Le protocole HDLC - Télécom SudParisgardie/MScCCN/HDLC/HDLC-lecture.pdf · GET/INT/LOR/RIP 8 of 33 October 15, 2007. TRANSPARENCY The aim of transparency is to transmit any bit

HDLC PROTOCOL

Michel GARDIE

INT/LOR/RIP October 15, 2007

The version of this document is temporary. There are still several mistakes. I'm sorry for that.

email: [email protected]

© 1985 FRANCE TELECOM INT/DIT/TI© 1992 INT/DSR/TI© 1994 INT/LOR/AIGRI© 2000 GET/INT/LOR/RIP© 2001 GET/INT/LOR/RIP© 2005 GET/INT/LOR/RIP© 2006 GET/INT/LOR/RIP© 2007 GET/INT/LOR/RIP

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GET/INT/LOR/RIP 2 of 33 October 15, 2007

HDLC

ISO 3309 Data communication – High-level data link control procedures – Frame structure.

ISO 4335 Data communication – High-level data link control procedures – Elements of procedure.

ISO 6256 Data communication – HDLC unbalanced classes of procedures.

ISO 8885 Information processing systems – Data communication -- High-level data link control procedures – General purpose XID frame information field content and format.

ISO/IEC 13239 Information technology – Telecommunications and information exchange between systems – High-level data link control (HDLC) procedures. 2002. (Revises 3309, 4335, 6256, 8885)

HDLC is used in X.25 networks, has been adapted to create the PPP protocol (Internet), or the LAPDm protocol (data link layer on the GSM radio interface).


HIGH-LEVEL DATA LINK CONTROL

HDLC

Overview

● Synchronous

● Bit-oriented

● Full-duplex

● Transparent

● Point-to-point or multipoint links

Features

● One frame structure

● Several elements of procedure


FRAME STRUCTURE

Definition

Data and control information are transmitted within a single structure: the frame

Frame structure

flag: sequence for synchronization and delimitation

address: address of the secondary station

control: controls the protocol behavior

information: user data field

FCS: frame check sequence


Flag01111110

Flag01111110

Address8 bits

FCS16 bits

Information0 ≤ n < ∞ bits

Control8 bits

FRAME FIELDS

Flag

● delimitation

start of frame

end of frame

● synchronization

● inter-frame filling

Address

● Address of the secondary station

a secondary station receives command frames

Control

● identifies the different frames

● numbers the information frames

● acknowledges the information frames

● supervises the data link


FRAME FIELDS

Information field

● optional

● sequence of n bits

● user defined

FCS

● allows detection of transmission errors

● cyclic redundancy check

● polynomial: x16 + x12 + x5 + 1


FRAME CHARACTERISTICS

Transparency

● Allows to transmit any bit pattern

● Principle (between two [non included] flags)

Emission: inserts 1 "zero" after 5 consecutive "ones"

Reception: suppresses every "zero" following 5 consecutive "ones"

Frame inter-filling

● Maintains the synchronization

● Continuous emission of flags

Frame abandon

● Forces the receiver to ignore a frame

● Sequence of M bits: M ≥ 7


TRANSPARENCY

The aim of transparency is to transmit any bit pattern between two flags.

HDLC uses a special bit pattern (01111110) as a delimiter. This special pattern is called a flag.A receiver enters the reception state (and remains in this state) when it receives any bit pattern different from the delimitation pattern. It quits the reception state when it detects a new flag.However, the delimitation pattern can be found anywhere between two real flags:

● it can be an ASCII character (tilde; "~") whose binary coding is 01111110● it can be the association of two adjacent bytes giving the illusion of a flag.

For instance, let us consider the two following bytes: 10010011 and 11110011. When these byte are put together, we obtain the following binary stream: 1001001111110011 where a "flag" seems to appear.We absolutely must remove these pseudo-flags when sending a frame. On the reception side, we will have to restore the original bit stream.

The principle to do that is simple, and is applied only between two flags.On the sender side, we systematically add one "0" after five consecutive "1"s.On the receiver side, each time we receive five consecutive "1"s, we apply the following algorithm:

1. if the next bit is a "0", this bit is immediately and systematically removed2. if the next bit is a "1", there are two possibilities:

a) the next bit after this "1" is a "0": we are detecting a real flag. This is the end of the frame;

b) the next bit after this "1" is still a "1": we are detecting an abandon sequence. The frame must be discarded.


FRAME CHARACTERISTICS

Invalid frames

● Frame too short: less than 32 bits

● Frame too Long1

Extensions

● Address field (2 or 3 bytes)

● Control field (2 bytes)

1 Example: the final flag has not been detected by the receiver, and its reception buffer is full.


ELEMENTS OF PROCEDURE

The elements of procedure define how to use the basic element of the protocol: the

frame.

