CSCI 4550/8556 Computer NetworksDirect Point to Point CommunicationDirect Point to Point CommunicationConnections in a Point-to-point NetworkSlide 5Reducing the number of communication channelsGrowth of LAN TechnologiesLocality of ReferenceLAN TopologiesStar TopologyStar Topology in PracticeSlide 12Ring TopologySlide 14Bus TopologyWhy Have Multiple Topologies?EthernetEthernet SpeedsEthernet OperationCSMACollision DetectionEthernet Collision DetectionRecovery from a CollisionExponential BackoffWireless LANsLimited Wireless ConnectivityCSMA/CACollisionsLocalTalkToken RingTransmission Around a Token RingUsing the TokenThe Token and SynchronizationIBM Token RingFDDIFDDI and ReliabilityATM - A Star NetworkATM DetailsATM Illustrations - 1ATM Illustrations - 2SummaryCSCI 4550/8556Computer NetworksComer, Chapter 8:LAN Technology and Network TopologiesDirect Point to Point CommunicationDirect Point to Point CommunicationComputers connected by communication channels that each connect exactly two computers Forms mesh or point-to-point network Allows flexibility in communication hardware, packet formats, etc. Provides security and privacy because communication channel is not sharedConnections in a Point-to-point NetworkThe number of wires grows as square of number of computers For n computers, connections = (n 2 - n) / 2.Connections in a Point-to-point NetworkConnections between buildings can be prohibitive.Adding a new computer requires n-1 connections.Reducing the number of communication channelsLANs developed in late 1960s and early 1970s Key idea - reduce number of connections by sharing connections among many computersComputers take turnsTDM Must include techniques for synchronizing useGrowth of LAN TechnologiesLAN technologies reduce cost by reducing number of connections But ... attached computers compete for use of shared connection Local communication almost exclusively LAN Long distance almost exclusively point-to-point SMDSATMLocality of ReferencePrinciple of locality of reference helps predict computer communication patterns: Spatial (or physical) locality of reference - computers likely to communicate with other computers that are located nearby Temporal locality of reference - computers are likely to communicate with the same computers repeatedly Thus LANs are effective because of spatial locality of reference, and temporal locality of reference may give insight into which computers should be on a LANLAN TopologiesNetworks may be classified by shape Three most popular: Star RingBusStar TopologyAll computers attach to a single central point.The central point is sometimes called a hub.Star Topology in PracticePrevious diagram is idealized; usually, connecting cables run in parallel to computers:Star Topology in PracticeThe result often looks like this:Ring TopologyComputers connected in a closed loopFirst passes data to second, second passes data to third, and so onIn practice, there is a short connector cable from the computer to the ringRing connections may run past offices with connector cable to socket in the office:Ring TopologyBus TopologySingle cable connects all computers. Each computer has connector to shared cable. Computers must synchronize and allow only one computer to transmit at a time.Why Have Multiple Topologies?Each topology has advantages and disadvantages: Ring: eases synchronization; may be disabled if any cable is cut. Star: easier to manage and more robust; requires more cables. Bus: requires fewer cables; may be disable if cable is cut.UNO has used all three topologies; star topology is most widely used in PKI.Industry is settling on star topology as most widely usedEthernetWidely used LAN technology Invented at Xerox PARC (Palo Alto Research Center) in 1970s Defined in a standard by Xerox, Intel and Digital - DIX standard Standard now managed by IEEE - defines formats, voltages, cable lengths, ... Uses bus topology Single coax cable - the ether Multiple computers connect One Ethernet cable is sometimes called a segment Limited to 500 meters in length Minimum separation between connections is 3 metersEthernet SpeedsOriginally Ethernet ran at 3Mbps.The previous standard speed was 10Mbps. Most new installations now use Fast Ethernet, which operates at 100Mbps.Ethernet is also now commonly used at higher speeds (e.g. 1000Mbps) on backbone cables.Ethernet OperationOne computer transmits at a time.The signal is a modulated carrier which propagates from the transmitter in both directions along the length of the segment.CSMAThere is no central control managing when computers transmit on ether. Ethernet employs CSMA to coordinate transmission among multiple attached computers. Carrier Sense with Multiple Access Multiple access - multiple computers are attached and any of these can be a transmitter.Carrier sense – the computer wanting to transmit tests ether for a carrier before transmitting.Collision DetectionEven with CSMA, two computers may transmit simultaneously. Both check the ether at same time, find it idle, and begin transmitting. The window for transmission depends on the speed of propagation in the ether.Signals from two computers will interfere with each other if they are present at the same place at the same time. Overlapping frames is called a collision . Collisions cause no harm to hardware.Data from both frames is garbled.Ethernet Collision DetectionEthernet interfaces include hardware to detect transmissions: They monitor the outgoing signal (listen while sending) A garbled signal is interpreted as a collision. After a collision is detected, the computer immediately stops transmitting.So, Ethernet uses CSMA/CD to coordinate transmissions.Recovery from a CollisionThe computer that detects a collision sends a special signal to force all other interfaces to detect collision. That computer then waits for ether to be idle before transmitting again If both computers involved in the collision wait same length of time, the frames will collide again.The standard specifies the maximum delay, and both computers choose a random delay less than the maximum.After waiting, the computers use carrier sense to avoid subsequent collisions. The computer with the shorter delay will go first. But other computers (not involved in the collision) may transmit first.Exponential BackoffEven with random delays, collisions may occur. This is especially likely with busy network segments. Computers double their delay period with each subsequent collision. This reduces the