西电初试专业课讲义计算机网络15-lanoverview

1、COMPUTER NETWORKSChapter 15Local Area Network OverviewLAN ApplicationsPersonal computer LANsBack end networksStorage Area NetworksHigh speed office networksBackbone LANsLAN ArchitectureTopologiesTransmission mediumMedium access controlTopologiesTreeBusRingStarChoice of TopologyReliabilityExpandabili

2、tyPerformanceNeeds considering in context of:MediumWiring layoutAccess controlBus and TreeMultipoint mediumTransmission propagates throughout medium Heard by all stationsNeed to identify target stationEach station has unique addressFull duplex connection between station and tapAllows for transmissio

3、n and receptionNeed to regulate transmissionTo avoid collisionsTo avoid hoggingData in small blocks - framesTerminator absorbs frames at end of mediumRing TopologyRepeaters joined by point to point links in closed loopReceive data on one link and retransmit on anotherLinks unidirectionalStations att

4、ach to repeatersData in framesCirculate past all stationsDestination recognizes address and copies frameFrame circulates back to source where it is removedMedia access control determines when station can insert frameStar TopologyEach station connected directly to central nodeUsually via two point to

5、 point linksCentral node can broadcastPhysical star, logical busOnly one station can transmit at a timeCentral node can act as frame switchTransmission MediaChoice of MediaConstrained by LAN topologyCapacityReliabilityTypes of data supportedEnvironmental scopeAvailable MediaVoice grade unshielded tw

6、isted pair (UTP)Shielded twisted pair and baseband coaxialBroadband cableHigh performance UTPOptical fiberProtocol ArchitectureLower layers of OSI modelIEEE 802 reference modelPhysicalLogical link control (LLC)Media access control (MAC)Physical LayerEncoding/decodingPreamble generation/removalBit tr

7、ansmission/receptionTransmission medium and topologyLogical Link Control LayerInterface to higher levelsFlow and error controlTransmission of link level PDUs between two stationsMust support multiaccess, shared mediumRelieved of some link access details by MAC layerAddressing involves specifying sou

8、rce and destination LLC usersReferred to as service access points (SAP)Typically higher level protocolLLC Service & ProtocolLLC Services based on HDLCUnacknowledged connectionless serviceConnection mode serviceAcknowledged connectionless serviceLLC ProtocolUnnumbered information PDUs to support unac

9、knowledged connectionless service (type 1)Asynchronous balanced mode to support connection mode LLC service (type 2 operation)Two new unnumbered information PDUs to support acknowledged connectionless service (type 3)Media Access Control LayerMAC FunctionsAssembly of data into frame with address and

10、 error detection fieldsDisassembly of frameAddress recognitionError detectionGovern access to transmission mediumWhereCentralDistributedHowSynchronousAsynchronousAsynchronous MACRound robinGood if many stations have data to transmit over extended periodReservationGood for stream trafficContentionGoo

11、d for bursty trafficAll stations contend for timeDistributedSimple to implementEfficient under moderate loadTend to collapse under heavy loadMAC Frame FormatMAC layer receives data from LLC layerMAC layer detects errors and discards framesLLC optionally retransmits unsuccessful framesMAC Frame Field

12、sMAC controlDestination MAC addressSource MAC addressLLCCRCLAN InterconnectionBridgeAbility to expand beyond single LANBridge is simplerConnects similar LANsIdentical protocols for physical and link layersMinimal processingRouter more general purposeInterconnect various LANs and WANsFunctions of a B

13、ridgeRead all frames transmitted on one LAN and accept those address to any station on the other LANUsing MAC protocol for second LAN, retransmit each frameDo the same the other way roundCharacteristics of a BridgeNo modification to content or format of frameNo encapsulationExact bitwise copy of fra

14、meMinimal buffering to meet peak demandContains routing and address intelligenceMust be able to tell which frames to passMay be more than one bridge to crossMay connect more than two LANsBridging is transparent to stationsAppears to all stations on multiple LANs as if they are on one single LANWLAN

