课件版1计算机网络16-highspeedlans

1、COMPUTER NETWORKSChapter 16High Speed LANsIntroductionRange of technologiesFast and Gigabit EthernetFiber ChannelHigh Speed Wireless LANsApplicationsOffice LANs used to provide basic connectivitySpeed and power of PCs has risenMIS organizations recognize LANs as essential Centralized server farmsPow

2、er workgroupsSmall number of cooperating usersHigh-speed local backboneIEEE802.3 MACIEEE 802.3 -EthernetRandom Access Stations access medium randomlyAlohaSlotted Aloha ContentionStations content for time on mediumCSMACSMA/CD (Carriers Sense Multiple Access with Collision Detection)ALOHAPacket RadioS

3、ending:When station has frame, it sendsStation listens for max round trip time plus small incrementIf ACK, fine. If not, retransmitIf no ACK after repeated transmissions, give upReceiving:If frame OK and address matches receiver, send ACKFrame may be damaged by noise or by another station transmitti

4、ng at the same time (collision)Any overlap of frames causes collisionMax utilization 18%Slotted ALOHATime in uniform slots equal to frame transmission timeNeed central clock (or other sync mechanism)Transmission begins at slot boundaryFrames either miss or overlap totallyMax utilization 37%CSMAPropa

5、gation time is much less than transmission timeAll stations know that a transmission has started almost immediatelyAlgorithmFirst listen for clear medium (carrier sense)If medium idle, transmitIf two stations start at the same instant, collisionWait reasonable time (round trip plus ACK contention)No

6、 ACK then retransmitMax utilization depends on propagation time (medium length) and frame lengthLonger frame and shorter propagation gives better utilizationThree typesNon-persistent CSMA1-persistent CSMAp-persistent CSMANon-persistent CSMAIf medium is idle, transmit; otherwise, go to 2If medium is

7、busy, wait amount of time drawn from probability distribution (retransmission delay) and repeat 1Random delays reduces probability of collisionsConsider two stations e ready to transmit at same time While another transmission is in progressIf both stations delay same time before retrying, both will

8、attempt to transmit at same timeCapacity is wasted because medium will remain idle following end of transmissionEven if one or more stations waiting1-persistent CSMAIf medium idle, transmit; otherwise, go to step 2If medium busy, listen until idle; then transmit immediatelyTo avoid idle channel time

9、Stations are selfishIf two or more stations waiting, collision guaranteedp-persistent CSMAIf medium idle, transmit with probability p, and delay one time unit with probability (1 p)Time unit typically maximum propagation delayIf medium busy, listen until idle and repeat step 1If transmission is dela

10、yed one time unit, repeat step 1Compromise that attempts to reduce collisionsLike non-persistentAnd reduce idle timeLike 1-persistentWhat is an effective value of p?Value of p?Avoid instability under heavy loadn stations waiting to sendEnd of transmission, expected number of stations attempting to t

11、ransmit is number of stations ready times probability of transmittingnpIf np 1 on average there will be a collisionRepeated attempts to transmit almost guaranteeing more collisionsRetries compete with new transmissions Eventually, all stations trying to sendContinuous collisions; zero throughputSo n

12、p 1 for expected peaks of nIf heavy load expected, p smallHowever, as p made smaller, stations wait longerAt low loads, this gives very long delaysCSMACSMA/CDWith CSMA, collision occupies medium for duration of transmission(如果产生冲突无法立刻消除，除非等到没有ACK返回的时候，效率低下)Stations listen whilst transmittingIf mediu

13、m idle, transmit, otherwise, step 2If busy, listen for idle, then transmitIf collision detected, jam then cease transmissionAfter jam, wait random time then start from step 1 先听后发，边听边发，冲突停止，随机延迟后重发CSMA/CD冲突检测(CD)原理 带冲突检测的监听算法把浪费带宽的时间减少到检测冲突的时间，对局域网来说这个时间是很短的。在下一页图中画出了基带系统检测冲突需要的最长时间。这个时间发生在网络中相距最远的两

14、个站(A和D)之间。在t0时刻A开始发送。假设经过一段时间(网络最大传播时延)，D开始发送。D立即就会检测到冲突，并能很快停止。但A仍然感觉不到冲突，并继续发送。再经过一段时间 ，A才会收到冲突信号，从而停止发送。可见在基带系统中检测冲突的最长时间是网络传播延迟的两倍(2 )，我们把这个时间叫做冲突窗口。网络中根据设计的数据速率和最大网段长度规定了最小帧长:CSMA/CD的实现载波监听的实现: 对于基带系统，是检测电压脉冲序列。由于以太网的编码采用Manchester编码，这种编码的特点是每比特(无论是0或1)中间都有电压跳变，监听站可以把这种跳变信号当作代表信道忙的载波信号。对于宽带系统，监

15、听站接收RF载波以判断信道是否空闲。冲突检测的实现: 对于基带系统，是把直流电压加到信号上来检测冲突的。每个站都测量总线的直流电平，由于冲突而叠加的直流电平比单个站发出的信号强，所以IEEE 802标准规定：如果发送站电缆接头出的信号强度超过了单个站发送的最大强度，则说明检测到了冲突。然而，信号在电缆上传播时会有衰减，如果电缆太长，就会使冲突信号到达远端时的幅度小于规定的CD门限值。为此，标准限制了电缆长度为500m或者200m。对于宽带系统，检测冲突把接收的数据与发送的数据逐位比较。当一个站向入径上发送时，同时(考虑了传播和端头的延迟后)从出径上接收数据，通过比较发现是否有冲突。对于双绞线星

