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

2. EIE325: Telecommunication Technologies

3. EIE325: Telecommunication Technologies

4. HDLC Syntax Stations: Link 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
EIE325: Telecommunication Technologies

5. HDLC Station Types Primary station Secondary station Combined 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
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6. HDLC Link Configurations
Unbalanced
One primary and one or more secondary stations
Supports full duplex and half duplex
Balanced
Two combined stations
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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
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8. HDLC Transfer Modes Asynchronous Balanced Mode (ABM)
Balanced configuration
Either station may initiate transmission without receiving permission
Most widely used
No polling overhead
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9. HDLC Transfer Modes Asynchronous Response Mode (ARM)
Unbalanced configuration
Secondary may initiate transmission without permission form primary
Primary responsible for line
Rarely used
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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
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11. Frame Structure Synchronous transmission All transmissions in frames
Single frame format for all data and control exchanges
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12. Frame Structure Diagram
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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
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
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14. Example with possible errors
Bit Stuffing

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 ( ) is broadcast
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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
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17. Control Field Diagram
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18. Poll/Final Bit Use depends on context Command frame Response frame
P bit
1 to solicit (poll) response from peer
Response frame
F bit
1 indicates response to soliciting command
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19. Information Field Only in information and some unnumbered frames
Must contain integral number of octets
Variable length
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20. Frame Check Sequence Field
FCS
Error detection
16 bit CRC
Optional 32 bit CRC
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21. HDLC Operation
Exchange of information, supervisory and unnumbered frames
Three phases
Initialisation
Data transfer
Disconnect
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22. EIE325: Telecommunication Technologies

23. HDLC commands
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25. E.g.: Setup and disconnect
SABME: Set ABM, 7-bit sequence numbers
UA: Unnumbered ACK
DISC: Disconnect
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26. E.g.: Exchange I,n,m: Information
n & m are sender and receiver sequence numbers
RR: Receive ready
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27. E.g.: Busy RNR: Receive not ready
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28. E.g.: Reject recovery REJ: Reject
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29. E.g.: Timeout recovery
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30. Telecommunication Technologies
Week 9
Synchronisation

31. 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)
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32. 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)
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33. 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
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34. Asynchronous (diagram)
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35. 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.
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36. Asynchronous - Behavior
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)
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37. Asynchronous Errors Timing errors Framing errors
extreme discrepancy between sender and receiver’s clocks.
Framing errors
Erroneous start bits
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38. 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)
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39. 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 patterns ending in
More efficient (lower overhead) than asynchronous
E.g. HDLC
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40. Synchronous e.g. HDLC
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41. 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!
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42. Telecommunication Technologies
Week 9
Interfacing

43. 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
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44. Interfacing
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45. Characteristics of Interface
Mechanical
Connection plugs
Electrical
Voltage, timing, encoding
Functional
Data, control, timing, grounding
Procedural
Sequence of events
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46. Two examples
Modem
ISDN
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47. 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
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48. Mechanical Specification

49. Data Pins
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50. Control Pins
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51. Timing Pins
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52. Electrical Specification
Signaling specification
Values interpreted as data or control, depending on circuit
More than -3v is binary 1, more than +3v is binary 0 (NRZ-L)
Signal rate < 20kbps
Distance <15m
For control, more than-3v is off, +3v is on
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53. Electrical Specification1
+3V
??????
-3V
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