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/decoding Characteristics of machinery,electr
3、icity,function and procedures Transmission/reception of bit The functions of LLC and MAC are similar to the Data Link Layer OSI Local Area Network5.1 LAN ArchitecturerLogical Link Control Sublayer(LLC)mLLC sublayer provides assurance mechanisms and flow controlmechanism,while MAC sublayer does not p
4、rovide.mIP protocol does not require the functions of LLC.mLLC sublayer hides differences between different 802MAC and provide a single format and interface for the network layer.mFor the same LLC,several MAC options are available.Local Area Network5.1 LAN ArchitecturerMedium Access Control Sublayer
5、(MAC)mData Frames AssemblymData Frames DisassemblymAddress ResolutionmError Detection mChannel Assignment 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 ArchitectureLocal Area Network5.1
6、LAN ArchitecturerDescription of LLC framemNo delimiters and checksum fieldmLLC frame is in the data field of MAC framemIt has source address(Source Service Access Point),so that it can be applicable to point-point,point-multipoint and broadcast communications.Local Area Network5.1 LAN ArchitecturerC
7、ontrol field of LLC and HDLC are the samemThe control field divides the LLC frame into information frame,management control frame,unnumbered frame,respectively,to complete different functions.m802.2 standard in general is based on the HDLC data link control protocol.Local Area Network5.1 LAN Archite
8、cturerThe place of IEEE802 Standard in the network architecture 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 composed as follows:m802.1:Basic description and interface primiti
9、ves 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 Network5.1 LAN Architecturem802.6:Define specifications of metropolitan area network(MAN)medium access control sublayer and ph
10、ysical layerm802.8:Define the optical fiber technologym802.10:Define the security specification of LANm802.11:Define the Wireless LAN technology Local Area Network5.2 LAN Technologies rChannel AssignmentmThere are two methods of channel assignmentmstatic Allocation Frequency Division Multiplexing:FD
11、M(Wavelength Division Multiplexing:WDM)Time Division Multiplexing:TDM Advantage: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 site number and traffic.Local Area Network5.2 LAN Technologies mDynamic Allo
12、cation Key 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 Technologies rThe original Ethernet is that many computers are connected to a bus
13、.Initially it is thought that this connection method is simple and reliable because there is no active device on the bus.Local Area NetworkB sends data to D C D A EMatching resistor(To absorb the signal on the bus)Matching resistorNot acceptNot acceptNot acceptacceptBOnly D accepts the data sent by
14、B5.2 LAN Technologies 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
15、delivery that is 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
16、the frame and send 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 Access Multiple users share one linemCarrier Sense Before access transmission channel,the si
17、te monitors whether 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 i
18、s also called”Listen 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 a
19、n algorithm to determine 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 CSMAmPrinciple If the site has data to send,it monitors the channel first.If the site finds
20、that the channel is idle,it sends the data.If the channel busy,the site continues to listen until the time it finds that the channel is idle,and then completes the sending.If there is a conflict,the site waits a random time,and then restarts the sending process.Local Area Network5.2 LAN Technologies
21、 r1-persistent CSMAmThis protocol is called the 1-persistent CSMA,because once the site finds that the channel is idle,its probability of sending data is 1.mAdvantages It reduces the idle time of channel.mDisadvantages:It increases the probability of conflict.Local Area Network5.2 LAN Technologies r
22、nonpersistent CSMAmPrinciple If the site has data to send,it monitors the channel first.If the site finds that the channel is idle,it sends.If the channel is busy,the site waits a random time,and then restart the sending process.mAdvantages It reduces the probability of conflict.mDisadvantages:It in
23、creases the channel idle time and data transmission delay.Channel efficiency is lower and delay may be longer than 1 persistent CSMA low.Local Area Network5.2 LAN Technologies rp-persistent CSMAmIt Applies to sub-slot channel.mPrinciple If the site has data to send,it monitors the channel first.If t
24、he medium becomes idle,the sender transmits a frame with aprobabilityp and chooses to transmit the frame in the next availabletime 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
25、 the probability q once again(Repeat this process),until the data to be sent or the slots are occupied by other sitesLocal Area Network5.2 LAN Technologies rp-persistent CSMAmPrinciple If the site listens to the channel is busy in the beginning,it will wait until the next time slot,and then begins t
26、he above-mentioned processes.If there is a conflict,the site waits a random time,and then restarts the sending process.Local Area Network5.2 LAN Technologies rCarrier Sense Multiple Access Protocol with Collision Detection(CSMA/CD)Local Area Network5.2 LAN Technologies Local Area Network5.2 LAN Tech
27、nologies rThe reasons of the introduction of CSMA/CDmWhen two frames are in conflict,the sending of two damaged frames is no sense,and the channel can not be used by other sites.For a limited channel,this is a great waste.If there is a conflict,every side will immediately stop sending.It can enhance
28、 the utilization of the channel,resulting in the CSMA/CDLocal Area Network5.2 LAN Technologies rCarrier Sense Multiple Access Protocol with Collision Detection(CSMA/CD)mPrinciple The site sends data with the CSMA protocol.During the sending,if the site detects a conflict,it will finish sending immed
29、iately,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 adopts 1-CSMA/CDLocal Area Network5.2 LAN Technologies rWorking statemtransmission
30、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 LAN Technologies Local Area Network站点要求发送有载波？发送一帧信息有冲突？发出阻塞信号冲突 16？延时处理放弃发送另作处理一次发
31、送结束图4.3 CSMA/CD发送过程流程图YNYNNY5.2 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 access poi
32、nt 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(512bit 1
33、0M/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 BackoffmAfter the i
34、-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 Area Ne
35、twork5.2 LAN Technologies rIEEE 802.5 Token RingmIBM put forward the standard IEEE 802.5 token ring network first in the 1980s.Rate:1M/4M/16M Topology:Ring Medium:twisted pair,coaxial,fiber optic Encoding:ManchesterLocal Area Network5.2 LAN TechnologiesrIEEE 802.5 Token RingmToken ring network is th
36、at 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 Are
37、a 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 PPPTMRRR
38、 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 disting
39、uish 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.mToken Ri
40、ng 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 tok
41、en,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 time 1
42、0ms).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 destinati
43、on 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 along
44、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 copied At
45、 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 reservatio
46、nsmEach 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 subst
47、itute 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 Network Token frame is damaged,there is
48、 no token in ring,all sites can not send data Because the data frame is rashness sound effects,it have not been recoveredLocal Area NetworkmTo deal with these situations,monitoring sites were introduced Monitoring sites set a timer,when the token passing.If the token is not returned within the speci
49、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
50、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 frame Contains only SD,ED,does not contain i