The rest of this document presents the following points:

● the different types of frame

information

supervision

unnumbered

● the rules of restart

● the transmission controls

● the "command" and "response" frames


FRAME TYPES

N(S) number of the sent frame

N(R) number of the next expected frame

SS bits defining the supervision functions

UUUUU bits defining the extra functions

P/F P (poll) = request of an explicit response

F (final) = indication of an explicit response


F A C Info FCS F

8 7 6 5 4 3 2 1

N(R) N(S) 0

1

1

0

1

P/F

P/F

P/F

N(R)

U U U U U

S S

FRAME TYPES

Information

● The information field is not empty and its content is defined by the user.

● The frame numbers are modulo 8 2

N(S) number of the sent frame

N(R) number of the next expected frame

Supervision

● No information field

● Number transmitted by the frame: N(R)

● This type of frame is used to:

acknowledge information frames

reject information frames

specify the state of the receiving station

2 It is possible to use a modulus of 128. This is negotiated when establishing the connection.


FRAME TYPES

Unnumbered

● No information field

● No frame number

● Basic control of the data link

initialization (connection)

connection reject

disconnection

indication of protocol errors


SUPERVISION FUNCTIONS

S4, S3 = 0,0 RR Receiver Ready

S4, S3 = 1,0 REJ Reject

S4, S3 = 0,1 RNR Receiver Not Ready

S4, S3 = 1,1 SREJ Selective Reject


F A C Info FCS F

8 7 6 5 4 3 2 1

N(R) 10P/F S S

SUPERVISION FRAMES

All the frames below acknowledge information frames whose number is less or equal

to N(R) – 1 (modulo n).

RR RECEIVER READY

● The receiver is ready to process new information frames.

● The previous received information frames have been correctly processed and their content has been delivered to the user.

REJ REJECT

● The receiver requests the retransmission of all information frames starting from the number N(R).


SUPERVISION FRAMES

RNR RECEIVER NOT READY

● The receiver temporarily cannot receive extra information frames.

● The previous frames (up to N(R) - 1) were correctly received but their content could not be delivered to the user.

SREJ SELECTIVE REJECT

● The receiver requests the retransmission of the information frame whose number is N(R).

● The next received frames (after N(R)) have been correctly received, and have been stored; the content of these frames has not been delivered to the user.

● The use of this supervision frame is optional.


EXTRA FUNCTIONS

FORMAT OF THE CONTROL FIELD OF THE UNNUMBERED FRAMES

The table below only gives the most common frames.

Table 1: Bits of the control field

8 7 6 5 4 3 2 1 Meaning

0 0 0 P/F 1 1 1 1SARM

Set Asynchronous Response Mode

0 0 1 P/F 1 1 1 1SABM

Set Asynchronous Balanced Mode

0 1 0 P/F 0 0 1 1DISC

Disconnect

0 1 1 P/F 0 0 1 1UA

Unnumbered Acknowledgment

0 0 0 P/F 1 1 1 1DM

Disconnected Mode

1 0 0 P/F 0 1 1 1CMDR / FRMR

Command Reject / Frame Reject

The CMDR / FRMR frames contain an information field explaining the reason of the reject.


EXTRA FUNCTIONS

LAPB (LINK ACCESS PROTOCOL BALANCED)

SABM

● Frame nature: command.

● Select the balanced mode (LAPB).

● The stations can both manage the data link.

● The acceptation of the connection is done with a UA frame.

● The rejection of the connection is done with a DM frame.

DISC

● Frame nature: command.

● Generates the logical disconnection of the data link between two stations.

● The receiving station must answer with a UA.


EXTRA FUNCTIONS

UA

● Frame nature: response.

● Accepts a command.

FRMR


● Rejects a command:

after an error;

if the requested command is not implemented.

DM


● Two functions:

Indicates that a station is not logically connected.

Rejects a command


RULES OF RESTART

Some error conditions or abnormal behavior may occur. The rules of restart resolve these problems.

OVERFLOW

● A station, which cannot receive any more information frames, sends a RNR frame with N(R) indicating the first unaccepted frame.

● The station sends a RR frame as soon as it is ready again to receive new information frames.

TRANSMISSION ERROR

● Any frame, whose FCS computation indicates a transmission error, must be discarded.

● No other specific action should be undertaken by the receiver.

WRONG N(S) (REJ VERSION)

● Any information frame whose N(S) does not contain the expected value must be discarded; the next frames must be ignored too.

● The station receiving such a frame must then send a REJ frame with N(R) indicating the expected frame number.


RULES OF RESTART

WRONG N(S) (SREJ VERSION)

● Any information frame whose N(S) does not contain the expected value must be discarded. The eventual next information frames should be stored.

● A station receiving such a frame must then send a SREJ frame with N(R) indicating the expected frame number.

COMMAND REJECT

● After receiving a CMDR / FRMR frame, the data link is supposed to be disconnected.

RESET

● Clears a data link.

● The SABM and UA frames are exchanged without prior disconnection.

● The N(S) and N(R) counters are reset; the pending information frames are discarded.


RULES OF RESTART

RESTART AFTER TIME-OUT

● A station which does not receive any acknowledgment before T1 seconds, must retransmit the first unacknowledged information frame. The P/F bit of this information frame must be set.

