Chapter 10Data Link Control Protocols

AgendaDefinitionFunctionsAttributesConceptsTypesConversion

DefinitionProtocols: set of rules or guidelines for communication between DTEsData link protocol: rules for operating the circuit and sending message over it

FunctionsCommunication startupCharacter identification and framingMessage identificationLine controlError controlTermination

Data Link Protocols AttributesTransparency (DTE any bit pattern as data)Code independenceEfficiency (control codes)

Protocol ConceptsLine AccessContentionPolling Token passingMessage format

ContentionCarrier sense multiple access with collision detection (CSMA/CD) by XEROXFor:Point-to-point circuitsTraffic light for multipoint circuitsFast speed of circuitOffice application & LANNot forManufacturing due to inconsistent response time

PollingCharacteristicsOne master and slaversSpecial polling characterTypesRoll call polling (polling list)Fast select polling (several terminals)Hub polling (station to station)Token passing (token)Token (particular sequence of bits, Free or busy)

Message FormatStart of header (SOH)Header (Address, sequence, date and time)Start of text (STX)TextEnd of text (ETX) or text lengthTrailerBlock check character (BCC)Synchronization character (SYN)

Asynchronous Data Link Control Protocols - CharacteristicsStart/stop bitsParity checkingCharacter orientedLess expensive and less complicated equipmentFor personal computers

Asynchronous Data Link Control Protocols - IXMODEM (simple, less reliable error checking)XMODEM-CRC (more reliable)XMODEM-1K (more efficient)YMODEM (reliable, multiple files transfer)YMODEM-G (fast)ZMODEM (fast, good failure recovery)X.PC (pack switching network, multiple sessions on one circuit)

Asynchronous Data Link Control Protocols - IIKERMIT (reliable, fast file transfer, PC & mainframe)Serial Line Internet Protocol (SLIP)Full-duplexIP over asynchronous dial-up or leased linesNo error correctionPoint-to-point Protocol (PPP)PC to a TCP/IP networkFull-duplex for synchronous and asynchronous transmissionAuthentication, compression, error correction, & packet sequencing

Synchronous Data Link Protocols - ClassificationCharacter-oriented protocolsSpecial character for start and end of messageBinary Synchronous Communication Protocol (BSC or BISYNC)Byte-count-oriented protocolsSpecial character for start of the header, count field, message, block check character (BCC)DECs Digital Data Communication Message Protocol (DDCMP)Bit-oriented protocolsUse flag character for start and end of messageIBMs Synchronous Data Link Control (SDLC)ISOs High-Level Data Link control (HDLC)

Binary Synchronous CommunicationsBy IBMFor 6-bit transcode (SBT), ASCII, EBCDICSYN at start and middle of transmissionPoint to point and multipoint (polling)ARQ approach for error checking (ACK1, ACK0, NAK)Pros:Transparency and non-transparency modesEfficient, understandable, and widely usedPoint-to-point & multipoint operationsCons:Code dependentHalf-duplex protocolCumbersome for transparency mode

Digital Data Communications Message Protocol (DDCMP)UsageDigital Network Architecture (DNA)High-bandwidth and high-latency (satellite) linksData message and control messageError checkingUse block check characterUnique and increasing sequence number for sending messageACK indicates the last message receivedPro:Efficient at transparency modeVariable length of dataOnly one control character: SOHEasy implement for full-duplex and message sequence

High-level Data Link Control (HDLC) - IOperating modesNormal response mode (NRM) for a primary node and one or more secondary modes on a circuit (polling)Asynchronous balanced mode (ABM) for nodes with peers (most used for efficiency with no polling in full duplex)Asynchronous response mode (ARM) for primary and secondary nodes with same transmission right (rarely used)FramesSupervisory or S format: control acknowledgeUnnumbered or U format: (operation mode, start, termination)Information or I format: (data)

High-level Data Link Control (HDLC) - IFlag01111110Bit stuffing 0 after five 1s by hardwareFor synchronizationOther fieldsAddress fieldControl field: type of frame, sequence number for information framesInformation field: multiple of 8 bitsFrame check sequence (FCS) field: error checking Frame flowInitialization phase, data transfer phase, and disconnect phase

Other Data Link Control ProtocolsLink access procedure, balanced (LAPB)Full-duplex, point-to-point modeFor X.25 DTE and packet switching networkLink access procedure, D-channel (LAPD)ISDN networkLink access procedure for frame mode bearer service (LAPF)High-speed packet switching network on low error rate digital circuitsAsynchronous transfer modeData transfer on high s-speed, digital, error free network

Protocol ConversionMethodsDedicated hardware protocol convertersAdd-in circuit board protocol convertersSoftware protocol converters Gateway FunctionsCode translationControl characters, format, and error checking

Points to RememberDefinitionFunctionsAttributesConceptsTypesConversion

DiscussionSelect proper protocol for the network of a bank state you reasons