现代计算机网络讲义4(英语)+控制访问子层

1、*1The Medium Access ControlSublayerChapter 4*24.1 The Channel Allocation ProblemnSharing Broadcasting MediumnMultiplexingnStatic Channel AllocationnDynamic Channel Allocation*34.1.1 Static Channel AllocationnFDM/TDMnConstant number of usersnHeavy and steady load of traffic nInefficient for burst dat

2、a nQueue theory nMean time delay, T,nPoisson arrival and service timesnChannel capacity C bps,nArrival rate of frames/sec, 1/ bits/framenservice rate is C frames/sec.nT=1/(C- )nTFDM=1/(C/N /N) = NT*44.1.2 Dynamic Channel Allocation in LANs and MANsnStation Model.nThe arrival rate of new frames is ,

3、a constant . nOnce a frame has been generated, the station is blocked and does nothing until the frame has been successfully transmitted. nSingle Channel Assumption.nCollision Assumption.n(a) Continuous Time.(b) Slotted Time.n(a) Carrier Sense.(b) No Carrier Sense.*54.2 Multiple Access ProtocolsnALO

4、HAnCarrier Sense Multiple Access ProtocolsnCollision-Free ProtocolsnLimited-Contention ProtocolsnWavelength Division Multiple Access ProtocolsnWireless LAN Protocols*64.2.1 ALOHA(1)nPure ALOHAnFrames are transmitted at completely arbitrary times.nListen when sending to detect collision nIf collision

5、, delay a random time and retransmit *74.2.1 ALOHA (2)nVulnerable period for the shaded frame.*84.2.1 ALOHA (3)nEfficiency of pure ALOHAnFrame time:the amount of time needed to transmit a standard frame.nS: throughput, the number of new frames generated per frame time, 0S1nG: the number of frames (n

6、ew and old) waited for send per frame time S1/e as k-?. nThe probability that the contention interval has exactly j slots in it is A(1 - A) j 1nThe mean number of slots per contention is given by4.3.5 Ethernet Performance (1)011)1 (jjAAjA*31nEach slot has a duration 2, the mean contention interval w

7、 = 2/A. nAssuming optimal p, so w = 2e = 5.4 .nIf the mean frame takes P sec to transmit, when many stations have frames to send,nChannel efficiency = P/ (P+2/A) = 1/(1+2e/P) = 1/(1+BLe/cF) nFrame length FnNetwork bandwidth BnP = F/B.nCable length LnSpeed of signal propagation c4.3.5 Ethernet Perfor

8、mance (2)*324.3.5 Ethernet Performance (3)nEfficiency of Ethernet at 10 Mbps with 512-bit slot times.*334.3.6 Switched Ethernet*nA simple example of switched Ethernet.*344.3.9 IEEE 802.2: Logical Link ControlnPosition of LLC. nThree service options:nunreliable datagram servicenacknowledged datagram

9、servicenreliable connection-oriented service. nProtocol formats.nThe LLC header contains three fields:a destination access point,a source access point,a control field.*354.4 Wireless LANsnThe 802.11 Protocol StacknThe 802.11 Physical LayernThe 802.11 MAC Sublayer ProtocolnThe 802.11 Frame Structuren

10、Services4.4.1 802.11 Architecture and Protocol Stack (1)nInfrastructure modenAd-hoc modeAccessPointClientTo Network*374.4.1 The 802.11 Architecture and Protocol Stack（2）nPart of the 802.11 protocol stack.*384.4.2 The 802.11 Physical LayernDifferent transmission techniquesnSpeedn802.11nn108Mbpsn2.4Gh

11、z & 5.8GhznMIMO + OFDMn1Km*394.4.3 The 802.11 MAC Sublayer Protocol (1)nSingle MAC to support multiple PHYsnSingle and multiple channel PHYsnPHYs with different “Medium Sense” characteristicsnHidden station problem and exposed station problem.nDCF (Distributed Coordination Function). No central

