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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
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