Data Link Protocols Asynchronous Protocols XMODEM frame ...

Data Link ProtocolsData Link Protocolsผศ.ดร. อนันต ผลเพิ่มAsst.Prof. Anan Phonphoem, Ph.D.anan@cpe.ku.ac.thhttp://www.cpe.ku.ac.th/~ananComputer Engineering DepartmentKasetsart University, Bangkok, ThailandAsynchronousProtocols• Xmodem• Ymodem• Zmodem• BLAST• KermitCharacter-orientedSynchronousProtocolsBit-orientedAsynchronous ProtocolsXMODEM frame• Long, long…time ago• Not complex and easy to implement• Slow• Required start/stop bit and space• Now mainly used in modem• Replaced by high speed synchronousHalf-duplex, stop-and-wait ARQ protocol3of 534of 53AsynchronousProtocols• Xmodem• Ymodem• Zmodem• BLAST• KermitData Link ProtocolsCharacter-oriented(Byte-oriented)• BSCSynchronousProtocolsBit-orientedSynchronous Protocols• Character-oriented protocol• Based on one byte (8-bit)• Use ASCII for control character• Not efficient seldom used• Bit-oriented protocol• Based on individual bits• One or multiple bits for control• More efficient6of 531

IBM’s Binary SynchronousCommunication (BSC)• Character-oriented protocol• Half-duplex, stop-and-wait ARQ• 2 frame types• Data frame(data transmission)• Control frame(connect/disconnect and flow/error control)A simple BSC data frameSYN = Synchronous idle = 0010110STX = Start of text = 0000010ETX = End of text = 00000117of 538of 53A BSC frame with a headerA multiblock frameHeader Fields:• address (sender/receiver)• #frame identifier (0/1 for stop-and-wait ARQ)ITB = Intermediate text block9of 5310 of 53Multiframe transmissionControl frameETB = End of transmission Block11 of 53Note: Control Frame is used to send command* Establish connection* Maintaining flow & error control* terminating connection12 of 532

Control framesControl frames13 of 5314 of 53Control framesData Transparency• BSC is designed for text message• Now, non-text message (graphics,…)• Problem?• BSC control character problem• Data transparency: should be able tosend any data15 of 5316 of 53Byte stuffingData Link ProtocolsAsynchronousProtocolsSynchronousProtocols• Xmodem• Ymodem• Zmodem• BLAST• KermitCharacter-oriented(Byte-oriented)Bit-orientedDLE = data link escape17 of 53• BSC18 of 533

Bit-oriented protocol• Represent more information into shorterframe• Avoid the transparency problemsBit-orientedProtocolsSDLC HDLC LAPs LANsSDLC: Synchronous data link control – IBMHDLC: High-level data link control – ISOLAPs : Link access procedure19 of 53HDLC• Support half/full – duplex over point-topointand multipoint links• HDLC system characterization• Station types• Configurations• Communication modes• FramesHDLC station types• Primary station• The station that controls the medium by sending“command”• Secondary station• The station that “response” to the primary station• Combined station• The station that can both command and response21 of 5322 of 53HDLC configurationsHDLC Configurations:Unbalanced (master/slave)• The relationship of hardware devices ona link• 3 configurations of all stations(primary/secondary/combined)• Unbalanced• Symmetrical• Balanced23 of 5324 of 534

HDLC Configurations:SymmetricalHDLC Configurations:Balanced25 of 5326 of 53HDLC communication modesMode : describe “Who controls the link”NRM: Normal response mode (master/slave)ARM: Asynchronous response mode(secondary can initiate if idle, all transmissions are made to primary station)ABM: Asynchronous balanced mode (point-to-point equal)27 of 53HDLC frame• 3 frame types• Information frame (I-frame)• Supervisory frame (S-frame)For ACK, Flow/Error controls• Unnumbered frame (U-frame)For Mode setting, Initialize, Disconnect28 of 53HDLC FrameHDLC Frame29 of 5330 of 535

HDLC Frame: Flag fieldBit Stuffing• How to differentiate data and flag?• Adding one extra 0 whenever there are fiveconsecutive 1s in the data31 of 5332 of 53HDLC: Bit stuffingHDLC frame: Address field• Primary station creates a frame destination address• Secondary station creates a frame source address• Can be one byte or more33 of 5334 of 53HDLC Frame: Address fieldHDLC Frame: Control field35 of 5336 of 536

HDLC frame: Poll / FinalHDLC Frame: Information field37 of 5338 of 53HDLC Frame: FCS fieldHDLC: S-FrameFCS: Frame check sequence39 of 5340 of 53HDLC: Use of P/F fieldHDLC: Use of P/F fieldPiggybacking:data + ack41 of 5342 of 537

HDLC: Use of P/F fieldHDLC: Use of P/F field43 of 5344 of 53HDLC: S-FrameAcknowledgementHDLC: S-FramePositive Acknowledgement• RR• Receiver sends “Positive Ack” (no data to send)• N(R) = seq of next frame• RNR• Receiver sends “Positive Ack”• N(R) = seq of next frame• Receiver tells sender that sender cannot send anyframe until ‘RR’ frame is received45 of 5346 of 53HDLC: S-FrameNegative AcknowledgementHDLC: U-Frame control field• Reject (REJ)• Go-back-n ARQ• N(R) = # of damage frame (and follow)• Selective-Reject (SREJ)• N(R) = # of damage frameFor session management and control information47 of 5348 of 538

HDLC: U-Frame control fieldHDLC: Polling example49 of 5350 of 53HDLC: Selecting exampleHDLC: Peer-to-peer example51 of 53SABM: Set asynchronous balanced modeUA: Unnumbered ack52 of 53HDLC: Peer-to-peer example53 of 539