12、controller (similar to Ethernet). *40 Wireless LAN Protocols (1)nThe CSMA/CA (Multiple Access with Collision Avoidance) protocol 802.11. nPhysical channel sensing: nwhen a station wants to transmit, it senses the channel. nIf it is idle, it just starts transmitting.nIf the channel is busy, the sende

13、r defers until it goes idle and then starts transmitting. nIf a collision occurs, wait a random time, using binary exponential backoff algorithm, and then try again later.*41Wireless LAN Protocols (2)nAll radio transmitters have some fixed rangenHidden station problem (Cs transmitting will interfere

14、 at B)nExposed station problem.*42Wireless LAN Protocols (3)nSender output a short frame to stimulate the receiver and to get channel for sending data frame nA sending RTS to B, and send data frame when received CTSnB responding with a CTS to A, receive data frame from AnC hears the RTS , does not i

15、nterfere with the CTS, free to transmit while the data frame is being sent. nD does not hear the RTS but does hear the CTS, silent until data frame is finished.nE hears RTS and CTS, like D, silent during A sending data frame.*43nDCF uses CSMA/CA (CSMA with Collision Avoidance) protocol. nVirtual cha

16、nnel sensing, based on MACAW n( example) A wants to send to B. nC is a station within range of A.n D is a station within range of B but not within range of A.nNAV (Network Allocation Vector) , not transmitted 4.4.3 The 802.11 MAC Sublayer Protocol (2)*444.4.3 The 802.11 MAC Sublayer Protocol (3)nThe

17、 use of virtual channel sensing using CSMA/CA.*454.4.3 The 802.11 MAC Sublayer Protocol (4)nNoisy channel, fragmentnA fragment burst.*46nSaving Power MechanismnAP broadcasts beacon frames periodic (10 to 100 times per second), contains system parameters nClients enters power-save mode, dozes and wak

18、es up for every beacon nAP buffers the traffic for clients and sends traffic map in beacon framenClients can sent a poll message to the AP to get the message nAP then send the buffered trafficnClients go back to sleep until the next beacon arrived.4.4.3 The 802.11 MAC Sublayer Protocol (5)*474.4.3 T

19、he 802.11 MAC Sublayer Protocol (6)nInterframe spacing in 802.11.*48nSIFS (Short InterFrame Spacing). To allow the parties in a single dialog the chance to go first. nthe receiver send a CTS to respond to an RTSnthe receiver send an ACK for a fragment or full data framenthe sender of a fragment burs

20、t transmit the next fragmentnAIFS (Arbitration InterFrame Spacing). nAIFS1, the AP send high-priority frame.nAIFS4, the AP send low-priority frame.nDIFS (DCF InterFrame Spacing), any station may attempt to acquire the channel to send a new frame. The usual contention rules apply.nEIFS (Extended Inte

21、rFrame Spacing)na station that has just received a bad or unknown frame to report the bad frame. 4.4.3 The 802.11 MAC Sublayer Protocol (7)*494.4.4 The 802.11 Frame Structure(1)*The 802.11 data frame.*50nFrame Control field has 11 subfields. nProtocol version: allows two versions of the protocol to

22、operate at the same time in the same cell. nType (data, control, or management) and Subtype fields (e.g., RTS or CTS). nTo DS and From DS bits indicate the frame is going to or coming from the intercell distribution system (e.g., Ethernet).n MF bit means that more fragments will follow. nRetry bit m

23、arks a retransmission of a frame sent earlier. nPower management bit is used by the base station to put the receiver into sleep state or take it out of sleep state. nMore bit indicates that the sender has additional frames for the receiver. nW bit specifies that the frame body has been encrypted usi

24、ng the WEP (Wired Equivalent Privacy) algorithm. nO bit tells the receiver that a sequence of frames with this bit on must be processed strictly in order.4.4.4 The 802.11 Frame Structure(2)*51nDuration field - tells how long the frame and its acknowledgement will occupy the channel. nhow other stati

