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Telecommunication Technologies

Telecommunication Technologies. Week 9 HDLC (ISO 33009, ISO 4335). HDLC (High level Data Link Control) ISO 33009, ISO 4335. Stations: Primary, Secondary, Combined Link Balance or Unbalanced Transfer Modes NRM, ABM, ARM Frames I-frames, S-frames, U-frames. HDLC Syntax.

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Telecommunication Technologies

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  1. Telecommunication Technologies Week 9 HDLC (ISO 33009, ISO 4335)

  2. EIE325: Telecommunication Technologies

  3. EIE325: Telecommunication Technologies

  4. HDLC (High level Data Link Control) ISO 33009, ISO 4335 Stations: Primary, Secondary, Combined Link Balance or Unbalanced Transfer Modes NRM, ABM, ARM Frames I-frames, S-frames, U-frames HDLC Syntax EIE325: Telecommunication Technologies

  5. HDLC Station Types • Primary station • Controls operation of link • Frames issued are called commands • Maintains separate logical link to each secondary station • Secondary station • Under control of primary station • Frames issued called responses • Combined station • May issue commands and responses EIE325: Telecommunication Technologies

  6. HDLC Link Configurations • Unbalanced • One primary and one or more secondary stations • Supports full duplex and half duplex • Balanced • Two combined stations • Supports full duplex and half duplex EIE325: Telecommunication Technologies

  7. HDLC Transfer Modes • Normal Response Mode (NRM) • Unbalanced configuration • Primary initiates transfer to secondary • Secondary may only transmit data in response to command from primary • Used on multi-drop lines • Host computer as primary • Terminals as secondary EIE325: Telecommunication Technologies

  8. HDLC Transfer Modes • Asynchronous Balanced Mode (ABM) • Balanced configuration • Either station may initiate transmission without receiving permission • Most widely used • No polling overhead EIE325: Telecommunication Technologies

  9. HDLC Transfer Modes • Asynchronous Response Mode (ARM) • Unbalanced configuration • Secondary may initiate transmission without permission form primary • Primary responsible for line • Rarely used EIE325: Telecommunication Technologies

  10. HDLC Frame Types • Three types of frames • I-frames : Information (data) • S-frames : Supervisory (ARQ) • U-frames : Unnumbered (other) • Frame structure the same in each case EIE325: Telecommunication Technologies

  11. Frame Structure • Synchronous transmission • All transmissions in frames • Single frame format for all data and control exchanges EIE325: Telecommunication Technologies

  12. Frame Structure Diagram EIE325: Telecommunication Technologies

  13. Flag Fields • Delimit frame at both ends • 01111110 • May close one frame and open another • Receiver hunts for flag sequence to synchronise • Bit stuffing used to avoid confusion with data containing 01111110 • 0 inserted after every sequence of five 1s • If receiver detects five 1s it checks next bit • If 0, it is deleted • If 1 and seventh bit is 0, accept as flag • If sixth and seventh bits 1, sender is indicating abort EIE325: Telecommunication Technologies

  14. Bit Stuffing • Example with possible errors

  15. Address Field • Identifies secondary station that sent or will receive frame • Usually 8 bits long • May be extended to multiples of 7 bits • LSB of each octet indicates that it is the last octet (1) or not (0) • All ones (11111111) is broadcast EIE325: Telecommunication Technologies

  16. Control Field • Different for different frame type • Information - data to be transmitted to user (next layer up) • Flow and error control piggybacked on information frames • Supervisory - ARQ when piggyback not used • Unnumbered - supplementary link control • First one or two bits of control field identify frame type EIE325: Telecommunication Technologies

  17. Control Field Diagram EIE325: Telecommunication Technologies

  18. Poll/Final Bit • Use depends on context • Command frame • P bit • 1 to solicit (poll) response from peer • Response frame • F bit • 1 indicates response to soliciting command EIE325: Telecommunication Technologies

  19. Information Field • Only in information and some unnumbered frames • Must contain integral number of octets • Variable length EIE325: Telecommunication Technologies

  20. Frame Check Sequence Field • FCS • Error detection • 16 bit CRC • Optional 32 bit CRC EIE325: Telecommunication Technologies

  21. HDLC Operation • Exchange of information, supervisory and unnumbered frames • Three phases • Initialisation • Data transfer • Disconnect EIE325: Telecommunication Technologies

