High-level data link control

High-Level Data Link Control ( HDLC ) is an ISO standardized network protocol ISO / IEC 13239: 2002. It is within the ISO / OSI model Layer 2, the data link layer to integrate. The basic structure of HDLC is based on the SDLC protocol from IBM; there is also the proprietary Cisco HDLC.

properties

The HDLC standard consists of:

Control of the transmission section
Detection of transmission errors and sequence errors by block check (CRC-16) and sequence number control
Error correction through block repetition
Flow control with window mechanism
Forwarding of uncorrectable errors and protocol errors to the next higher layer ( switching layer , network layer)

Cisco HDLC:

Like HDLC, but by inserting a proprietary field (2 bytes between the "Control" and "Information" fields) the Layer 3 protocol of the following data is specified and a multi-protocol environment is possible.
Communication between Cisco HDLC and other HDLC devices is not possible.

The HDLC data format is typical for a bit-oriented protocol without the possibility of authentication . It enables point-to-point connections and point-to-multipoint connections.

Block structure

It consists of the opening flag (block delimitation) '01111110' (hexadecimal 0x7E), which is represented with 8 bits. This is followed by the address field with another 8 bits or, alternatively, an extended address field with a multiple of 8 bits, identified by a 0 in bit position 1. In third place is the control field with 8 or 16 bits. Then follows the information field (data field), which has a variable length (multiple of 8 bits). The frame check sequence field (Block test) consists of 16 bits and contains a checksum of the data transmitted ( CCITT CRC-16 ) whose binary digits after the calculation and prior to the transmission at the transmitter with 0xFFFF XOR be ORed. Alternatively, the shift register, which is used for the CRC calculation, can also be filled with ones before the calculation. This modification creates a more robust checksum, which also protects against adding and deleting zeros. After division, the receiver compares with 0001 1101 0000 1111 instead of 0. Finally, the closing flag (block delimitation) follows with another 8 bits ('01111110').

Flag	Address	Control	information	Frame check sequence		Flag
01111110	XXXXXXXX	XXXXXXXX	...	XXXXXXXX	XXXXXXXX	01111110

In order to avoid that within the data area or the checksum, the flag Opening or Closing flag occurs, bit stuffing ( bit stuffing ) or zero insertion applied. This means that after the '1' appears five times, a '0' is inserted within the frame in order to prevent confusion with a flag. On the recipient side, a '0' is simply deleted again after the '1' has occurred five times.

However, there are two special characters that can be used:

Coding	meaning
01111111	frame abortion
111111111111111	channel not active

Block types

There are three different data units that differ in the structure of the control field :

I-frames ( information frames ) - for data transmission

Bit 1	2	3	4th	5	6th	7th	8th
0	Send sequence number			Poll / final bit	Receive sequence number

S-frames ( supervisory frames ) - to control the data flow

Bit 1	2	3	4th	5	6th	7th	8th
1	0	Function bits		Poll / final bit	Receive sequence number

The function bits of the S-frame are coded as follows:

Coding	command	meaning
00	Receive-ready	ready to receive further data; acknowledges successful receipt of the previous packets
01	Receive-Not-Ready	do not send any further data
10	Reject	repeat all data from the specified sequence number
11	Selective reject	repeat the data frame with the specified sequence number

U-frames ( Unnumbered frames ) - to control the connection

Bit 1	2	3	4th	5	6th	7th	8th
1	1	Function bits		Poll / final bit	Function bits

The first 2 and the following 3 function bits are combined to form a command / reply code.

Functional Bits	command	Command (C) / Reply Code (R)	meaning	command	Command (C) / Reply Code (R)	meaning
10,000	SIM	C / -	Set Init.Mode	RIM	- / R	Request Init.Mode
11,000	SARM	C / -	Set async. Response mode	DM	- / R	Disconnected mode
00 010	DISC	C / -	Disconnect	RD	- / R	Request disconnect
10 001	CMDR	- / R	Command Reject	FRMR	- / R	Frame reject
00 000	UI	C / R	Unnumbered information
00 100	UP	C / -	Unnumbered poll
00 110	UA	- / R	Unnumbered Ack.
11 100	SABM	C / -	Set async. Balanced mode
00 001	SNRM	C / -	Set Normal Response Mode
11 101	XID	C / R	Exchange identification

Operating modes

HDLC knows three different operating modes:

Normal Response Mode (NRM): Primary station → secondary station ( half duplex )
Asynchronous Response Mode (ARM): Primary station → secondary station ( full duplex )
Asynchronous Balanced Mode (ABM): Both stations equivalent ( full duplex )

In the NRM, a control station sends to one or more subsequent stations. The following stations only send the control station on request ( polling ). A set final bit in the message of a subsequent station signals the end of its transmission. In the ARM, which is rarely used in practice, the following stations also have the option of sending data to the control station without polling it. The prerequisite for this is that the line is free. Finally, in the ABM only point-to-point connections between exactly two stations are possible. In contrast to the first two cases, the data exchange takes place symmetrically.

HDLC and variants are used with X.25 , GSM , ISDN , Frame Relay and PPP .

Related protocols and variants

Based on HDLC, a transmission can be divided over several physical lines to increase the data throughput. While the individual connections are each secured via an HDLC, the coordination of these is carried out using the multilink procedure (MLP) . A comparable application is channel bundling in ISDN .

Specifications

RFC 4349 High-Level Data Link Control (HDLC)