25、ons manage the NAV mechanism. nAddress fields all in standard IEEE 802 format: nFirst two are source and destination addressesnThe other two addresses are used for the source and destination base stations for intercell traffic. nSequence field - allows fragments to be numbered. Of the 16 bits availa

26、ble, 12 identify the frame and 4 identify the fragment.nData field contains the payload, up to 2312 bytes, followed by the usual Checksum.nManagement frames have a format similar to that of data frames, except without one of the base station addresses, because management frames are restricted to a s

27、ingle cell. nControl frames are shorter still, having only one or two addresses, no Data field, and no Sequence field. nThe key information here is in the Subtype field, usually RTS, CTS, or ACK.4.4.4 The 802.11 Frame Structure(3)*52nInteresting Research TopicsnPrivacy, authentication.(802.11i)nPowe

28、r saving in ad-hoc mode.n802.20, Mobile Broadband Wireless Access, MBWAnSoftware Defined Radio*53The 802 Wireless Space*544.5 Broadband Wirelessn802.16, approved in April 2002. (WiMax)nAir Interface for Fixed Broadband Wireless Access Systems, OfficiallynWireless MAN (Metropolitan Area Network) nWir

29、eless local loopnComparison of 802.11 and 802.16nThe 802.16 Protocol StacknThe 802.16 Physical LayernThe 802.16 MAC Sublayer ProtocolnThe 802.16 Frame Structure*554.5.1 Comparison of 802.11 and 802.16nWireless, Not mobilenDistances can be several kilometers nEach cell have many more usersnMore spect

30、rum is neededn10-to-66 GHz frequency range, directional beams ,802.16n2-to-11GHz,802.16an802.20, Mobile Broadband Wireless Access (MBWA)n802.22, Wireless Regional Area Networks (WRAN)nBased on Cognitive RadiosnUsing the white space between TV channels.*564.5.2 The 802.16 Protocol StacknThe 802.16 Pr

31、otocol Stack.nTransmission convergence sublayer to hide the different from the data link layer. (ATM service or Packet service)nMAC sublayer common part, allocate and manage channel, to provide QoSnSecurity sublayer, authentication, connection establishment, key exchange and encryption.nService-spec

32、ific convergence sublayer takes place of LLC, to interface to the network layer. *574.5.3 The 802.16 Physical Layer (1)nThe 802.16 transmission environment.nmultiple antennasnthree different modulation schemes,nflexible way to allocate the bandwidth. FDD & TDD*584.5.3 The 802.16 Physical Layer (

33、2)*nTDD(time division duplexing), frames and time slots.*594.5.4 802.16 MAC Sublayer (1)nConnection orientednMAC frames occupy an integral number of physical layer time slots. nEach frame is composed of sub-framesnthe downstream and upstream maps, tell what is in which time slot and which time slots

34、 are free.*604.5.4 802.16 MAC Sublayer (2)nService ClassesnConstant bit rate servicenReal-time, periodic fixed size packets (e.g. T1 or VoIP)nReal-time variable bit rate servicenReal-time, periodic variable sizes packets (e.g MPEG)nNon-real-time variable bit rate servicenVariable sized packets with

35、loose delay requirements (e.g. FTP)nBest efforts service*614.5.5 The 802.16 Frame Structure*n(a) A generic frame. n(b) A bandwidth request frame.Advance On 802.16n802.16e, Mobile Broadband Wireless Access System n802.16j, Multihop relayn802.16m, Advanced Air Interface with data rates of 100 Mbit/s m

36、obile & 1 Gbit/s fixed *634.6 BluetoothnA cable replacement technologyn1Mb/s symbol rate, 10+ meters range, single chip radio + baseband, 2.4GHznLow power, low price, small device n802.15.1nBluetooth ArchitecturenBluetooth ApplicationsnThe Bluetooth Protocol Stack*644.6.1 Bluetooth Architecturen

