation and maintenance require a great

deal of expertise• Cost

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Asynchronous Transfer Mode (ATM)

• Asynchronous Transfer Mode (ATM) (also called cell relay) is a technology originally designed for use in wide area networks that is now often used in backbone networks.

• ATM backbone switches typically provide point-to-point full duplex circuits at 155 Mbps (total of 310 Mbps).

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ATM vs. Switched Ethernet• ATM is a switched network, but differs from

switched Ethernet in four ways:1. ATM uses small, fixed-length packets of 53 bytes

(called cells). Ethernet frames are variable and can be up to about 1 kilobyte in length.

2. ATM provides no error correction on the user data. Switched Ethernet does error correction.

3. ATM uses virtual channels instead of the fixed addresses used by traditional data link layer protocols such as switched Ethernet (see Fig. 7-17).

4. ATM prioritizes transmissions based on Quality of Service (QoS), while switched Ethernet does not.

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Figure 7-17 Addressing & Forwarding with ATM Virtual Circuits

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ATM’s Virtual Circuits

• ATM is connection-oriented, meaning all packets travel in order through the same virtual circuit.

• There are two types of ATM virtual circuits:– Permanent Virtual Circuits (PVCs) - defined

when the network is established or modified.– Switched Virtual Circuits (SVCs) - defined

temporarily for one transmission and deleted when the transmission is completed.

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LAN Encapsulation• The first step in LAN Encapsulation is to create an

ATM virtual circuit identifier for the virtual circuit that will connect the “gateway” ATM edge switch to the ATM edge switch nearest the frame’s destination (see Figure 7-18)

• Once the virtual circuit is ready, the Ethernet frame is broken up into a series of ATM cells and sent over the ATM backbone using the ATM virtual circuit identifier.

• At the receiving edge switch the frame is reassembled. Unfortunately LAN has very high overhead and so network performance suffers as a consequence.

12 - 53Figure 7-18 ATM Encapsulation

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ATM to the Desktop• ATM-25 is a low-speed option that provides

point-to-point full duplex circuits at 25.6 Mbps in each direction. It is an adaptation of token ring that runs over cat 3 cable and can even use token ring hardware if modified.

• ATM-51 is designed for the desktop allowing 51.84 Mbps from computers to the switch.

• Both these ATMs appear to be good choices for desktop connections when ATM backbone networks are used. However, industry has been very slow to accept either and have instead moved to Fast Ethernet which is both cheaper and faster.