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Wide Area Networks

Computer Networks

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Motivation

Connect multiple sitesSpan geographic distancesCross public right-of-way

streetsbuildingsrailroads

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Building blocks

Point-to-point long distance connectionsleased circuitsprovided by telecommunications companies

Packet switchesIP routersATM switchesmany types...

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Long-Distance Transmission Technology

Lease transmission facilities from telecommunications companiesGenerally point-to-pointNOT part of the conventional telephone

systemCopper, fiber, microwave or satelliteAnalog or digital

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WAN Transmission Equipment

AnalogMODEM required at each end

DigitalCSU/DSU required at each end

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CSU/DSUs

Performs two functions, usually in a single box

Needed because telecommunications digital encoding differs from computer industry digital encoding

DSUtranslates between the two encodings

CSUterminates lineallows for maintenance

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CSU/DSU Illustrated

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Standards for WAN Circuits

Specified by telephony industry in each region

Can differ around the worldNorth America and Japan examples

DS0, DS1, DS3, T1, T3, Fractional T1OC1, OC3, OC12, OC48, SONET

Rest of the world examplesE1, E2, E3SDH

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Inverse Multiplexing

Recall multiplexingInstead of intertwining, you combineCombines two or more circuitsProduces intermediate capacity circuitSpecial hardware required

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Inverse Multiplexing Illustrated

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Early WAN Technologies

ARPANEThistorically important in packet switchingfast when invented, slow by current standardsprecursor to today’s Internet

X.25 protocol suiteearly commercial servicestill usedmore popular in Europeservice by telecommunications companies

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Recent WAN Technologies

SMDSoffered by telecommunications companiesnot as popular as frame relay

Frame relaywidely used commercial serviceoffered by telecommunications companies

ATMoffered by telecommunications companiesdesigned for both WAN and LAN

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Future WAN Technologies

Packet over SONETprovided by telecommunications companiesremoves intermediate complexity

Packet over DWDMprovided by telecommunications companies

and long haul fiber optic providers

Ethernet?

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Asynchronous Transfer Mode (ATM)

Connection oriented using virtual circuits

Use of fixed sized packets called cellsDefines service categories for QoS

networkingOperates on LANs using LAN

Emulation (LANE)Usually used for high-speed WAN

connectivity

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Virtual Circuits

Circuit ID instead of full host address pairs

Switches swap IDs using translation tables

Switch stores connection state for hosts

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Cell Format

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Service Types

CBR constant bit rateVBR-NRT variable bit rate - non real timeVBR-RT variable bit rate - real timeABR available bit rateUBR unspecified bit rate

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LANE and MPOA

OverviewBackwards compatibility of ATM nodes with LANs such as Token Ring and

Ethernet. Must support the broadcast nature of LANs with the virtual circuit nature of ATM.

LAN Emulation Client (LEC)Provides abstraction for LAN protocols to ATM network

LAN Emulation Server (LES)Manages and supports LECs

LAN Emulation Configuration Server (LECS)Maintains emulated LANs (ELANs) and supports LECs

Broadcast and Unknown Server (BUS)Distributes broadcasts/multicasts

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How ATM is Used in a LAN

1. LEC - Implemented at each client (bridge listens to all)2. LEC contacts LECS to going a ELAN via a pre-configured

SVC or discovered dynamically via integrated local management interface (ILMI)

3. LECS assign LEC to a ELAN and direct them to a LES4. LEC setups up connection to LES and registers addresses5. LES assigns a BUS to the LEC6. LEC queries LES for a MAC address to ATM address

translation7. LECS responds directly or forwards query to another LEC8. MAC broadcasts are sent to BUS so that an associated ATM

broadcast can be sent to all stations in the ELAN.

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ATM Summary

The aim was to combine the flexibility of packet data networks (e.g. Internet) with the per-user quality of service

guarantees of a circuit switched network (e.g. telephone network). A

monumental task! Although an important technology, it may follow the path of similar technologies (e.g. Token

Ring). Stay tuned.

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Final Thoughts

Local loopISDNDigital Subscriber Line (DSL)Cable modemsCellularCIR/CAREverything over IP