High-Level Data Link Control

High - Level Data Link Control (HDLC ) is a normalized by the ISO network protocol ISO / IEC 13239:2002. It is within the ISO / OSI model Layer 2 to integrate the data link layer. HDLC is based in its basic structure on the IBM SDLC protocol, in addition there are the proprietary Cisco HDLC.

Properties

The HDLC standard consists of:

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

Cisco HDLC:

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

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

Block structure

It consists of the opening flag ( block boundary ) '01111110 ' (hexadecimal 0x7E ), which is shown with 8 bits. This is followed by the Address field ( the address field ) with another 8 bits or alternatively an extended address field with a multiple of 8 bits, characterized by a 0 1 at the bit position in third place comes the Control field ( control field ) with 8 or 16 bits. Then followed by the information field (data field ), the variable multiple of 8-bit length. The frame check sequence field (Block test) consists of 16 bits, and contains a check sum of the transferred data (CCITT CRC -16) the binary digits are XORed according to the calculation and in front of the transmission at the transmitter with 0xFFFF. Alternatively, the shift register, which is used for the CRC calculation, also be filled prior to the calculation with ones. This modification results in a more robust checksum that protects against appending or deleting zeros. The receiver compares after dividing with 0001 1101 0000 1111, instead of 0 Finally follows the closing flag (block boundary ) with another 8 bits ( '01111110 ').

To avoid that flag the opening within the data range or the checksum or closing flag occurs, bit stuffing is ( bit stuffing ) or zero insertion applied. This means that within the scope of the appearance after five '1 ' '0' is inserted in order to prevent any confusion with a flag. On the receiver side a '0 ' after five occurrence of '1' is simply deleted.

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

Block types

There are three data units which are different in the structure of the control field ( control field):

I- frame ( information frames) - for data transmission

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

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

U- Frame (Unnumbered frames) - for controlling the connection

The first 2 and the following 3 function bits are assembled into one command / reply code.

Bits

Reply code (R )

Reply code (R )

Modes

HDLC has three different operating modes:

• Normal Response Mode (NRM ) Primary station → secondary station (half duplex)

• Primary station → secondary station (full duplex)

• Both stations equivalent (full duplex)

In NRM sends a control station to one or more slave stations. This only send on request ( polling) the control station data to these. In ARM, which is rarely used in practice, the consequence stations also have the option to send even without polling the control station data to these. The only prerequisite is that the line is free. In ABM finally only point-to -point connections between exactly two stations are possible. The data exchange is here, in contrast to the first two cases, symmetrically.

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

Related protocols and variants

Sitting up to HDLC, a transmission for increasing the data throughput on a plurality of physical lines are divided. While the individual compounds are in each case secured via a HDLC, the coordination of the multi-link procedure (MLP) is performed. A similar application is the channel bonding in the ISDN.