Telecommunication
Technologies
Week 9
HDLC
(ISO 33009, ISO 4335)
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
HDLC Syntax
 Stations:

 HDLC (High level
Data Link Control)
 ISO 33009, ISO 4335
Primary, Secondary,
Combined
 Link

Balance or
Unbalanced
 Transfer Modes

NRM, ABM, ARM
 Frames

EIE325: Telecommunication Technologies
I-frames, S-frames, Uframes
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
Frame Structure
 Synchronous transmission
 All transmissions in frames
 Single frame format for all data and
control exchanges
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Frame Structure Diagram
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
Bit Stuffing
 Example with
possible errors
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
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
Control Field Diagram
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
Information Field
 Only in information and some
unnumbered frames
 Must contain integral number of octets
 Variable length
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Frame Check Sequence Field
 FCS
 Error detection
 16 bit CRC
 Optional 32 bit CRC
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
HDLC Operation
 Exchange of information, supervisory
and unnumbered frames
 Three phases



Initialisation
Data transfer
Disconnect
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
HDLC commands
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
E.g.: Setup and disconnect
 SABME: Set ABM, 7bit sequence
numbers
 UA: Unnumbered
ACK
 DISC: Disconnect
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
E.g.: Exchange
 I,n,m: Information
 n & m are sender and
receiver sequence
numbers
 RR: Receive ready
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
E.g.: Busy
 RNR: Receive not
ready
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
E.g.: Reject recovery
 REJ: Reject
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
E.g.: Timeout recovery
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Telecommunication
Technologies
Week 9
Synchronisation
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
Asynchronous (diagram)
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
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
EIE325: Telecommunication Technologies
 Simple
 Cheap
 Overhead of 2 or 3
bits per char (~20%)
 Good for data with
large gaps
(keyboard/terminal)
Maciej Ogorzałek, PolyU, EIE
Asynchronous Errors
 Timing errors

extreme discrepancy between sender and
receiver’s clocks.
 Framing errors

Erroneous start bits
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
Synchronous e.g. HDLC
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
Telecommunication
Technologies
Week 9
Interfacing
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
Interfacing
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Characteristics of Interface
 Mechanical

Connection plugs
 Electrical

Voltage, timing, encoding
 Functional

Data, control, timing, grounding
 Procedural

Sequence of events
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Two examples
 Modem
 ISDN
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
Maciej Ogorzałek, PolyU, EIE
Mechanical Specification
Data Pins
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Control Pins
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Timing Pins
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
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
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE
Electrical Specification
1
+3V
??????
-3V
0
EIE325: Telecommunication Technologies
Maciej Ogorzałek, PolyU, EIE