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

1、Chapter 5 Local Area NetworkThe Medium Access Control SublayerLocal Area NetworkChapter 5 Local Area NetworkTh5.1 LAN ArchitectureIEEE 802 Reference ModelAfter the establishment of LAN Standards Committee (IEEE 802 Committee) in February 1980, it has developed a series of LAN standards known as IEEE

2、 802 Standards.Local Area Network5.1 LAN ArchitectureIEEE 802 R5.1 LAN ArchitectureIEEE802 Reference ModelLocal Area Network5.1 LAN ArchitectureIEEE802 ReChapter-5-Local-Area-Network-高级计算机网络-教学课件(英文版)Chapter-5-Local-Area-Network-高级计算机网络-教学课件(英文版)5.1 LAN ArchitectureMedium Access Control Sublayer (MA

3、C)Data Frames AssemblyData Frames DisassemblyAddress ResolutionError Detection Channel Assignment to solve the channel competition (the main function of MAC)Local Area Network5.1 LAN ArchitectureMedium Acc5.1 LAN ArchitectureLocal Area Network5.1 LAN ArchitectureLocal Area5.1 LAN ArchitectureLocal A

4、rea Network5.1 LAN ArchitectureLocal Area5.1 LAN ArchitectureLocal Area Network5.1 LAN ArchitectureLocal Area5.1 LAN ArchitectureDescription of LLC frameNo delimiters and checksum fieldLLC frame is in the data field of MAC frameIt has source address (Source Service Access Point) , so that it can be

5、applicable to point - point, point - multipoint and broadcast communications.Local Area Network5.1 LAN ArchitectureDescriptio5.1 LAN ArchitectureControl field of LLC and HDLC are the sameThe control field divides the LLC frame into information frame, management control frame, unnumbered frame, respe

6、ctively, to complete different functions.802.2 standard in general is based on the HDLC data link control protocol.Local Area Network5.1 LAN ArchitectureControl fi5.1 LAN ArchitectureThe place of IEEE802 Standard in the network architecture Local Area Network5.1 LAN ArchitectureThe place 5.1 LAN Arc

7、hitectureIEEE 802 Standard defines several types of LAN, including definitions and descriptions of the physical layer and MAC sublayer. It is composed as follows:802.1:Basic description and interface primitives definition802.2:Logical Link Control (LLC) sublayer802.3:LAN with CSMA / CD technologies8

8、02.4:LAN with token bus technology802.5:LAN with token ring technologyLocal Area Network5.1 LAN ArchitectureIEEE 802 S5.1 LAN Architecture802.6: Define specifications of metropolitan area network (MAN) medium access control sublayer and physical layer802.8: Define the optical fiber technology802.10:

9、 Define the security specification of LAN802.11: Define the Wireless LAN technology Local Area Network5.1 LAN Architecture802.6: Def5.2 LAN Technologies Channel AssignmentThere are two methods of channel assignmentstatic AllocationFrequency Division Multiplexing: FDM (Wavelength Division Multiplexin

10、g: 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 site number and traffic.Local Area Network5.2 LAN Technologies Channel A5.2 LAN Technologies Dynamic Al

11、locationKey issues: How to resolve the channel contention , minimize conflicts and introduce MAC layerWhich node sends data?If there is a conflict when it sends data?How to deal with a conflict?Local Area Network5.2 LAN Technologies Dynamic A5.2 LAN Technologies The original Ethernet is that many co

12、mputers are connected to a bus. 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 acceptaccept

13、BOnly D accepts the data sent by B5.2 LAN Technologies The origi5.2 LAN Technologies Ethernet broadcast modeEthernet communicationUnacknowledged ConnectionlessThe reason for this is the good quality of LAN channel because the channel error probability is very small.Local Area Network5.2 LAN Technolo

14、gies Ethernet 5.2 LAN Technologies Ethernet ServicesEthernet services are not reliable delivery that is best efforts to delivery.When the destination station receives data frame with error, it will discard this frame. Error correction is determined by the high level.If the level finds the missing of

15、 some data to retransfer, but the Ethernet doesnt know that this is a retransmission of the frame and send it as a new data frame.Local Area Network5.2 LAN Technologies Ethernet 5.2 LAN Technologies Carrier Sense Multiple Access Protocol with collision detection (CSMA/CD)Carrier Sense Multiple Acces

