Blue White Lightning - Télécom SudParisgardie/MScCCN/HDLC/HDLC.pdf · resume (RR) DTE NETWORK DLDTreq DLDTreq DLDTind DLDTind I 1,0 RNR 2 I 0,0 RR 2. October 10, 2009 Telecom &

HDLC

High-level Data Link Control

Michel Gardie

Telecom & Management SudParis 2October 10, 2009

Summary● Overview and features

● The frame structure Information frames Supervision frames

● Restart rules● Flow control● Exchange modes


Overview● Synchronous

● Bit-oriented

● Full duplex

● Transparent

● Point-to-point or multipoint links


Features● One frame structure

● Several elements of procedure


Frame structure● Data and control information are transmitted

within a single structure:

The frame

Flag0111111

0

Flag0111111

0

Address8 bits

FCS16 bits

Information0 ≤ n < ∞ bits

Control8 bits


Frame fields● Flag

Sequence for synchronisation and delimitation Eight-bit pattern: 01111110

● Address Address of the secondary station A secondary station receives “command” frames

and transmits “response” frames


Frame fields (2)● Information

Contains the user data The field has no maximum length defined

● FCS Frame check sequence 16-bit field; contains a CRC (CRC-16) Used to detect transmission errors


Frame fields (3)● Control

Controls the protocol behaviour Identifies the frames Numbers the information frames Acknowledges the information frames Supervises the data link


Frame characteristics● Transparency

Allows to transmit any bit pattern Insertion of one “0” after five “1”s Suppression of every “0” after five “1”s

The flags must not be modified!

● Frame inter-filling Maintains the synchonisation Continuous transmission of flags


Frame characteristics (2)● Frame abandon

Forces the receiver to discard a frame Sequence of M bits, with M ≥ 7

● Invalid frames Shorter than 32 bits Frame “too long”


Frame types● N(S): number of the

frame sent

● N(R): number of the next expected frame

● P/F: poll/final bit

● S, U: supervision and extra functions

8 7 6 5 4 3 12

N(R)

N(R)

P/F N(S)

S S

U U U U U

0

1

1

0

1

P/F

P/F

F A C INFO FCS F

Information

Supervision

Unnumbered


Information frames● The information field is not empty and its

content is defined by the user

● The frame counters are modulo 8 N(S) number of the sent frame N(R) number of the next expected frame

● The frame can acknowledge other information frames


Supervision frames● No information field

● A counter is 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


Supervision frames (2)● All the supervision frames acknowledge

information frames whose number N(S) is less or equal to N(R) – 1 (modulo n).


Supervision functions● S4, S3 = 0,0

RR Receiver Ready

● S4, S3 = 1,0REJ Reject

● S4, S3 = 0,1RNR Receiver Not Ready

● S4, S3 = 1,1SREJ Selective Reject

8 7 6 5 4 3 12

N(R) S S 10P/F

F A C INFO FCS F


RR Receiver Ready● The receiver is ready to process new information

frames.

● The previous received information frames (up to N(R) - 1) have been correctly processed and their content has been delivered to the user.


RR example● The receiver can receive

several information frames before sending an RR frame.

● N(R) is the next expected frame.

NETWORKDTE

DLDTreq

DLDTreq DLDTind

DLDTind

I 1,0

RR 2

I 0,0


REJ Reject● The receiver requests the retransmission of all

information frames starting from the number N(R).

● The frames received up to N(R) - 1 are acknowledged.

● All the frames received from N(R) are discarded.


REJ example● Frame N(S)=1: bad FCS

● Frame N(S)=2: discarded

● REJ 1 requests the retransmission from 1 and acknowledged the frame N(S)=0.

NETWORKDTE

DLDTreq

DLDTreqDLDTind

DLDTind

REJ 1

I 0,0

DLDTreqI 1,0

I 2,0

I 1,0

I 2,0

DLDTind


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.


RNR example● The upper layers

cannot receive data

● The frames are acknowledged (RNR)

● After reception by the upper layers, the normal transfer can resume (RR)

NETWORKDTE

DLDTreq

DLDTreq

DLDTind

DLDTind

I 1,0

RNR 2

I 0,0

NETWORKDTE

DLDTreq

DLDTreq

RR 2


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.


Restart rules● Some error conditions or abnormal behavior may

occur. The rules of restart resolve these problems.

Wrong N(S) Transmission error Overflow etc.


Wrong N(S)● 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.


Wrong N(S) [SREJ]● 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.


Transmission error● Any frame, whose FCS computation indicates a

transmission error, must be discarded.

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


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.