15、bridgeBridge Protocol ArchitectureIEEE 802.1DMAC level Station address is at this levelBridge does not need LLC layerIt is relaying MAC framesRouting StrategyFixed RoutingSpanning TreeTypes of BridgeSource Route Bridge IBM Token RingSpanning Tree Bridge (Transparent Bridge)EthernetFixed RoutingRouti

16、ng selected for each source-destination pair of LANsDone in configurationUsually least hop routeOnly changed when topology changesBridge must decide whether to forward frameBridge must decide which LAN to forward frame onComplex large LANs need alternative routesLoad balancingFault toleranceSpanning

17、 TreeEach bridge assigned unique identifierAddress learning works for tree layoutExchange between bridges to establish spanning treeAutomatically update in response to changesThree mechanismsFrame forwardingAddress learningLoop resolutionFrame ForwardingMaintain forwarding database for each portList

18、 station addresses reached through each portFor a frame arriving on port X:Search forwarding database to see if MAC address is listed for any port except XIf address not found, forward to all ports except XIf address listed for port Y, check port Y for blocking or forwarding stateBlocking prevents p

19、ort from receiving or transmittingIf not blocked, transmit frame through port YAddress Learning & Loop ResolutionCan preload forwarding databaseWhen frame arrives at port X, it has come from the LAN attached to port XUse the source address to update forwarding database for port X to include that add

20、ressHubEach station connected to hub by two linesTransmit and receiveHub acts as a repeaterWhen single station transmits, hub repeats signal on outgoing line to each stationLine consists of two unshielded twisted pairsLimited to about 100 mHigh data rate and poor transmission qualities of UTPOptical

21、 fiber may be usedMax about 500 mPhysically star, logically busTransmission from any station received by all other stationsIf two stations transmit at the same time, collisionMultiple levels of hubs cascadedEach hub may have a mixture of stations and other hubs attached to from belowBus and HubBus c

22、onfigurationAll stations share capacity of bus (e.g. 10Mbps)Only one station transmitting at a timeHub uses star wiring to attach stations to hubTransmission from any station received by hub and retransmitted on all outgoing linesOnly one station can transmit at a timeTotal capacity of LAN is 10 Mbp

23、sImprove performance with layer 2 switchLayer 2 SwitchCentral hub acts as switch ing frame from particular station switched to appropriate output lineUnused lines can switch other trafficMore than one station transmitting at a timeBenefitsNo change to attached devices to convert bus LAN or hub LAN t

24、o switched LANFor Ethernet LAN, each device uses Ethernet MAC protocol Device has dedicated capacity equal to original LANLayer 2 switch scales easilyAdditional devices attached to switch by increasing capacity of layer 2Types of Layer 2 SwitchStore-and-forward switchAccepts frame on input lineBuffe

25、rs it briefly, check integrity Then routes it to appropriate output lineDelay between sender and receiverBoosts integrity of networkCut-through switchTakes advantage of destination address appearing at beginning of frameSwitch begins repeating frame onto output line as soon as it recognizes destinat

26、ion addressHighest possible throughput Risk of propagating bad framesSwitch unable to check CRC prior to retransmissionBridge vs Layer 2 SwitchBridge Bridge frame handling done in softwareBridge only analyzes and forwards one frame at a timeBridge uses store-and-forward operationBridge suffered comm

27、erciallyNew installations typically include layer 2 switches with bridge functionality rather than bridgesSwitchSwitch performs address recognition and frame forwarding in hardwareSwitch has multiple parallel data pathsCan handle multiple frames at a timeSwitch can have cut-through operationProblems

28、 with Layer 2 SwitchProblemBroadcast overloadLack of multiple linksSet of devices and LANs connected by layer 2 switches have flat address spaceIf any device issues broadcast frame, that frame is delivered to all devices attached to network connected by layer 2 switches and/or bridgesSolution: break

29、 up network into subnetworks connected by routersMAC broadcast frame limited to devices and switches contained in single subnetworkIP-based routers employ sophisticated routing algorithms Another DrawbackRouters do all IP-level processing in software-Low speedSolution: Layer 3 switchImplement packet

