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 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