  22. EIE325: Telecommunication Technologies

  23. HDLC commands EIE325: Telecommunication Technologies

  24. E.g.: Setup and disconnect • SABME: Set ABM, 7-bit sequence numbers • UA: Unnumbered ACK • DISC: Disconnect EIE325: Telecommunication Technologies

  25. E.g.: Exchange • I,n,m: Information • n & m are sender and receiver sequence numbers • RR: Receive ready EIE325: Telecommunication Technologies

  26. E.g.: Busy • RNR: Receive not ready EIE325: Telecommunication Technologies

  27. E.g.: Reject recovery • REJ: Reject EIE325: Telecommunication Technologies

  28. E.g.: Timeout recovery EIE325: Telecommunication Technologies

  29. Telecommunication Technologies Week 9 Synchronisation

  30. Asynchronous and Synchronous Transmission • Timing problems require a mechanism to synchronise the transmitter and receiver • data rate • bit duration • inter-frame spacing • Two solutions • Asynchronous (data not contiguous) • Synchronous (data blocks contiguous) EIE325: Telecommunication Technologies

  31. Presumptions • Serial, not Parallel Transmission • One bit per signal element • Sender and receiver’s clocks are different • Errors in timing (sampling) as well as amplitude (quantisation) EIE325: Telecommunication Technologies

  32. Asynchronous • Data transmitted one character (5-8 bits) at a time • Timing only needs maintaining within each character • Resynchronise for each character • Idle (binary 1) between characters • Start bit is a binary 0 EIE325: Telecommunication Technologies

  33. Asynchronous (diagram) EIE325: Telecommunication Technologies

  34. Asynchronous Timing Error • 10kbps transmission … bit duration of 0.1ms. • Over 8 bits a 5% timing error is acceptable • A 6% timing error is not. EIE325: Telecommunication Technologies

  35. In a steady stream, interval between characters is uniform (length of stop element) In idle state, receiver looks for transition 1 to 0 Then samples next seven intervals (char length) Then looks for next 1 to 0 for next char Simple Cheap Overhead of 2 or 3 bits per char (~20%) Good for data with large gaps (keyboard/terminal) Asynchronous - Behavior EIE325: Telecommunication Technologies

  36. Asynchronous Errors • Timing errors • extreme discrepancy between sender and receiver’s clocks. • Framing errors • Erroneous start bits EIE325: Telecommunication Technologies

  37. Synchronous - Bit Level • Block of data transmitted without start or stop bits • Clocks must be synchronised • Can use separate clock line • Good over short distances • Subject to impairments • Embed clock signal in data • Manchester encoding • Carrier frequency (analog) EIE325: Telecommunication Technologies

  38. Synchronous - Block Level • Need to indicate start and end of block • Use preamble and “postamble” • e.g. series of SYN (hex 16) characters • e.g. block of 11111111 patterns ending in 11111110 • More efficient (lower overhead) than asynchronous • E.g. HDLC EIE325: Telecommunication Technologies

  39. Synchronous e.g. HDLC EIE325: Telecommunication Technologies

  40. Echo Cancellation • Transceiver: Transmitter/Receiver • Two parties transmit on the same data path simultaneously • Each is aware of their own transmission and can subtract that from the resultant to receive the other! EIE325: Telecommunication Technologies

  41. Telecommunication Technologies Week 9 Interfacing

  42. Interfacing • Data processing devices (or data terminal equipment, DTE) do not (usually) include data transmission facilities • Need an interface called data circuit terminating equipment (DCE) • e.g. modem, NIC • DCE transmits bits on medium • DCE communicates data and control info with DTE • Done over interchange circuits • Clear interface standards required EIE325: Telecommunication Technologies

  43. Interfacing EIE325: Telecommunication Technologies

  44. Characteristics of Interface • Mechanical • Connection plugs • Electrical • Voltage, timing, encoding • Functional • Data, control, timing, grounding • Procedural • Sequence of events EIE325: Telecommunication Technologies

  45. Two examples • Modem • ISDN EIE325: Telecommunication Technologies

  46. E.g.: V.24/EIA-232-F • ITU-T V.24 specifies functional and procedural • EIA-232-F (USA) (originally RS-232): • Mechanical ISO 2110 • Electrical V.28 • Functional V.24 • Procedural V.24 EIE325: Telecommunication Technologies

  47. Mechanical Specification

  48. Data Pins EIE325: Telecommunication Technologies

  49. Control Pins EIE325: Telecommunication Technologies

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