Chapter-5-Local-Area-Network-高级计算机网络-教学(英文版)课件.ppt

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1、Chapter 5 Local Area NetworkThe Medium Access Control SublayerLocal Area Network5.1 LAN ArchitecturerIEEE 802 Reference ModelmAfter the establishment of LAN Standards Committee(IEEE 802 Committee)in February 1980,it has developed a series of LAN standards known as IEEE 802 Standards.Local Area Netwo

2、rk5.1 LAN ArchitecturerIEEE802 Reference ModelLocal Area Network5.1 LAN ArchitecturerFunctions of each layermThe lowest level of IEEE802 reference model corresponds to the physical layer of OSI model,and consists of the following functions:Signal encoding/decodingCharacteristics of machinery,electri

3、city,function and proceduresTransmission/reception of bitThe functions of LLC and MAC are similar to the Data Link Layer OSI Local Area Network5.1 LAN ArchitecturerMedium Access Control Sublayer(MAC)mData Frames AssemblymData Frames DisassemblymAddress ResolutionmError Detection mChannel Assignment

4、to solve the channel competition(the main function of MAC)Local Area Network5.1 LAN ArchitectureLocal Area Network5.1 LAN ArchitectureLocal Area Network5.1 LAN ArchitecturerDescription of LLC framemNo delimiters and checksum fieldmLLC frame is in the data field of MAC framemIt has source address(Sou

5、rce Service Access Point),so that it can be applicable to point-point,point-multipoint and broadcast communications.Local Area Network5.1 LAN ArchitecturerControl field of LLC and HDLC are the samemThe control field divides the LLC frame into information frame,management control frame,unnumbered fra

6、me,respectively,to complete different functions.m802.2 standard in general is based on the HDLC data link control protocol.Local Area Network5.1 LAN ArchitecturerIEEE 802 Standard defines several types of LAN,including definitions and descriptions of the physical layer and MAC sublayer.It is compose

7、d as follows:m802.1:Basic description and interface primitives definitionm802.2:Logical Link Control(LLC)sublayerm802.3:LAN with CSMA/CD technologiesm802.4:LAN with token bus technologym802.5:LAN with token ring technologyLocal Area Network5.2 LAN Technologies rChannel AssignmentmThere are two metho

8、ds of channel assignmentmstatic AllocationFrequency Division Multiplexing:FDM(Wavelength Division Multiplexing:WDM)Time Division Multiplexing:TDMAdvantage:It is suitable for situations of less users,fixed site number,large user traffic.Disadvantage:It can not be flexible to adapt to changes of the s

9、ite number and traffic.Local Area Network5.2 LAN Technologies mDynamic AllocationKey issues:How to resolve the channel contention,minimize conflicts and introduce MAC layermWhich node sends data?mIf there is a conflict when it sends data?mHow to deal with a conflict?Local Area Network5.2 LAN Technol

10、ogies rEthernet broadcast moderEthernet communicationmUnacknowledged ConnectionlessmThe reason for this is the good quality of LAN channel because the channel error probability is very small.Local Area Network5.2 LAN Technologies rEthernet ServicesmEthernet services are not reliable delivery that is

11、 best efforts to delivery.mWhen the destination station receives data frame with error,it will discard this frame.Error correction is determined by the high level.mIf the level finds the missing of some data to retransfer,but the Ethernet doesnt know that this is a retransmission of the frame and se

12、nd it as a new data frame.Local Area Network5.2 LAN Technologies rCarrier Sense Multiple Access Protocol with collision detection(CSMA/CD)mCarrier Sense Multiple Access Protocol:CSMAmMultiple AccessMultiple users share one linemCarrier SenseBefore access transmission channel,the site monitors whethe

13、r there is carrier on the channel.If a carrier is sensed,it shows that a user is using the channel,and the site doesnt send the frame to avoid conflict.Local Area Network5.2 LAN Technologies rCarrier Sense Multiple Access Protocol(CSMA)mCarrier Sense Multiple Access(CSMA)strategy is also called”List

14、en first,Speak last”.mThe station listens to the media first to determine whether there are other station that is transferring.mIf media is idle,the station can transmit,otherwise,the station will backoff time and try again later.Local Area Network5.2 LAN Technologies rCSMA needs an algorithm to det

15、ermine how to deal with when the media is busy.There are three common algorithms.mnon-persistent CSMAm1-persistent CSMAmp-persistent CSMALocal Area Network5.2 LAN Technologies r1-persistent CSMAmThis protocol is called the 1-persistent CSMA,because once the site finds that the channel is idle,its pr

16、obability of sending data is 1.mAdvantagesIt reduces the idle time of channel.mDisadvantages:It increases the probability of conflict.Local Area Network5.2 LAN Technologies rnonpersistent CSMAmPrincipleIf the site has data to send,it monitors the channel first.If the site finds that the channel is i