37、A master node and up to seven active slave nodesnMaster sets the clock and frequency hopping patternnMaster determines the bit rate of slave nodesnTwo piconets can be connected to form a scatternet.nSharing of common master or slave.*654.6.2 Bluetooth ApplicationsnThe Bluetooth profiles.nDescribe th

38、e parts of the technology can be used to fulfill a desired function for a Bluetooth devicenthe basis upon which the real applications are builtn13 applications supported and different protocol stacks provided for each one.GenericOptional*664.6.3 The Bluetooth Protocol Stack(1)nThe 802.15 version of

39、the Bluetooth protocol architecture.*67nPhysical radio (RF), radio transmission and modulation. 4.6.3 The Bluetooth Protocol Stack(2)nBaseband (BB), piconet/channel definition and sharing.nTDMA*68nLink managernEstablishment of logical channelsnPower managementnAuthenticationnQuality of service.nLogi

40、cal link control adaptation protocol(L2CAP), analogous to the LLCnMiddleware layer, a mix of different protocols.n802 LLCnRfcomm, emulates the standard serial portnTelephony protocol, for the three speech-oriented profiles, manages call setup and termination.nService discovery protocol, to locate se

41、rvices within the network.nApplications and profiles.4.6.3 The Bluetooth Protocol Stack(3)*694.6.4 ZIGBEE(1)nIEEE Std 802.15.4nWPAN for low data rate (LR-WPAN)nLow complexitynMulti-month to multi-year battery lifenPeer-to-peer and star topologiesnData rates from 20 kb/s (868 MHz) to 250 kb/s (2450 M

42、Hz)nApplicationsnSensors, interactive toys (joysticks etc.), remote controls*704.6.4 ZIGBEE(2)nTwo types of devicesnFull-function (FFD) and reduced-function (RFD)nFFD can perform as PAN coordinatornControls an optional superframe structurenProvides beacons for synchronizationnProvides guaranteed tim

43、e slots for low-latency applications*714.6.4 ZIGBEE(3)nPHY layer*724.6.4 ZIGBEE(4)nMAC SuperframenCAP, Contention Access Period, nodes compete for channel access using a slotted CSMA-CA protocolnCFP, Contention Free Period, nodes transmit without contending nGTS, Guaranteed Time Slots, assigned and

44、administered by the PAN coordinator4.6.4 ZIGBEE(5)nMAC protocolnWhen an end-device needs to send data to a coordinator it must wait for the beacon to synchronize and later contend for channel access.nWhen a coordinator needs to send data to an end-device , the coordinator stores the message and anno

45、unces pending delivery in the beacon. nEnd-devices usually sleep and wake up periodically to see if they have to receive messages from the coordinator by waiting for the beacon. When they notice that a message is available, they request it explicitly during the CAP. nWhen a coordinator wishes to tal

46、k to another coordinator it must synchronize with its beacon and act as an end-device.*73*744.6.4 ZIGBEE(6)nZigbee Protocol Stack nNetwork Layer ResponsibilitiesnStarting a networknJoining and Leaving NetworknConfigurationnAddressingnSynchronizationnSecuritynRouting*754.7 Data Link Layer SwitchingnT

47、o use bridges to connect multiple LANsnDifferent department set up different LANnLANs may be spread over several buildingnTo split a LAN to accommodate loadnBridges from 802.x to 802.ynLocal InternetworkingnSpanning Tree BridgesnRemote BridgesnVirtual LANsnBridges used to connect LANs and stations*7

48、6nProtocol processing at a bridge*77*784.7.1 Bridges from 802.x to 802.y(1)nOperation of a LAN bridge from 802.11 to 802.3.*794.7.1 Bridges from 802.x to 802.y (2)nThe difficulties when trying to build a bridgenA different frame format.nA different data rate.nA different maximum frame lengthnSecurit

