LAN technologies and network topology

LANs and shared mediaLocality of reference

Star, bus and ring topologiesMedium 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

• 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– 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 on-going 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

• Locality of reference

• Shared medium

• Star, bus and ring topologies

• Medium access control protocols