16、型网，冲突检测的方法更简单。这种情况下，HUB监视输入端的活动，若有两处以上的输入端出现信号，则认为发生冲突，并立即产生一个“冲突出现”的特殊信号CP，向所有输出端广播。二进制指数退避 检测到冲突发送干扰信号后退一段时间重新发送。后退时间的多少对网络的稳定工作有很大影响。特别在负载很重的情况下，为了避免很多站连续发生冲突，需要设计有效的后退算法。一般选择二进制指数后退算法，后退时延的取值范围与重发次数n形成二进制指数关系。 其中第一式是在区间0，2n中取一均匀分布的随机整数，第二式是计算出随机后退时延。当然，还可以有其他的后退算法，但二进制指数后退算法考虑了网络负载的变化情况。事实上，后退次数

17、的多少往往与负载大小有关。而二进制指数后退算法的优点是把后退时延的平均取值与负载的大小联系起来。 IEEE 802.3选用二进制指数退避的1坚持算法具有较高效率轻负载时，信道一空闲就能够立刻传输重负载时，能够保持一定的稳定性10Mbps Ethernet10Base510Base210Base-T10Base-FMediumCoaxialCoaxialUTP850nm fiberSignalingBasebandManchesterBasebandManchesterBasebandManchesterManchesterOn/OffTopologyBusBusStarStarNodes100

18、30-33100Mbps Fast EthernetUse IEEE 802.3 MAC protocol and frame format100BASE-X use physical medium specifications from FDDITwo physical links between nodesTransmission and reception100BASE-TX uses STP or Cat. 5 UTP100BASE-FX uses optical fiber100BASE-T4 can use Cat. 3, voice-grade UTP Uses four twi

19、sted-pair lines between nodesData transmission uses three pairs in one direction at a timeStar-wire topologySimilar to 10BASE-T100Base-TX100Base-FX100Base-T4Medium2 pair, STP2 pair, Cat 5 UTP2 optical fiber4 pair, cat 3,4,5SignalingMLT-3MLT-34B5B,NRZI8B6T,NRZ100Mbps Fast EthernetFull Duplex Operatio

20、nTraditional Ethernet half duplexEither transmit or receive but not both simultaneously100-Mbps Ethernet in full-duplex mode, theoretical transfer rate 200 MbpsMust use switching hubEach station constitutes separate collision domainIn fact, no collisionsCSMA/CD algorithm no longer needed802.3 MAC fr

21、ame format usedAttached stations can continue CSMA/CDMixed ConfigurationsFast Ethernet supports mixture of existing 10-Mbps LANs and newer 100-Mbps LANsE.g. 100-Mbps backbone LAN to support 10-Mbps hubsGigabit EthernetIdentical CSMA/CD and MAC format with 10,100MbpsTwo differences:Carrier extensionA

22、t least 4096 bit-times long (512 for 10/100)Frame burstingGigabit Ethernet Physical1000Base-SXShort wavelength, multimode fiber1000Base-LXLong wavelength, Multi or single mode fiber1000Base-CXCopper jumpers 25m, shielded twisted pair1000Base-T4 pairs, cat 5 UTPSignaling - 8B/10B10Gbps EthernetMaximu

23、m link distances cover 300 m to 40 kmFull-duplex mode only10GBASE-S (short):850 nm on multimode fiberUp to 300 m10GBASE-L (long)1310 nm on single-mode fiberUp to 10 km10GBASE-E (extended)1550 nm on single-mode fiberUp to 40 km10GBASE-LX4:1310 nm on single-mode or multimode fiberUp to 10 kmWavelength

24、-division multiplexing (WDM) bit stream across four light wavesToken Ring (802.5)Each repeater connects to two others via unidirectional transmission linksSingle closed pathData transferred bit by bit from one repeater to the nextRepeater regenerates and retransmits each bitRepeater performs data in

25、sertion, data reception, data removalRepeater acts as attachment pointPacket removed by transmitter after one trip round ringRing Repeater StatesListen State FunctionsScan passing bit stream for patternsAddress of attached stationToken permission to transmitCopy ing bit and send to attached stationW

26、hilst forwarding each bitModify bit as it passese.g. to indicate a packet has been copied (ACK)Transmit State FunctionsStation has dataRepeater has permissionMay receive ing bitsIf ring bit length shorter than packetPass back to station for checking (ACK)Bypass StateSignals propagate past repeater w

27、ith no delay (other than propagation delay)Partial solution to reliability problemImproved performance802.5 MAC ProtocolSmall frame (token) circulates when idleStation waits for tokenChanges one bit in token to make it SOF for data frameAppend rest of data frameFrame makes round trip and is absorbed

28、 by transmitting stationStation then inserts new token when transmission has finished and leading edge of returning frame arrivesUnder light loads, some inefficiencyUnder heavy loads, round robin802.5 Physical LayerData Rate4Mbps16Mbps100Mbps100Mbps1000MbpsMediumUTP,STP,FiberUTP,STP,FiberUTP,STPFiberFiberSignalingDifferential ManchesterDifferential ManchesterMLT-34B5B,NRZI8B10TMax Frame BYTE455018200182001820018200Access ControlTP or DTRTP or DTRDTRDTRDTRTP令牌传递接入控制 DTR 专用令牌环FDDI光纤分布式数据接口FDDI是一个基于令牌环标准的令牌传送环协议。一个FDD