49、y, 802.11 to 802.3 nQuality of service*804.7.2 Local Internetworking(1)nLearning bridgenThe routing procedure for an incoming framenIf destination and source LANs are the same, discard the framenIf the destination and source LANs are different, forward the framenIf the destination LAN is unknown, us

50、e floodingnA hash table lists each possible destination and tell which output line (LAN) it belongs onnBackward learning*814.7.3 Spanning Tree Bridges (1)nTwo parallel learning bridges.nCycle goes on forever for unknown destination frame.*824.7.3 Spanning Tree Bridges (2)nInterconnected LANs. nA spa

51、nning tree covering the LANs. nThere is exactly one path from every LAN to every other LAN.4.7.4 Ethernet Ring Protocols (1)*83nDrawbacks of STP The convergence time is too longnSTP (spanning-tree protocol) is built for the redundant links and loop avoidance network. When topology changes, STP takes

52、 about 30-50 seconds to convergenRSTP (Rapid STP) improves the speed of convergence for bridged network from 30-50 seconds to about 4 seconds, by immediately transitioning root and designated ports to the forwarding state.nERP only spends 50-200 ms converging but it uses a ring topology instead of t

53、ree topologyY(J)S APS Slide 844.7.4 Ethernet Ring Protocols(2)nOpen loop methodsnG.8032 (ERPS)nrSTP (ex 802.1w)nRFER (RAD)nERP (NSN)nRRST (based on RSTP)nREP (Cisco)nRRSTP (Alcatel)nRRPP (Huawei)nEAPS (Extreme, RFC 3619)nEPSR (Allied Telesis)nPSR (Overture)nClosed loop methodsnRPR (IEEE 802.17)nCLEE

54、R and NERT (RAD)CompanyD-LinkCisco3 COMFoundryHPExtremeAllied TelesisProtocolERPS(ITU-T G.8032)REPRRPPMRP/ MRPIIRRPP EAPS(RFC-3619)EPSRNameEthernet Ring Protection SwitchingResilient Ethernet Protocol Rapid Ring Protection ProtocolMetro Ring Protocol Rapid Ring Protection ProtocolEthernet Automatic

55、Protection SwitchingEthernet Protection Switching RingRing TopologiesSingle ring/ Multiple ringsSingle ring/ More complex ringsSingle ring/Two or more ringsSingle ring/Overlapping ringsSingle ring/Two or more ringsSingle ringsSingle ringConvergence Time50-200ms50-250ms 200ms 50ms 200msFaster than RS

56、TP50ms4.7.4 Ethernet Ring Protocols (3)nRapid Ring Protection Protocol (RRPP)nA link layer protocol dedicated to Ethernet ringsnTo prevent broadcast storms when the Ethernet ring is healthynTo rapidly restore the communication paths after a link is disconnected. nCompared with STP, RRPP has the foll

57、owing advantages:nFast topology convergence within 50 ms. nConvergence time independent of Ethernet ring size.nOn intersecting rings, the topology change of an RRPP ring does not cause topology changes in the other rings, and therefore, data transmission is more stable. *854.7.4 Ethernet Ring Protoc

58、ols(4)nThe elements of an RRPP domain elements:nRRPP domain(single RRPP ring or multiple connected RRPP rings)nRRPP rings (Ring 1 -primary ring, Ring 2 - subring)nRRPP control VLANsnRRPP protected VLANsnMaster nodes(S1,S6)nTransit nodes(S2,S3,S4,S5)nEdge nodes(S3)nAssistant-edge nodes(S2)nPrimary co

59、ntrol VLAN(VLAN 3)nSecondary control VLAN(VLAN 4) *864.7.4 Ethernet ring protocols(5)nRing status detectionnMaster node sends Hello messages and each transit node on the ring in turn to check ring completeness.*874.7.4 Ethernet ring protocols(6)nFault detectionnPolling mechanism, If polling fail to reach the secondary port of the master node within a specifi