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Wide Area Networks. Computer Networks. Motivation. Connect multiple sites Span geographic distances Cross public right-of-way streets buildings railroads. Building blocks. Point-to-point long distance connections leased circuits provided by telecommunications companies Packet switches

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Wide Area Networks

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Wide area networks

Wide Area Networks

Computer Networks

John Kristoff



  • Connect multiple sites

  • Span geographic distances

  • Cross public right-of-way

    • streets

    • buildings

    • railroads

John Kristoff

Building blocks

Building blocks

  • Point-to-point long distance connections

    • leased circuits

    • provided by telecommunications companies

  • Packet switches

    • IP routers

    • ATM switches

    • many types...

John Kristoff

Long distance transmission technology

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

John Kristoff

Wan transmission equipment

WAN Transmission Equipment

  • Analog

    • MODEM required at each end

  • Digital

    • CSU/DSU required at each end

John Kristoff

Csu dsus


  • Performs two functions, usually in a single box

  • Needed because telecommunications digital encoding differs from computer industry digital encoding

  • DSU

    • translates between the two encodings

  • CSU

    • terminates line

    • allows for maintenance

John Kristoff

Csu dsu illustrated

CSU/DSU Illustrated

John Kristoff

Standards for wan circuits

Standards for WAN Circuits

  • Specified by telephony industry in each region

  • Can differ around the world

  • North 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

John Kristoff

Inverse multiplexing

Inverse Multiplexing

  • Recall multiplexing

  • Instead of intertwining, you combine

  • Combines two or more circuits

  • Produces intermediate capacity circuit

  • Special hardware required

John Kristoff

Inverse multiplexing illustrated

Inverse Multiplexing Illustrated

John Kristoff

Early wan technologies

Early WAN Technologies


    • 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

John Kristoff

Recent wan technologies

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

John Kristoff

Future wan technologies

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?

John Kristoff

Asynchronous transfer mode atm

Asynchronous Transfer Mode (ATM)

  • Connection oriented using virtual circuits

  • Use of fixed sized packets called cells

  • Defines service categories for QoS networking

  • Operates on LANs using LAN Emulation (LANE)

  • Usually used for high-speed WAN connectivity

John Kristoff

Virtual circuits

Virtual Circuits

  • Circuit ID instead of full host address pairs

  • Switches swap IDs using translation tables

  • Switch stores connection state for hosts

John Kristoff

Cell format

Cell Format

John Kristoff

Service types

Service Types

  • CBRconstant bit rate

  • VBR-NRTvariable bit rate - non real time

  • VBR-RTvariable bit rate - real time

  • ABRavailable bit rate

  • UBRunspecified bit rate

John Kristoff

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

John Kristoff

How atm is used in a lan

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.

John Kristoff

Atm summary

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.

John Kristoff

Final thoughts

Final Thoughts

  • Local loop

  • ISDN

  • Digital Subscriber Line (DSL)

  • Cable modems

  • Cellular


  • Everything over packets

John Kristoff

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