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