hdlc protocol

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: michel.gardie@int-edu.eu 

© 1985 FRANCE TELECOM INT/DIT/TI 

© 1992 INT/DSR/TI 

© 1994 INT/LOR/AIGRI 

© 2000 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 -- Highlevel 

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 

01111110 

Address 

8 bits 

Control 

8 bits 

Information 

0 ≤ n < ∞ bits 

FCS 

16 bits 

Flag 

01111110 

flag: 

address: 

control: 

information: 

FCS: 

sequence for synchronization and delimitation 

address of the secondary station 

controls the protocol behavior 

user data field 

frame check sequence 


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: x 16 + x 12 + x 5 + 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 removed 

2. 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 Long 1 

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 

F A C Info FCS F 

8 7 6 5 4 3 2 1 

N(R) P/F N(S) 0 

N(R) 

P/F 

S 

S 

0 

1 

U U U P/F U U 

1 

1 

N(S) 

N(R) 

SS 

UUUUU 

P/F 

number of the sent frame 

number of the next expected frame 

bits defining the supervision functions 

bits defining the extra functions 

P (poll) = request of an explicit response 

F (final) = indication of an explicit response 


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 

F A C Info FCS F 

8 7 6 5 4 3 2 1 

N(R) P/F S S 0 1 

S 4, S 3 = 0,0 RR Receiver Ready 

S 4, S 3 = 1,0 REJ Reject 

S 4, S 3 = 0,1 RNR Receiver Not Ready 

S 4, S 3 = 1,1 SREJ Selective Reject 


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 1 

SARM 

Set Asynchronous Response Mode 

0 0 1 P/F 1 1 1 1 

SABM 

Set Asynchronous Balanced Mode 

0 1 0 P/F 0 0 1 1 

DISC 

Disconnect 

0 1 1 P/F 0 0 1 1 

UA 

Unnumbered Acknowledgment 

0 0 0 P/F 1 1 1 1 

DM 

Disconnected Mode 

1 0 0 P/F 0 1 1 1 

CMDR / FRMR 

Command Reject / Frame Reject 

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



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. 



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. 



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. 



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/F 3 bit set, must send a 

response frame with the P/F 4 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: 

Address 

Bit number 

1 2 3 4 5 6 7 8 

hexadecimal 

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 

DTE 

W = 3 

NETWORK 

DLDTreq 

I 0,0 

DLDTreq 

DLDTind 

I 0,0 

DLDTreq 

I 0,0 

DLDTind 

DLDTreq 

RR 2 

DLDTind 

I 0,0 

DLDTind 

RR 2 

W = 3 the window size is 3. 

DLDTreq : 

DLDTind : 

I,0,2 : 

RR,2 : 

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

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

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

supervision frame RR, N(R)=2 


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 

INFORMATION 

SOURCE 

ACKNOWLEDGMENT 

DESTINATION 

COMMAND 

PRIMARY 

RESPONSE 

SECONDARY 

PRIMARY 

SOURCE 

SELECTING 

ACCEPTANCE 

INFORMATION 


SECONDARY 

DESTINATION 

PRIMARY 

DESTINATION 

POLLING 

ACCEPTANCE 

INFORMATION 


SECONDARY 

SOURCE 



ASYMMETRIC POINT-TO-POINT 

Station A 

Station B 

PRIMARY 

SECONDARY 

SOURCE 

I 

SEL 

POLL 

I 

ACK 

I 

DEST. 

ACK 

DEST. 

I 

I 

I 

SOURCE 

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)". 



SYMMETRIC POINT-TO-POINT 

Station A 

Station B 

PRIMARY 

I 

I 

I 

SECONDARY 

SOURCE 

ACK 

DEST. 

SECONDARY 

ACK 

PRIMARY 

DEST. 

I 

I 

I 

SOURCE 

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. 


EXCHANGE MODE 

BALANCED POINT-TO-POINT 

Station A 

Station B 

SOURCE 

DEST. 

Combined station 

Combined station 

DEST. 

SOURCE 

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)". 


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