Mesh networks LAN technologies and network topology • Early local networks used dedicated links between each pair of computers LANs and shared media • Some useful properties Locality of reference – hardware and frame details can be tailored for Star, bus and ring topologies each link Medium access control protocols – easy to enforce security and privacy Disadvantages of meshes Links between rooms/buildings • Poor scalability • Many links would follow the same physical path Shared Communication Channels Locality of reference • Shared LANs invented in the 1960s • LANs now connect more computers than any other form of network • Rely on computers sharing a single medium • The reason LANs are so popular is due to • Computers coordinate their access the principle of locality of reference • Low cost – physical locality of reference - computers more • But not suitable for wide area - likely to communicate with those nearby communication delays inhibit coordination – temporal locality of reference - computer is more likely to communicate with the same computers repeatedly 1 LAN topologies • LANs may be categorised according to topology ring bus star Pros and cons Example bus network: Ethernet • Star is more robust but hub may be a • Single coaxial cable - the ether - to which bottleneck computers connect • Ring enables easy coordination but is • IEEE standard specifies details sensitive to a cable being cut – data rates • Bus requires less wiring but is also sensitive – maximum length and minimum separation to a cable being cut – frame formats – electrical and physical details Conceptual flow of Ethernet data 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 • Transmitter has exclusive use of the medium 2 Collision detection Collision recovery • Collisions can occur if computers decide to transmit at the same time • Computers must wait after collision before retransmission • Each computer also senses for garbled transmission - a collision • Choose random delay up to specified max • Ethernet mechanism is called - carrier • Double the delay for each subsequent sense multiple access with collision detect - collision - binary exponential backoff CSMA/CD • Difference between bandwidth and • Example of a medium access control (MAC) throughput protocol Throughput vs. offered traffic What to do if the medium is busy? • Non persistent CSMA (deferential) • More offered traffic results in more collisions, more backing-off and eventually – if medium is idle, transmit congestion and reduced throughput – 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) Wireless LANs and CSMA/CA Example bus network - • Collision detection does not work because a LocalTalk transmission from one computer may only be • LAN technology for Apple computers received by its immediate neighbours • Solution is collision avoidance • MAC protocol is CSMA/CA (collision – sender sends small request message to receiver avoidance) – receiver responds with a ‘clear to send’ message that • Each computer first sends a small message received by all adjacent computers to reserve the bus 3 • Special message called the token grants Example ring network - permission (needs bit stuffing) IBM Token Ring • Computer grabs token, removes it, sends one frame, checks for errors then replaces it • 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 Example ring network - FDDI Example star network - ATM • Overcomes token ring susceptibility to failure through two counter-rotating cables • 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 4 .