Restart after time-out (1)● The first frame

occurence is lost (bad FCS)

● After a time-out (T1), the frame is sent again

● The frame is normally acknowledged

NETWORKDTE

DLDTreq

DLDTind

I 0,0

I 0,0 P/F

T1

RR 1 P/F


Restart after time-out (2)● The first frame

occurrence is correct

● The RR is lost (bad FCS)

● After T1, the second frame occurrence is discarded (wrong N(S))

NETWORKDTE

DLDTreq

DLDTind

I 0,0

I 0,0 P/F

T1 RR 1

REJ 1 P/F


Bit P/F● A station which receives a command frame with

the P/F bit set, must send a response frame with the P/F bit also set.

● The P/F bit is called P (Poll) in a command frame (e.g. Information frames)

● The P/F bit is called F (Final) in a response frame (e.g. RR, RNR or REJ frames)


Flow control● 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.


Flow control (cont’d)● The receiver can acknowledge one, several, or all

the information frames previously received.

● When a station acknowledges some information frames, it opens more or less the window of the peer station:

RR or REJ acknowledge the information frames and open the window.

RNR acknowledges the information frames but does not open the window.


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.


Flow control example● 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,2,0 → information frame, N(S)=2, N(R)=0

● RR,2 → supervision frame RR, N(R)=2

NETWORKDTE

DLDTreq

DLDTreq

DLDTreq

DLDTreq

DLDTind

DLDTind

DLDTind

DLDTind

W = 3

I 0,0

I 1,0

I 2,0

RR 2

I 3,0

RR 4


Unnumbered frames● No information field

● No frame counter

● Basic control of the data link initialization (connection) connection reject disconnection indication of protocol errors


Unnumbered frames examples

8 7 6 5 4 3 2 1 Meaning

0 0 0 P/F 1 1 1 1 SARMSet Asynchronous Response Mode

0 0 1 P/F 1 1 1 1SABMSet Asynchronous Balanced Mode

0 1 0 P/F 0 0 1 1DISCDisconnect

0 1 1 P/F 0 0 1 1UAUnnumbered Acknowledgment

0 0 0 P/F 1 1 1 1DMDisconnected Mode

1 0 0 P/F 0 1 1 1CMDR / FRMRCommand Reject / Frame Reject


SABM● Frame nature: command.

● Select the balanced mode (LAPB), and establishes the connection

● 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.


SARM● Frame nature: command.

● Select the symmetric mode (obsolete), and establishes the connection.

● 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.


UA● Frame nature: response

● Accepts a command: Accepts a connection Accepts a disconnection


DISC● Frame nature: command.

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

● The receiving station must answer with a UA.


DM● Frame nature: response.

● Two functions: Indicates that a station is not logically connected. Rejects a command

● Note: DM and SARM share the same binary coding. The address field is used to make the distinction between these two frames.


FRMR/CMDR● Frame nature: response.

● Rejects a command: after an error; if the requested command is not implemented.


Some parameters● 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.


Some parameters (cont’d)● N1

Maximum size of 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


Exchange modes definitions

SOURCE DESTINATION

INFORMATION

ACKNOWLEDGMENT

PRIMARY SECONDARY

COMMAND

RESPONSE


Assymetric point-to-point

PRIMARY SECONDARY

SOURCE

Station A Station B

DEST. SOURCE

SEL I

POLL ACK

ACK

II

I I

I

DEST.


Symetric point-to-point

PRIMARY SECONDARY

DEST.SOURCE

Station A Station B

DEST. SOURCE

I

ACK

ACK

II

I I

I

PRIMARYSECONDARY


Balanced point-to-point

DEST.

Station A Station B

SOURCE

SOURCE

DEST.

Com

b ine

d s

tati o

n

Com

b ine

d s

tati o

n


Credits● These slides have been realized with OpenOffice.org 3.1.1

● Creation date: 17 October 2008

● Last modification date: 10 October 2009

● Drawings:

Trash/dustbin: http://pleindetrucs.com/galerie/details.php?image_id=13872

Other drawings made with OOo 3.1.1 draw.


Public Documentation licence Notice

● The contents of this Documentation are subject to the Public Documentation License Version 1.0 (the "License"); you may only use this Documentation if you comply with the terms of this License. A copy of the License is available at http://www.openoffice.org/licenses/PDL.html.

● The Original Documentation is HDLC, High-level Data Link Control. The Initial Writer of the Original Documentation is Michel Gardie Copyright © 2007. All Rights Reserved. (Initial Writer contact(s): [email protected]).

● Contributor(s): none.

● NOTE: The text of this Appendix may differ slightly from the text of the notices in the files of the Original Documentation. You should use the text of this Appendix rather than the text found in the Original Documentation for Your Modifications.

Documents

Blue White Lightning - Télécom SudParisgardie/MScCCN/HDLC/HDLC.pdf · resume (RR) DTE NETWORK DLDTreq DLDTreq DLDTind DLDTind I 1,0 RNR 2 I 0,0 RR 2. October 10, 2009 Telecom &