eie325: telecommunication technologiesmaciej ogorza ł ek, polyu, eie telecommunication technologies...

EIE325: Telecommunication Technologies Maciej Ogorzałek, PolyU, EIE

Telecommunication Technologies

Week 9

HDLC(ISO 33009, ISO 4335)


HDLC Syntax

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 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


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


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


HDLC Transfer Modes

Asynchronous Balanced Mode (ABM) Balanced configuration Either station may initiate transmission

without receiving permission Most widely used No polling overhead


HDLC Transfer Modes

Asynchronous Response Mode (ARM) Unbalanced configuration Secondary may initiate transmission

without permission form primary Primary responsible for line Rarely used


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


Frame Structure

Synchronous transmission All transmissions in frames Single frame format for all data and

control exchanges


Frame Structure Diagram


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

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


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


Control Field Diagram


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


Information Field

Only in information and some unnumbered frames

Must contain integral number of octets Variable length


Frame Check Sequence Field

FCS Error detection 16 bit CRC Optional 32 bit CRC


HDLC Operation

Exchange of information, supervisory and unnumbered frames

Three phases Initialisation Data transfer Disconnect


HDLC commands


E.g.: Setup and disconnect

SABME: Set ABM, 7-bit sequence numbers

UA: Unnumbered ACK

DISC: Disconnect


E.g.: Exchange

I,n,m: Information n & m are sender

and receiver sequence numbers

RR: Receive ready


E.g.: Busy

RNR: Receive not ready


E.g.: Reject recovery

REJ: Reject


E.g.: Timeout recovery



Week 9

Synchronisation


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)


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)


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


Asynchronous (diagram)


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.


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)


Asynchronous Errors

Timing errors extreme discrepancy between sender and

receiver’s clocks. Framing errors

Erroneous start bits


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)


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


Synchronous e.g. HDLC


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!



Week 9

Interfacing


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


Interfacing


Characteristics of Interface

Mechanical Connection plugs

Electrical Voltage, timing, encoding

Functional Data, control, timing, grounding

Procedural Sequence of events


Two examples

Modem ISDN


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

Mechanical Specification


Data Pins


Control Pins


Timing Pins


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


Electrical Specification

0

1

??????

+3V

-3V

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