Local Area Networks

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 Poor scalability 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 Media Access Control (MAC) Low cost But not suitable for wide area – communication delays prevent 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

Advantages & Disadvantages Star is more robust but hub may be a bottleneck, and use up the most wiring Ring enables easy coordination but is sensitive to a cable cut Bus requires less wiring but is also sensitive to a cable cut

Example Bus Network - Ethernet Single coaxial cable - the ether - to which computers connect Transmitter has exclusive use of the medium IEEE standard (802.3) specifies details data rates maximum length and minimum separation electrical and physical details frame formats Assignment: IEEE 802 standards

Ethernet Wiring

Original Thick Ethernet Wiring 10Base5 Coaxial cable and AUI (Attachment Unit Interface) connector Network hardware involves two components network interface card transceiver (transmitter-receiver)

Connection Multiplexing Allows multiple computers to share a transceiver and makes wiring simpler

Thin Ethernet Wiring 10Base2 Thinner wire, no transceiver and simple BNC connector

Thin Ethernet Wiring

Twisted Pair Ethernet 10BaseT Twisted pair cable connects all computers to a central hub

Ethernet Wiring

Wiring Schemes

Comparing Wiring Schemes Thicknet can change computers without disruption Accessing remote transceivers can be difficult Thinnet easier access greater possibility of disconnection Twisted pair cheapest cable/connection failure only affects one machine Is 10BaseT Ethernet a bus or a star physically it is a star logically it is a bus Physical topology can differ from logical topology!!

Network Interface Hardware Network Adapter Card or Network Interface Card (NIC) handles details of transmission independently of the CPU specific to a particular technology sometimes the NIC may need to connect to other hardware in order to connect to the network

NIC Connectors NICs often support all three kinds of connections

Hardware Addressing A LAN provides connectivity for all computers but most communications are one-to-one to avoid unnecessary interruptions, each NIC is assigned a unique numerical address (48 bits) called its hardware address, physical address or MAC address Each frame transmitted across the network includes a destination and a source MAC address field the destination MAC address is decode to determine whether to accept it the source address field is used when a reply is needed

Other Addressing Modes Broadcasting some applications require that a sender transmits a frame to all stations on the network use a special reserved broadcast address as destination all NICs are configured to accept frames with their own address and the broadcast address Promiscuous Mode NICs can often be configured to accept all packets useful for network analysers (sniffers). has security implications

Ethernet Transmission Manchester encoding uses rising and falling edges to encode data Ethernet uses a standard frame format with 48 bits address and a 16 bit frame type field Frame type field used by receivers to decide follow-on processing required Standard frame types are defined by Ethernet standards Preambles consist of alternating 1’s and 0’s that are used for clock synchronization

Explicit Frame Types

Ethernet Coordination The computers can detect when a signal is on the Ether – carrier sense Can transmit only when the Ether is free – carrier sense with multiple access (CSMA), thus preventing a computer interrupting an ongoing transmission Collision can still occur if computers sense the media as free and proceed to transmit at the same time Each computer also senses the transmission to detect for a collision (if signal is garbled) Whole mechanism is called - carrier sense multiple access with collision detect - CSMA/CD

Collision Recovery Computers must wait after collision before retransmission Choose random delay up to specified maximum Double the delay sets for each subsequent collision – binary exponential backoff More offered traffic results in more collisions, more backing off, results in congestion and reduced throughput.

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 Another example is wireless LAN hidden station problem exposed station problem

(a)The hidden terminal problem. (b) The exposed station problem. CSMA Problems in Wireless Networks (a)The hidden terminal problem. (b) The exposed station problem. From the book Computer Networks by Tanenbaum

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 data and generates a new token

IBM Token Ring (Continued) Special message called token grants permission Computer grabs token, removes it, sends one frame, checks for errors when the frame returns then regenerates a new token into the network

Example Ring Network - FDDI Fiber Distributed Data Interconnect Overcomes token ring weakness 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 Switch

LAN Extensions Fiber modems Repeaters extend connection between computer and transceiver Repeaters join LAN segments regenerate signals without knowledge of frames

Multiple Repeaters Ethernet standard says no more than four repeaters between two computers Fiber modems can be used between repeaters for long distance extensions Biggest problem with repeaters is that all signals including collisions and noise will be transmitted

Bridges Connect two segments, but work at the frame level Use promiscuous mode and forward all frames Don’t forward incorrect frames (e.g. collisions and noise)

Self-Learning Bridge Only forward a frame if necessary (frame filtering) destination is on the other segment broadcast address is used Bridge learns which segment a computer is on when that computer sends a frame When a frame is received by the bridge extracts source address and updates knowledge inspects destination address and consults the current knowledge for forwarding decisions

Bridge Example of Learning

Bridged Network Planning Parallelism for capacity and performance bridges allow communication on separate segments to occur at the same time computers that communicate frequently with one another should be attached to the same segment

Cycle of Bridges To prevent the problem of infinite loops, an algorithm call distributed spanning tree (DST) is used when a bridge first initializes DST enables a bridge to determine whether any other bridge is already performing forwarding operation in each LAN segment that it attaches If there is, some of its ports will be blocked, creating a logical tree topology on top of the physical one, thus preventing loops.

Switching Switch – a single electronic device that transfers frames between computers Whereas a hub simulates a shared medium, a switch simulated a bridged LAN with one computer per segment Advantage is greater data rate due to parallelism Hubs and switches are combined in LAN design to reduce cost