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UNIT-IV DATA COMMUNICATION TECHNIQUES

UNIT-IV DATA COMMUNICATION TECHNIQUES. Data Link Protocols. Asynchronous Protocols. Synchronous Protocols. Xmodem Ymodem Zmodem BLAST Kermit. Character-oriented. Bit-oriented. Asynchronous: treat each character in a bit stream independently

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UNIT-IV DATA COMMUNICATION TECHNIQUES

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  1. UNIT-IVDATA COMMUNICATION TECHNIQUES

  2. Data Link Protocols Asynchronous Protocols Synchronous Protocols • Xmodem • Ymodem • Zmodem • BLAST • Kermit Character-oriented Bit-oriented Asynchronous: treat each character in a bit stream independently Synchronous: take whole bit stream and chop it into characters of equal size

  3. The Use of the Word Asynchronous • Asynchronous Transmission • Generally refers to the transmission of characters with each character carrying information about timing • Asynchronous Communication • Refers to overall communication between two points • An example in this case would be ATM

  4. Asynchronous Transmission Applied to Characters Stop Bit Start Bit Character Frame Each character is individually timed.

  5. Asynchronous Transmission Applied to Packets Burst of Data Packets of data Packets of data A B Intermittent transmission of packets of data

  6. Asynchronous Transmission/Communication Application • Character by character transmission • Data packet transmission at present

  7. Speed Variations In Asynchronous Transmission • Low and high-speed transmissions are possible • Low speed • Almost all modem based communications fall into this category • High speed • Asynchronous Transfer Mode (ATM) • Internet is a good example where asynchronous communication is used predominantly to carry the information

  8. Asynchronous Protocols • 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 synchronous

  9. Data Link Protocols Asynchronous Protocols Synchronous Protocols • Xmodem • Ymodem • Zmodem • BLAST • Kermit Character-oriented (Byte-oriented) Bit-oriented • BSC

  10. Ymodem  data unit changes to 1024 bytes (Xmodem=128) use CRC16 multiple files accepted Zmodem  combination of X and Ymodem BLAST (Blocked Asynchronous Transmission)  better than Xmodem (full-duplex, sliding window flow conrol) Kermit (Columbia U)  most widely used asyn. Protocol (operation same as Xmodem)

  11. Synchronous 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 efficient

  12. Binary Synchronous Communication (BISYNC)OR (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)

  13. A simple BSC data frame SYN : Alert the receiver for the incoming frame BCC : can be LRC (longitudinal redundancy check) or CRC (cyclic redundancy check) This simple frame is seldom used SYN = Synchronous idle = 0010110 STX = Start of text = 0000010 ETX = End of text = 0000011

  14. A BSC frame with a header • Header Fields: • address (sender/receiver) • #frame identifier (0/1 for stop-and-wait ARQ)

  15. A multiblock frame ITB = Intermediate text block

  16. Probability of error: Frame size increases, error increases  multiple faults occurs  Difficult to detect errors (error cancel each others)  Message is divided in several blocks  Each block has STX, ITB and BCC  Ending with ETX (end of text)  Error detected, whole frame is discarded (needs retransmission)  ACK for entire frame  one frame is entire message

  17. Multiframe transmission ETB = End of transmission Block

  18. “Large Message” is broken down to multiple frame  need ETB (End of transmission Block)  need ETX (End of text)  Half-duplex so ACK 0 and ACK 1 alternately

  19. Control frame • Note: Control Frame is used to send command • * Establish connection • * Maintaining flow & error control • * terminating connection

  20. Control frames

  21. Control frames

  22. Control frames

  23. Data Transparency • BSC is designed for text message • Now, non-text message (graphics,…) • Problem? • BSC control character problem • Data transparency: should be able to send any data

  24. Byte stuffing DLE = data link escape

  25. Byte Stuffing 2 activities: - Defining the transparent text region with DLE - Preceding any DLE character within the transparent region (extra DLE) Problem still exist if text = DLE ?  Insert an addition DLE next to the character (DLE DLE)

  26. Data Link Protocols Asynchronous Protocols Synchronous Protocols • Xmodem • Ymodem • Zmodem • BLAST • Kermit Character-oriented (Byte-oriented) Bit-oriented • BSC

  27. Bit-oriented protocol • Represent more information into shorter frame • Avoid the transparency problems

  28. Bit-oriented Protocols SDLC HDLC LAPs LANs SDLC: Synchronous data link control – IBM HDLC: High-level data link control – ISO LAPs : Link access procedure

  29. HDLC • Support half/full – duplex over point-to-point and multipoint links • HDLC system characterization • Station types • Configurations • Communication modes • Frames

  30. HDLC 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 response

  31. HDLC configurations • The relationship of hardware devices on a link • 3 configurations of all stations (primary/secondary/combined) • Unbalanced • Symmetrical • Balanced

  32. HDLC Configurations: Unbalanced (master/slave)

  33. HDLC Configurations: Symmetrical

  34. HDLC Configurations:Balanced

  35. HDLC communication modes Mode : 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)

  36. HDLC 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, Disconnect

  37. HDLC Frame

  38. HDLC Frame

  39. HDLC Frame: Flag field Flag:  beginning and ending of a frame  Last flag can be the start of the next flag Flag  similar to “Control Character”  problem for transparency !!!  Bit Stuffing

  40. Bit Stuffing • How to differentiate data and flag? • Adding one extra 0 whenever there are five consecutive 1s in the data

  41. HDLC: Bit stuffing

  42. HDLC frame: Address field • Primary station creates a frame  destination address • Secondary station creates a frame  source address • Can be one byte or more

  43. HDLC Frame: Address field

  44. HDLC Frame: Control field

  45. N(R)  can be think as “ACK” if correct  N(R) = next frame seq else  N(R) = number of damaged frame (need reTx) In S-Frame  not transmit data, so do not need N(S)  S-Frame for response (return N(R) ) Code  flow and error control information

  46. HDLC frame: Poll / Final P/F: dual purposes 1) P/F = 0 no meaning (regular data) 2) P/F = 1 means “poll” when send by primary P/F = 1 means “final” when send by secondary

  47. HDLC Frame: Information field

  48. HDLC Frame: FCS field FCS: Frame check sequence

  49. HDLC: S-Frame

  50. HDLC: Use of P/F field

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