16、s Protocol: CSMAMultiple AccessMultiple users share one lineCarrier SenseBefore access transmission channel, the site 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 Ne

17、twork5.2 LAN Technologies Carrier S5.2 LAN Technologies Carrier Sense Multiple Access Protocol (CSMA)Carrier Sense Multiple Access (CSMA) strategy is also called ”Listen first , Speak last”.The station listens to the media first to determine whether there are other station that is transferring.If me

18、dia is idle, the station can transmit, otherwise, the station will backoff time and try again later.Local Area Network5.2 LAN Technologies Carrier S5.2 LAN Technologies CSMA needs an algorithm to determine how to deal with when the media is busy. There are three common algorithms.non-persistent CSMA

19、1-persistent CSMAp-persistent CSMALocal Area Network5.2 LAN Technologies CSMA need5.2 LAN Technologies 1-persistent CSMAPrincipleIf the site has data to send, it monitors the channel first.If the site finds that the channel is idle, it sends the data.If the channel busy, the site continues to listen

20、 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 1-persist5.2 LAN Technologies 1-persistent CSMAThis protocol is called the 1-persistent

21、 CSMA, because once the site finds that the channel is idle, its probability of sending data is 1.AdvantagesIt reduces the idle time of channel.Disadvantages:It increases the probability of conflict.Local Area Network5.2 LAN Technologies 1-persist5.2 LAN Technologies nonpersistent CSMAPrincipleIf th

22、e 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.AdvantagesIt reduces the probability of conflict.Disadvantages:It increases the channel idle time and

23、 data transmission delay.Channel efficiency is lower and delay may be longer than 1 persistent CSMA low.Local Area Network5.2 LAN Technologies nonpersis5.2 LAN Technologies p-persistent CSMAIt Applies to sub-slot channel.PrincipleIf the site has data to send, it monitors the channel first.If the med

24、ium 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 the

25、 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 p-persist5.2 LAN Technologies p-persistent CSMAPrincipleIf the site listens to the channel is busy in the beginning , it will wait until the next

26、time slot, and then begins the 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 p-persist5.2 LAN Technologies Carrier Sense Multiple Access Protocol with Collision Detection (CSMA / CD) Local

27、Area Network5.2 LAN Technologies Carrier S5.2 LAN Technologies Local Area Network5.2 LAN Technologies Local Are5.2 LAN Technologies The reasons of the introduction of CSMA / CDWhen two frames are in conflict, the sending of two damaged frames is no sense, and the channel can not be used by other sit

28、es. For a limited channel, this is a great waste. If there is a conflict, every side will immediately stop sending. It can enhance the utilization of the channel, resulting in the CSMA / CDLocal Area Network5.2 LAN Technologies The reaso5.2 LAN Technologies Carrier Sense Multiple Access Protocol wit

29、h Collision Detection (CSMA / CD) PrincipleThe site sends data with the CSMA protocol.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

30、 time, and then repeats the above-mentioned processes.Supplementary IEEE802.3 standard adopts 1-CSMA/CDLocal Area Network5.2 LAN Technologies Carrier S5.2 LAN Technologies Working statetransmission cycleCompetition CycleIdle cycleQuestionFor a site, how long it will take to determine a conflict?Loca

31、l Area Network5.2 LAN Technologies Working s5.2 LAN Technologies The worst case, the double time of the cable transmissionLocal Area Network5.2 LAN Technologies The worst5.2 LAN Technologies Local Area Network5.2 LAN Technologies Local Are5.2 LAN Technologies Supplementary SpecificationBecause it is

32、 carrier sense, so it corresponds to the simulation system.For the conflict detection, generally, if the signal swing exceeds the normal time , we consider there is a conflict.IEEE provides that if the access point of transmitter detects that the signal on cables exceeds the amplitude produced by th

33、e transmitter alone, then there is a conflict.Local Area Network5.2 LAN Technologies Supplemen5.2 LAN Technologies Binary Exponential BackoffThe time after the conflict is divided into time slots with the length of 51.2 microseconds (512bit 10M / S);After the first conflict , each site waits for 0 o