● The station can then resume the transmission after the reception of an acknowledgment with the P/F bit set.

BIT P/F

● A station which receives a command frame with the P/F3 bit set, must send a response frame with the P/F4 bit also set.

3 The P/F bit is called P (Poll) in a command frame (e.g. Information frames)4 The P/F bit is called F (Final) in a response frame (e.g. RR, RNR or REJ frames)


TRANSMISSION CONTROL

OVERVIEW

Any transmission occurs between a PRIMARY and a SECONDARY.

The primary station controls the data link.

The secondary station is tied to the primary station.

STATION

A station may be composed of:

● a primary

● a secondary

● a primary and a secondary


ADDRESS FIELD

The address field is used to make a distinction between the command and the

response frames.

CODING OF THE ADDRESS FIELD IN THE X.25 PROTOCOL

There are only two values with the following coding:

AddressBit number

1 2 3 4 5 6 7 8hexadecimal

A 0 0 0 0 0 0 1 1 03

B 0 0 0 0 0 0 0 1 01

The use of these values is explained below:

Sender ►

Frame nature

▼

Network DTE

Command A B

Response B A


FLOW CONTROL

The flow control is used to regulate the information flow between stations.

✔ The mechanism used is a window.

✔ A station can send up to W information frames before receiving any acknowledgment.

✔ The window is closed if the station has sent W frames and did not receive any acknowledgment.

✔ A station cannot send other information frames if its window is closed.

✔ The receiver can acknowledge one, several, or all the information frames previously received.

✔ When a station acknowledges some informations frames, it opens more or less the window of its peer.


FLOW CONTROL

W = 3 the window size is 3.

DLDTreq : primitive of service sent by the user of the level 2 (DTE side): DLDTreq = Data Link Data request.

DLDTind : primitive of service sent by the user of the level 2 (network side): DLDTind = Data Link Data indication.

I,0,2 : information frame, N(R)=0, N(S)=2

RR,2 : supervision frame RR, N(R)=2


NETWORKDTE

DLDTreq

DLDTreq

DLDTreq

DLDTreq

DLDTind

DLDTind

DLDTind

DLDTind

I 0,0

I 0,0

I 0,0

I 0,0

RR 2

RR 2

W = 3

LIST OF SOME PARAMETERS USED AT THE FRAME LEVEL

TIMER T1

● On timeout of this timer, the first unacknowledged information frame must be sent again.

● Standard values:

T1 = 100, 200, 400, 800, 1600, 2500 ms

TIMER T2

● Maximum time that a receiver can wait to acknowledge an information frame.

N2

● Maximum number of retransmission of the same information frame.

● If this value is reached, the link is considered to be out-of-order.

● Standard value: 10

N1

● Maximum size of an information frame.


EXCHANGE MODES

DEFINITIONS


SOURCE DESTINATION

INFORMATION

ACKNOWLEDGMENT

PRIMARY SECONDARY

COMMAND

RESPONSE

PRIMARY SECONDARY

SELECTING

ACCEPTANCE

DESTINATIONSOURCE

INFORMATION

ACKNOWLEDGMENT

PRIMARY SECONDARY

POLLING

ACCEPTANCE

DESTINATION SOURCEINFORMATION

ACKNOWLEDGMENT

EXCHANGE MODES

ASYMMETRIC POINT-TO-POINT

This mode is typically intended for multi-point links. Asymmetric point-to-point link

is an adaptation of multi-point links.

The information frames can be either "command" frames, or "response" frames.

This mode is selected with the command frame "SNRM (Set Normal Response

Mode)".


PRIMARY SECONDARY

DEST.SOURCE

Station A Station B

DEST. SOURCE

SEL I

POLL ACK

ACK

II

I I

I

EXCHANGE MODES

SYMMETRIC POINT-TO-POINT

This mode is selected with the command frame "SARM (Set Asynchronous Response

Mode)".

The information frames are always "command" frames.

The supervisions frames (RR, RNR, REJ...) are generally "response" frames.


PRIMARY SECONDARY

DEST.SOURCE

Station A Station B

DEST. SOURCE

I

ACK

ACK

II

I I

I

PRIMARYSECONDARY

EXCHANGE MODE

BALANCED POINT-TO-POINT

In this mode, the concepts of "primary" and "secondary" are replaced by the concept

of "combined station".

The command frames (INFO, SABM, DISC...) always carry the address of the remote

station.

The response frames (RR, RNR, UA, DM....) always carry the address of the local

station.

This mode is selected with the command frame "SABM (Set Asynchronous Balanced

Mode)".


DEST.

Station A Station B

SOURCE

SOURCE

DEST.

Com

bine

d st

atio

n

Com

bine

d st

atio

n

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Documents

Le protocole HDLC - Télécom SudParisgardie/MScCCN/HDLC/HDLC-lecture.pdf · GET/INT/LOR/RIP 8 of 33 October 15, 2007. TRANSPARENCY The aim of transparency is to transmit any bit