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Wide Area Networks

Computer Networks

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Motivation

Connect multiple sitesSpan geographic distancesCross public right-of-way

streets buildings railroads

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Building blocks

Point-to-point long distance connections leased circuits provided by telecommunications companies

Packet switches IP routers ATM switches many types...

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Long-Distance Transmission Technology

Lease transmission facilities from telecommunications companies Generally point-to-point NOT part of the conventional telephone

system Copper, fiber, microwave or satellite Analog or digital

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WAN Transmission Equipment

Analog MODEM required at each end

Digital CSU/DSU required at each end

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CSU/DSUs

Performs two functions, usually in a single boxNeeded because telecommunications digital

encoding differs from computer industry digital encoding

DSU translates between the two encodings

CSU terminates line allows 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 T1 OC1, OC3, OC12, OC48, SONET

Rest of the world examples E1, E2, E3 SDH

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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

ARPANET historically important in packet switching fast when invented, slow by current standards precursor to today’s Internet

X.25 protocol suite early commercial service still used more popular in Europe service by telecommunications companies

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Recent WAN Technologies

SMDS offered by telecommunications companies not as popular as frame relay

Frame relay widely used commercial service offered by telecommunications companies

ATM offered by telecommunications companies designed for both WAN and LAN

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Future WAN Technologies

Packet over SONET provided by telecommunications

companies removes intermediate complexity

Packet over DWDM provided by telecommunications

companies and long haul fiber optic providers

Ethernet?

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Asynchronous Transfer Mode (ATM)

Connection oriented using virtual circuitsUse 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-NRTvariable bit rate - non real timeVBR-RT variable bit rate - real timeABR available bit rateUBR unspecified bit rate

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LANE and MPOA

Overview Backwards 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 LES 4. LEC setups up connection to LES and registers addresses 5. LES assigns a BUS to the LEC 6. LEC queries LES for a MAC address to ATM address

translation 7. LECS responds directly or forwards query to another LEC 8. 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 packets