34、r 1 time slot and then start re-transmission.After 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 Binary Ex5.2 LAN Technologies Binary Exponential BackoffAfter the i-times conflict, the site

35、 randomly chooses a waiting time slots from 0 to 2i-1 before starting retransmission.After 10 conflicts, the site chooses to wait for a fixed number of time slots between 0 to 210-1.After 16 conflicts, the sending fails, and the site reports it to the upper layer.Local Area Network5.2 LAN Technologi

36、es Binary Ex5.2 LAN Technologies IEEE 802.5 Token RingIBM 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 Technologies IEEE 802.5.2 LAN TechnologiesIEEE 802.

37、5 Token RingToken ring network is that the token and frame are transmitted on the ring consisting of many individual point (loop interface).Token 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 Techno

38、logiesIEEE 802.55.2 LAN Technologies IEEE 802.5 Token RingLocal Area Network5.2 LAN Technologies IEEE 802.5.2 LAN Technologies IEEE 802.5 Token RingLocal Area Network5.2 LAN Technologies IEEE 802.Local Area NetworkFrame structure:SDFCSADAFSEDCRCDATACRC1 byteJK0JK000Start-bound6 byteDestination addre

39、ss6 byteSourceaddressdataunrestricted4 byteCRCcheckoutACSDEDAC3 byte tokenInformation frame:Access Control byte1 byte PPPTMRRR 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 c

40、opyX: Reserved bit1 byte End-boundJK1JK1IE I： The last frame or notE：Error in frame or notFrame control field FC is used to distinguish the data frame and the control frame, and to maintain the ring.FS: Set by the receiving site, indicating that the frame has been readLocal Area NetworkFrame struct5

41、.2 LAN Technologies 802.5 conflict avoidanceEach site in the ring has two states: listening and sending. The beginning of all sites are in listening state.Token Ring technology is based on three bytes of a small token frame with circling loop.Local Area Network5.2 LAN Technologies 802.5 con5.2 LAN T

42、echnologies 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 token, and sets a reminder in its NIC, that is changing the state to sending state, and then sends a frame.

43、 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 10ms).Data will continue to be passed along the ring, and its destination address is identified

44、by each site in listening state.Local Area Network5.2 LAN Technologies If there 5.2 LAN Technologies When a frame arrives receiving point, the site:The terminal set the FS-bit in A bytes from 0 to 1, indicating the existence of destination site.Detect error CRC, if checksum is correct, the terminal

45、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 When a fr5.2 LAN Technologies The frame continues along the ring, until it returned t

46、o 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 copiedAt the same time, th

47、e 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 The frame5.2 LAN Technologies Local Area Network5.2 LAN Tech

48、nologies Local Are1 Priorities and reservationsEach 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 se

49、nt, the high-priority site has the right to substitute 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 Network1 Priorities and reservationsL2 Monitoring siteSome problems may interr

50、upt the normal communication Token Ring NetworkToken frame is damaged, there is no token in ring, all sites can not send dataBecause the data frame is rashness sound effects , it have not been recoveredLocal Area Network2 Monitoring siteLocal Area NeTo deal with these situations, monitoring sites we

51、re introducedMonitoring sites set a timer, when the token passing. If the token is not returned within the specified 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. Wh

52、en a frame passes, monitoring sites check the bit. If the bit isnt set（for example 0）,we set the bit. If it is found that 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 s

53、iteLocal Area NetworkTo deal with these situations,3 frame typeData frameToken frameAbnormal termination frameContains only SD, ED, does not contain informationSend site generation, to terminate their transmissionMonitoring site generation, used to clear the old frame on the line Local Area Network3

54、 frame typeLocal Area Network4 Implementation of ringA ringToken Rings ring contains a series of segment which is 150 ohm, and consisting of Shielded Twisted Pair. The output port of a site and another site of input ports are linked by each segment to form a one-way communications loop.Frame in orde

55、r 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 Network4 Implementation of ringLocal B. SwitchTo solve the above problems of network nodes，each node is connected with an automatic switch.Ring does not directly through the computer, but by each switch.If 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.If a site which is bypassed needs to add ring, NIC also sends a signal to switch. Switches do switch.Local Area Netw