(Data link control (chapter 7
اسلاید 1: William Stallings Data and Computer CommunicationsChapter 7Data Link Control
اسلاید 2: Flow ControlEnsuring the sending entity does not overwhelm the receiving entityPreventing buffer overflowTransmission timeTime taken to emit all bits into mediumPropagation timeTime for a bit to traverse the link
اسلاید 3: Model of Frame Transmission
اسلاید 4: Stop and WaitSource transmits frameDestination receives frame and replies with acknowledgementSource waits for ACK before sending next frameDestination can stop flow by not send ACKWorks well for a few large frames
اسلاید 5: FragmentationLarge block of data may be split into small framesLimited buffer sizeErrors detected sooner (when whole frame received)On error, retransmission of smaller frames is neededPrevents one station occupying medium for long periodsStop and wait becomes inadequate
اسلاید 6: Stop and Wait Link Utilization
اسلاید 7: Sliding Windows Flow ControlAllow multiple frames to be in transitReceiver has buffer W longTransmitter can send up to W frames without ACKEach frame is numberedACK includes number of next frame expectedSequence number bounded by size of field (k)Frames are numbered modulo 2k
اسلاید 8: Sliding Window Diagram
اسلاید 9: Example Sliding Window
اسلاید 10: Sliding Window EnhancementsReceiver can acknowledge frames without permitting further transmission (Receive Not Ready)Must send a normal acknowledge to resumeIf duplex, use piggybackingIf no data to send, use acknowledgement frameIf data but no acknowledgement to send, send last acknowledgement number again, or have ACK valid flag (TCP)
اسلاید 11: Error DetectionAdditional bits added by transmitter for error detection codeParityValue of parity bit is such that character has even (even parity) or odd (odd parity) number of onesEven number of bit errors goes undetected
اسلاید 12: Cyclic Redundancy CheckFor a block of k bits transmitter generates n bit sequenceTransmit k+n bits which is exactly divisible by some numberReceive divides frame by that numberIf no remainder, assume no errorFor math, see Stallings chapter 7
اسلاید 13: Error ControlDetection and correction of errorsLost framesDamaged framesAutomatic repeat requestError detectionPositive acknowledgmentRetransmission after timeoutNegative acknowledgement and retransmission
اسلاید 14: Automatic Repeat Request (ARQ)Stop and waitGo back NSelective reject (selective retransmission)
اسلاید 15: Stop and WaitSource transmits single frameWait for ACKIf received frame damaged, discard itTransmitter has timeoutIf no ACK within timeout, retransmitIf ACK damaged,transmitter will not recognize itTransmitter will retransmitReceive gets two copies of frameUse ACK0 and ACK1
اسلاید 16: Stop and Wait - Diagram
اسلاید 17: Stop and Wait - Pros and ConsSimpleInefficient
اسلاید 18: Go Back N (1)Based on sliding windowIf no error, ACK as usual with next frame expectedUse window to control number of outstanding framesIf error, reply with rejectionDiscard that frame and all future frames until error frame received correctlyTransmitter must go back and retransmit that frame and all subsequent frames
اسلاید 19: Go Back N - Damaged FrameReceiver detects error in frame iReceiver sends rejection-iTransmitter gets rejection-iTransmitter retransmits frame i and all subsequent
اسلاید 20: Go Back N - Lost Frame (1)Frame i lostTransmitter sends i+1Receiver gets frame i+1 out of sequenceReceiver send reject iTransmitter goes back to frame i and retransmits
اسلاید 21: Go Back N - Lost Frame (2)Frame i lost and no additional frame sentReceiver gets nothing and returns neither acknowledgement nor rejectionTransmitter times out and sends acknowledgement frame with P bit set to 1Receiver interprets this as command which it acknowledges with the number of the next frame it expects (frame i )Transmitter then retransmits frame i
اسلاید 22: Go Back N - Damaged AcknowledgementReceiver gets frame i and send acknowledgement (i+1) which is lostAcknowledgements are cumulative, so next acknowledgement (i+n) may arrive before transmitter times out on frame iIf transmitter times out, it sends acknowledgement with P bit set as beforeThis can be repeated a number of times before a reset procedure is initiated
اسلاید 23: Go Back N - Damaged RejectionAs for lost frame (2)
اسلاید 24: Go Back N - Diagram
اسلاید 25: Selective RejectAlso called selective retransmissionOnly rejected frames are retransmittedSubsequent frames are accepted by the receiver and bufferedMinimizes retransmissionReceiver must maintain large enough bufferMore complex login in transmitter
