Download Wide Area Networks

Wide Area Networks
Computer Networks
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Motivation
Connect multiple sites
Span geographic distances
Cross 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 box
Needed 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 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
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Inverse Multiplexing
Recall multiplexing
Instead of intertwining, you combine
Combines two or more circuits
Produces intermediate capacity circuit
Special 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 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
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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
VBR-NRT
VBR-RT
ABR
UBR
constant bit rate
variable bit rate - non real time
variable bit rate - real time
available bit rate
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 loop
ISDN
Digital Subscriber Line (DSL)
Cable modems
Cellular
CIR/CAR
Everything over IP
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