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LAN technologies and network
topology
LANs and shared media
Locality of reference
Star, bus and ring topologies
Medium access control protocols
Mesh networks
• Early local networks used dedicated links
between each pair of computers
• Some useful properties
– hardware and frame details can be tailored for
each link
– easy to enforce security and privacy
Disadvantages of meshes
• Poor scalability
Links between rooms/buildings
• Many links
would follow
the same
physical path
Shared Communication Channels
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Shared LANs invented in the 1960s
Rely on computers sharing a single medium
Computers coordinate their access
Low cost
But not suitable for wide area communication delays inhibit coordination
Locality of reference
• LANs now connect more computers than
any other form of network
• The reason LANs are so popular is due to
the principle of locality of reference
– physical locality of reference - computers more
likely to communicate with those nearby
– temporal locality of reference - computer is
more likely to communicate with the same
computers repeatedly
LAN topologies
• LANs may be categorised according to
topology
star
ring
bus
Pros and cons
• Star is more robust but hub may be a
bottleneck
• Ring enables easy coordination but is
sensitive to a cable being cut
• Bus requires less wiring but is also sensitive
to a cable being cut
Example bus network: Ethernet
• Single coaxial cable - the ether - to which
computers connect
• IEEE standard specifies details
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data rates
maximum length and minimum separation
frame formats
electrical and physical details
Conceptual flow of Ethernet data
• Transmitter has exclusive use of the
medium
Ethernet coordination
• The computers can detect when a signal is
on the Ether - carrier sense
• Can only transmit when the Ether is free carrier sense with multiple access (CSMA)
• Prevents a computer interrupting an ongoing transmission
Collision detection
• Collisions can occur if computers decide to
transmit at the same time
• Each computer also senses for garbled
transmission - a collision
• Ethernet mechanism is called - carrier
sense multiple access with collision detect CSMA/CD
• Example of a medium access control (MAC)
protocol
Collision recovery
• Computers must wait after collision before
retransmission
• Choose random delay up to specified max
• Double the delay for each subsequent
collision - binary exponential backoff
• Difference between bandwidth and
throughput
Throughput vs. offered traffic
• More offered traffic results in more
collisions, more backing-off and eventually
congestion and reduced throughput
What to do if the medium is busy?
• Non persistent CSMA (deferential)
– if medium is idle, transmit
– if busy, wait a random time then try again
• 1-persistent CSMA (selfish)
– if idle, transmit
– if busy, listen until idle then transmit
• p-persistent (compromise)
– if idle transmit with probability p and delay one
time unit with probability (1-p)
Example bus network LocalTalk
• LAN technology for Apple computers
• MAC protocol is CSMA/CA (collision
avoidance)
• Each computer first sends a small message
to reserve the bus
Wireless LANs and CSMA/CA
• Collision detection does not work because a
transmission from one computer may only be
received by its immediate neighbours
• Solution is collision avoidance
– sender sends small request message to receiver
– receiver responds with a ‘clear to send’ message that
received by all adjacent computers
Example ring network IBM Token Ring
• MAC protocol based on token passing
• Computer must wait for permission before
transmitting
• Computer controls the ring until finished
• Data flows right round the ring
– receiver makes a copy
– transmitter checks for errors and then removes
• Special message called the token grants
permission (needs bit stuffing)
• Computer grabs token, removes it, sends
one frame, checks for errors then replaces it
Example ring network - FDDI
• Overcomes token ring susceptibility to
failure through two counter-rotating cables
Example star network - ATM
• Asynchronous
Transfer Mode
• Uses pairs of
optical fibres to
connect
computers to a
central hub
Summary
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Locality of reference
Shared medium
Star, bus and ring topologies
Medium access control protocols