اسلاید 26: Selective Reject - Diagram
اسلاید 27: High Level Data Link ControlHDLCISO 33009, ISO 4335
اسلاید 28: HDLC Station TypesPrimary stationControls operation of linkFrames issued are called commandsMaintains separate logical link to each secondary stationSecondary stationUnder control of primary stationFrames issued called responsesCombined stationMay issue commands and responses
اسلاید 29: HDLC Link ConfigurationsUnbalancedOne primary and one or more secondary stationsSupports full duplex and half duplexBalancedTwo combined stationsSupports full duplex and half duplex
اسلاید 30: HDLC Transfer Modes (1)Normal Response Mode (NRM)Unbalanced configurationPrimary initiates transfer to secondarySecondary may only transmit data in response to command from primaryUsed on multi-drop linesHost computer as primaryTerminals as secondary
اسلاید 31: HDLC Transfer Modes (2)Asynchronous Balanced Mode (ABM)Balanced configurationEither station may initiate transmission without receiving permissionMost widely usedNo polling overhead
اسلاید 32: HDLC Transfer Modes (3)Asynchronous Response Mode (ARM)Unbalanced configurationSecondary may initiate transmission without permission form primaryPrimary responsible for linerarely used
اسلاید 33: Frame StructureSynchronous transmissionAll transmissions in framesSingle frame format for all data and control exchanges
اسلاید 34: Frame Structure Diagram
اسلاید 35: Flag FieldsDelimit frame at both ends01111110May close one frame and open anotherReceiver hunts for flag sequence to synchronizeBit stuffing used to avoid confusion with data containing 011111100 inserted after every sequence of five 1sIf receiver detects five 1s it checks next bitIf 0, it is deletedIf 1 and seventh bit is 0, accept as flagIf sixth and seventh bits 1, sender is indicating abort
اسلاید 36: Bit StuffingExample with possible errors
اسلاید 37: Address FieldIdentifies secondary station that sent or will receive frameUsually 8 bits longMay be extended to multiples of 7 bitsLSB of each octet indicates that it is the last octet (1) or not (0)All ones (11111111) is broadcast
اسلاید 38: Control FieldDifferent for different frame typeInformation - data to be transmitted to user (next layer up)Flow and error control piggybacked on information framesSupervisory - ARQ when piggyback not usedUnnumbered - supplementary link controlFirst one or two bits of control filed identify frame typeRemaining bits explained later
اسلاید 39: Control Field Diagram
اسلاید 40: Poll/Final BitUse depends on contextCommand frameP bit1 to solicit (poll) response from peerResponse frameF bit1 indicates response to soliciting command
اسلاید 41: Information FieldOnly in information and some unnumbered framesMust contain integral number of octetsVariable length
اسلاید 42: Frame Check Sequence FieldFCSError detection16 bit CRCOptional 32 bit CRC
اسلاید 43: HDLC OperationExchange of information, supervisory and unnumbered framesThree phasesInitializationData transferDisconnect
اسلاید 44: Examples of Operation (1)
اسلاید 45: Examples of Operation (2)
اسلاید 46: Other DLC Protocols (LAPB,LAPD)Link Access Procedure, Balanced (LAPB)Part of X.25 (ITU-T)Subset of HDLC - ABMPoint to point link between system and packet switching network nodeLink Access Procedure, D-ChannelISDN (ITU-D)ABMAlways 7-bit sequence numbers (no 3-bit)16 bit address field contains two sub-addressesOne for device and one for user (next layer up)
اسلاید 47: Other DLC Protocols (LLC)Logical Link Control (LLC)IEEE 802Different frame formatLink control split between medium access layer (MAC) and LLC (on top of MAC)No primary and secondary - all stations are peersTwo addresses neededSender and receiverError detection at MAC layer32 bit CRCDestination and source access points (DSAP, SSAP)
اسلاید 48: Other DLC Protocols (Frame Relay) (1)Streamlined capability over high speed packet witched networksUsed in place of X.25Uses Link Access Procedure for Frame-Mode Bearer Services (LAPF)Two protocolsControl - similar to HDLCCore - subset of control
اسلاید 49: Other DLC Protocols (Frame Relay) (2)ABM7-bit sequence numbers16 bit CRC2, 3 or 4 octet address fieldData link connection identifier (DLCI)Identifies logical connectionMore on frame relay later
اسلاید 50: Other DLC Protocols (ATM)Asynchronous Transfer ModeStreamlined capability across high speed networksNot HDLC basedFrame format called “cell”Fixed 53 octet (424 bit)Details later
اسلاید 51: Required ReadingStallings chapter 7Web sites on HDLC, frame relay, Ethernet and ATM
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