Transcript Wide Area Networks

Wide Area Networks
Computer Networks
Autumn 2000
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Motivation
Ñ Connect multiple sites
Ñ Span geographic distances
Ñ Cross public right-of-way
Ñstreets
Ñbuildings
Ñrailroads
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Building blocks
Ñ Point-to-point long distance connections
Ñleased circuits
Ñprovided by telecommunications companies
Ñ Packet switches
ÑIP routers
ÑATM switches
Ñmany types...
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Long-Distance
Transmission Technology
Ñ Lease transmission facilities from
telecommunications companies
ÑGenerally point-to-point
ÑNOT part of the conventional telephone
system
ÑCopper, fiber, microwave or satellite
ÑAnalog or digital
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WAN Transmission
Equipment
Ñ Analog
ÑMODEM required at each end
Ñ Digital
ÑCSU/DSU required at each end
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CSU/DSUs
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Performs two functions, usually in a single box
Needed because telecommunications digital
encoding differs from computer industry digital
encoding
DSU
Ñ translates between the two encodings
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CSU
Ñ terminates line
Ñ allows for maintenance
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CSU/DSU Illustrated
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Standards for WAN
Circuits
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Specified by telephony industry in each region
Can differ around the world
North America and Japan examples
Ñ DS0, DS1, DS3, T1, T3, Fractional T1
Ñ OC1, OC3, OC12, OC48, SONET
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Rest of the world examples
Ñ E1, E2, E3
Ñ SDH
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Inverse Multiplexing
Ñ Recall multiplexing
Ñ Instead of intertwining, you combine
Ñ Combines two or more circuits
Ñ Produces intermediate capacity circuit
Ñ Special hardware required
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Inverse Multiplexing
Illustrated
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Early WAN Technologies
Ñ ARPANET
Ñ historically important in packet switching
Ñ fast when invented, slow by current standards
Ñ precursor to today’s Internet
Ñ X.25 protocol suite
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early commercial service
still used
more popular in Europe
service by telecommunications companies
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Recent WAN Technologies
Ñ SMDS
Ñ offered by telecommunications companies
Ñ not as popular as frame relay
Ñ Frame relay
Ñ widely used commercial service
Ñ offered by telecommunications companies
Ñ ATM
Ñ offered by telecommunications companies
Ñ designed for both WAN and LAN
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Future WAN Technologies
Ñ Packet over SONET
Ñprovided by telecommunications companies
Ñremoves intermediate complexity
Ñ Packet over DWDM
Ñprovided by telecommunications companies
and long haul fiber optic providers
Ñ Ethernet?
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Asynchronous Transfer
Mode (ATM)
Ñ Connection oriented using virtual circuits
Ñ Use of fixed sized packets called cells
Ñ Defines service categories for QoS
networking
Ñ Operates on LANs using LAN Emulation
(LANE)
Ñ Usually used for high-speed WAN
connectivity
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Virtual Circuits
Ñ Circuit ID instead of full host address pairs
Ñ Switches swap IDs using translation tables
Ñ Switch stores connection state for hosts
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Cell Format
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Service Types
Ñ CBR
constant bit rate
Ñ VBR-NRT variable bit rate - non real time
Ñ VBR-RT
variable bit rate - real time
Ñ ABR
available bit rate
Ñ UBR
unspecified bit rate
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LANE and MPOA
Ñ Overview
Ñ 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.
Ñ LAN Emulation Client (LEC)
Ñ Provides abstraction for LAN protocols to ATM network
Ñ LAN Emulation Server (LES)
Ñ Manages and supports LECs
Ñ LAN Emulation Configuration Server (LECS)
Ñ Maintains emulated LANs (ELANs) and supports LECs
Ñ Broadcast and Unknown Server (BUS)
Ñ Distributes broadcasts/multicasts
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How ATM is Used in a LAN
Ñ 1. LEC - Implemented at each client (bridge listens to all)
Ñ 2. LEC contacts LECS to going a ELAN via a pre-configured SVC or
discovered dynamically via integrated local management interface
(ILMI)
Ñ 3. LECS assign LEC to a ELAN and direct them to a LES
Ñ 4. LEC setups up connection to LES and registers addresses
Ñ 5. LES assigns a BUS to the LEC
Ñ 6. LEC queries LES for a MAC address to ATM address translation
Ñ 7. LECS responds directly or forwards query to another LEC
Ñ 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
Ñ Local loop
Ñ ISDN
Ñ Digital Subscriber Line (DSL)
Ñ Cable modems
Ñ Cellular
Ñ CIR/CAR
Ñ Everything over IP
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