View source for High-Level Data Link Control

'''High-Level Data Link Control''' (usually abbreviated to '''HDLC''') is a [[protocol]] used to transmit [[packet]]s over [[serial line]]s (both [[synchronous serial line|synchronous]] and [[asynchronous serial line|asynchronous]]). There are a large number of variants (using the fact that the HDLC specification does not fully specify the [[semantics]] of all the fields in the [[header]]), and also descendants. HDLC is based on the earlier [[SDLC]].

The start and end of the packet (named a 'frame' in HDLC terminology) are marked by a specific bit pattern, '1111110', the 'flag'; instances of that pattern in the user data are obscured by [[bit stuffing]] (on synchronous lines) or [[byte stuffing]] (on asynchronous lines). In the former, '111111' is replaced, during transmission, by '1111101'; the inserted '0' is removed on reception. In the latter, an 'escape [[octet]]', 0x7D, is sent, followed by a modified form of the original data octet (with bit 5 inverted) - this allows all reserved patterns (including flags and escapes) to appear in user data.

HDLC uses [[Non Return to Zero Inverted|NRZI]] encoding on synchronous lines, to ensure that enough transitions are present to keep the [[clock]] synchronized.

An HDLC frame contains these fields:

* Flag - 8 bits
* Address - 8 or more bits
* Control - 8 or 16 bits
* Data - Variable
* [[Checksum]] - 16 or 32 bits

The checksum covers the address, control and data fields. The frame is followed either by i) another frame, or ii) the idle pattern - back-to-back flags.

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==External links==

* [http://www.acacia-net.com/wwwcla/protocol/iso_4335.htm#I%20format ISO 4335 - HDLC]

[[Category: Serial Lines]]