17、dle,it sends.If the channel is busy,the site waits a random time,and then restart the sending process.mAdvantagesIt reduces the probability of conflict.mDisadvantages:It increases the channel idle time and data transmission delay.Channel efficiency is lower and delay may be longer than 1 persistent

18、CSMA low.Local Area Network5.2 LAN Technologies rp-persistent CSMAmIt Applies to sub-slot channel.mPrincipleIf the site has data to send,it monitors the channel first.If the medium becomes idle,the sender transmits a frame with aprobabilityp and chooses to transmit the frame in the next availabletim

19、e slot with probability q=1-p.If the next time slot is still idle,the sender will send the frame with the probability p and postpone the sending to the next time slot with the probability q once again(Repeat this process),until the data to be sent or the slots are occupied by other sitesLocal Area N

20、etwork5.2 LAN Technologies rCarrier Sense Multiple Access Protocol with Collision Detection(CSMA/CD)Local Area Network5.2 LAN Technologies Local Area Network5.2 LAN Technologies rCarrier Sense Multiple Access Protocol with Collision Detection(CSMA/CD)mPrincipleThe site sends data with the CSMA proto

21、col.During the sending,if the site detects a conflict,it will finish sending immediately,and send an pulse signal,so that all sites are aware of the conflict.After sending the pulse signal,the site waits a random time,and then repeats the above-mentioned processes.mSupplementary IEEE802.3 standard a

22、dopts 1-CSMA/CDLocal Area Network5.2 LAN Technologies rWorking statemtransmission cyclemCompetition CyclemIdle cyclerQuestionmFor a site,how long it will take to determine a conflict?Local Area Network5.2 LAN Technologies The worst case,the double time of the cable transmissionLocal Area Network5.2

23、LAN Technologies Local Area Network5.2 LAN Technologies rSupplementary SpecificationmBecause it is carrier sense,so it corresponds to the simulation system.mFor the conflict detection,generally,if the signal swing exceeds the normal time,we consider there is a conflict.mIEEE provides that if the acc

24、ess point of transmitter detects that the signal on cables exceeds the amplitude produced by the transmitter alone,then there is a conflict.Local Area Network5.2 LAN Technologies rBinary Exponential BackoffmThe time after the conflict is divided into time slots with the length of 51.2 microseconds(5

25、12bit 10M/S);mAfter the first conflict,each site waits for 0 or 1 time slot and then start re-transmission.mAfter the second conflict,each site randomly chooses to wait for 0,1,2 or 3 time slots and then start the retransmission.Local Area Network5.2 LAN Technologies rBinary Exponential BackoffmAfte

26、r the i-times conflict,the site randomly chooses a waiting time slots from 0 to 2i-1 before starting retransmission.mAfter 10 conflicts,the site chooses to wait for a fixed number of time slots between 0 to 210-1.mAfter 16 conflicts,the sending fails,and the site reports it to the upper layer.Local

27、Area Network5.2 LAN Technologies rIEEE 802.5 Token RingmIBM put forward the standard IEEE 802.5 token ring network first in the 1980s.Rate:1M/4M/16MTopology:RingMedium:twisted pair,coaxial,fiber opticEncoding:ManchesterLocal Area Network5.2 LAN TechnologiesrIEEE 802.5 Token RingmToken ring network i

28、s that the token and frame are transmitted on the ring consisting of many individual point(loop interface).mToken is a special bit combination patterns.If a site wants to send frames,it should seize the token and move out of the ring.Local Area Network5.2 LAN Technologies rIEEE 802.5 Token RingLocal

29、 Area Network5.2 LAN Technologies rIEEE 802.5 Token RingLocal Area NetworkLocal Area NetworkFrame structure:SDFCSADAFSEDCRCDATACRC1 byteJK0JK000Start-bound6 byteDestination address6 byteSourceaddressdataunrestricted4 byteCRCcheckoutACSDEDAC3 byte tokenInformation frame:Access Control byte1 byte PPPT

30、MRRR PPP：Priority T：flag bit，frame/token M：monitoring bit，Monitoring the frame with errorRRR：Reserve Frame control byte1 byte Frame StateACxxACxxA：Address RecognitionC：Frame copyX:Reserved bit1 byte End-boundJK1JK1IE I：The last frame or notE：Error in frame or notFrame control field FC is used to dis

31、tinguish the data frame and the control frame,and to maintain the ring.FS:Set by the receiving site,indicating that the frame has been read5.2 LAN Technologies r802.5 conflict avoidancemEach site in the ring has two states:listening and sending.The beginning of all sites are in listening state.mToke

32、n Ring technology is based on three bytes of a small token frame with circling loop.Local Area Network5.2 LAN Technologies If there is no information in the ring,3-byte token will rotate all the time.The site wanting to transmit must wait until it detect an idle token.At this moment,it reserves this