30、-forwarding logic of router in hardwareLayer 3 SwitchTwo categoriesPacket by packet layer 3 switchFlow based layer 3 switchOperates in same way as traditional routerOrder of magnitude increase in performance compared to software-based routerFlow-based switch tries to enhance performance by identifyi

31、ng flows of IP packetsSame source and destinationDone by observing ongoing traffic or using a special flow label in packet header (IPv6)Once flow is identified, predefined route can be establishedMAC Address广播地址0 xFFFF.FFFF.FFFF。广播地址可以让局域网上的所有设备接收到同一个帧。组播地址在以太网和FDDI中使用。可以让局域网上的部分是设备接收到同一个帧。在IP组播中也使用

32、组播地址。功能地址只在令牌环上使用。功能地址区分一个或多个接口提供的特定功能Vendor Code(24 bits)MACAddressMostSignificantByteLeastSignificantByteEthernetMost Significant Bit is lastToken RingMost Significant Bit is first冲突域和广播域冲突域(Collision Domain)：在网络中,所有直接连接在一起的节点都接收、发送帧的部分,此部分在冲突域中,多个网络节点竞争访问相同的物理媒介,称为冲突域。在重发器和集线器中都会传播冲突,但在局域网交换机网桥和路

33、由器中,不会传播冲突。一个冲突域里的每台主机都共享网络介质，同一时刻，同一冲突域中的主机只有一个能发送数据。广播域里的主机发送的数据包将被该域里的所有主机接受。可以结合集线器和交换机的工作方式来理解。交换机的一个端口是一个冲突域，所有的端口组成一个广播域。集线器的所有端口是一个冲突域，也是一个广播域。路由器的一个端口就是一个广播域，也是一个冲突域。冲突域网桥路由器交换机广播域VLAN (虚拟局域网)一个虚拟局域网是由一个或多个交换机创建的广播域，是对连接到的第二层交换机端口的网络用户的逻辑分段，不受网络用户的物理位置限制而根据用户需求进行网络分段。一个VLAN可以在一个交换机或者跨交换机实现。

34、基于端口的虚拟局域网基于协议的虚拟局域网基于MAC的虚拟局域网VLAN (虚拟局域网)限制广播域，控制广播风暴VLAN相当于OSI参考模型的第二层的广播域，能够将广播风暴控制在一个VLAN内部，划分VLAN后，由于广播域的缩小，网络中广播包消耗带宽所占的比例大大降低，网络的性能得到显著的提高。不同的VLAN之间的数据传输是通过第三层（网络层）的路由来实现因此使用VLAN技术，结合数据链路层和网络层的交换设备可搭建安全可靠的网络。网络管理员通过控制交换机的每一个端口来控制网络用户对网络资源的访问，同时VLAN和第三层第四层的交换结合使用能够为网络提供较好的安全措施。VLAN (虚拟局域网)VLA

35、N的实现静态静态实现是网络管理员将交换机端口分配给某一个VLAN，这是一种最经常使用的配置方式，容易实现和监视，而且比较安全。动态动态实现方式中，管理员必须先建立一个较复杂的数据库，例如输入要连接的网络设备的MAC地址及相应的VLAN号，这样当网络设备接到交换机端口时交换机自动把这个网络设备所连接的端口分配给相应的VLAN。动态VLAN的配置可以基于网络设备的MAC地址、IP地址、应用或者所使用的协议。基于端口基于MAC基于协议基于子网综合布线系统实用性支持多种数据传输灵活性可连接不同类型的设备开放性可支持不同厂商的 设备模块化插接件通用经济性整体投资低体系结构工作区子系统为用户提供一个既符合标准、又可以满足高速数据传输的标准信息插座，一般可采用5类双绞线RJ-45插座。水平子系统将工作区引至管理子系统；管理子系统分布在各个网络管理单元，管理各层的水平布线，连接相应的网络设备；干线子系统则将分配线架与主配线架用星形结构连接起来。设备子系统是网络管理的重要场所；建筑群子系统则连接各楼。工作区子系统工作区布线子系统由终端设备与信息插座之间的连线组成。标