33、 token,and sets a reminder in its NIC,that is changing the state to sending state,and then sends a frame.It sets the T-bit in AC bytes of the token from 0 to 1.The first two bytes become to a frame header,and is followed by the data frame.It sends this frame to the ring,and hold the token(Holding ti

34、me 10ms).Data will continue to be passed along the ring,and its destination address is identified by each site in listening state.Local Area Network5.2 LAN Technologies mWhen a frame arrives receiving point,the site:The terminal set the FS-bit in A bytes from 0 to 1,indicating the existence of desti

35、nation site.Detect error CRC,if checksum is correct,the terminal copies the frame.Set the FS-bit in C bytes from 0 to 1,indicating that the destination site has received the data,and it will transmit this frame during the receiving processLocal Area Network5.2 LAN Technologies The frame continues al

36、ong the ring,until it returned to its delivery location.Sending site reclaims the frame it has sent,and checks A-bit and C bit in FS.Destination site doesnt exist or is not in power.A=1，C=0,destination site exists,but it doesnt receive the frame.A=1,C=1,destination site exists and the frame is copie

37、dAt the same time,the frame removes from the ring,the licensee within the time allowed to continue to send the next frame until the end,and then put back again to the token ring.Send state for the listener state changeLocal Area Network5.2 LAN Technologies Local Area Networkr1 Priorities and reserva

38、tionsmEach site has a priority code.When a data frame or token frame is through a site,the site can write the priority code in the access control field(AC)of this frame to obtain the right to send.If both the high and low priority sites have data to be sent,the high-priority site has the right to su

39、bstitute the information in the AC written by the sites with a low priority.For the sites with the same priority,it adopts the principle of first come first serve.Local Area Networkr2 Monitoring sitemSome problems may interrupt the normal communication Token Ring NetworkToken frame is damaged,there

40、is no token in ring,all sites can not send dataBecause the data frame is rashness sound effects,it have not been recoveredLocal Area NetworkmTo deal with these situations,monitoring sites were introducedMonitoring sites set a timer,when the token passing.If the token is not returned within the speci

41、fied time,it is claim that token is damaged.It regenerates a token into the ring.Monitoring sites set up a bit in each data frame of AC field,to prevent permanent cycle of the frame.When a frame passes,monitoring sites check the bit.If the bit isnt set（for example 0）,we set the bit.If it is found th

42、at the bit has been set,it shows that the frame has been circling for a loop,and should be deleted.If a monitoring site fails,another site is designed as the backup monitoring siteLocal Area Networkr3 frame typemData framemToken framemAbnormal termination frameContains only SD,ED,does not contain in

43、formationSend site generation,to terminate their transmissionMonitoring site generation,used to clear the old frame on the line Local Area Networkr4 Implementation of ringrA ringmToken Rings ring contains a series of segment which is 150 ohm,and consisting of Shielded Twisted Pair.The output port of

44、 a site and another site of input ports are linked by each segment to form a one-way communications loop.mFrame in order to reach each site,it is checked at each site and regenerate.Then to the next site,the frame is regenerate at each site.Local Area NetworkrB.SwitchmTo solve the above problems of

45、network nodes，each node is connected with an automatic switch.mRing does not directly through the computer,but by each switch.mIf a site failure or shutdown,NIC sends a signal to switch.Then switches do switch to form a loop to bypass the site,called bypass.mIf a site which is bypassed needs to add

46、ring,NIC also sends a signal to switch.Switches do switch.Local Area NetworkLocal Area NetworkrC.Multistation access unitmIn the actual implementation，automatic switch can be integrated into a hub，this hub is called the Multistation access unit.mUse of MAU，network is ring in logical，the physical top

47、ology can be starLocal Area Network5.3.4 FDDIrFDDI（Fiber Distributed Data Interface）rCharacteristicsmUse multimode fiber as transmission mediummMAC protocol is similar with Token RingmThe rate of 100MmUse 4B5B encoding methodmMaximum distance of 200 kmmUp to 1000 sitesmUsually as the backbone of the

48、 network connection LANLocal Area NetworkrFDDI uses major-minor Bicyclic structure.The major ring for normal data transmission,and the minor ring is redundant backup ring.Local Area NetworkLocal Area NetworkLocal Area Network1 Access mode:Token PassingnIn FDDI,Access is controlled by the time.A site

49、 can send any number of frames in the allotted time interval,and the collateral condition is that real-time frame is the first to send.nTo achieve this mechanism，FDDI distinguish between two different frames，real-time frame S and non-real-time frame A.nThe remaining time which is allocated to It to

50、send a frame.Local Area NetworkLocal Area NetworkLocal Area NetworkrNodemFDDI defines three types of nodes:Dual Attachment Station(DAS),Single Attachment Station(SAS)and Dual Attachment Connector(DAC)Local Area NetworkDAS:DAS with two MIC(Media Interface Connector)simultaneously connected to two rin