Browse Prior Art Database

Recursive Extension of HDLC Modulus

IP.com Disclosure Number: IPCOM000052541D
Original Publication Date: 1981-Jun-01
Included in the Prior Art Database: 2005-Feb-11
Document File: 2 page(s) / 32K

Publishing Venue

IBM

Related People

Ngo-Mai, C: AUTHOR

Abstract

This article describes improvements to data transmission systems involv long propagation delays, and more particularly a method for recursive extension of the modulus of the sequence numbers in those systems which use the format and protocols of the ISO High Level Data Link Control (HDLC).

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Recursive Extension of HDLC Modulus

This article describes improvements to data transmission systems involv long propagation delays, and more particularly a method for recursive extension of the modulus of the sequence numbers in those systems which use the format and protocols of the ISO High Level Data Link Control (HDLC).

Most modern data communication systems use the HDLC protocols with the following format: F, A, C, (Information field), FCS, F

Information frames are transmitted using the control field (c) encoded as shown in Fig. 1, where N(S) = Send sequence number N(R) = Expected receive sequence number P/F = Poll/Final bit.

Each information frame is sequentially numbered, and "modulus" designates the maximum limit of these sequence numbers. The maximum modulus for the numbering of information frames is limited at present to 128 (extended format).

A simple method to permit recursive, unlimited extension of the modulus is proposed here.

In the 2-byte control field, when all the seven bits of N(S) are set to "1", this is the indication that the following byte is an extension to the control field, which in this case is comprised of 3 bytes, as illustrated in Fig. 2. Half of the extension byte, i.e., four bits, is used as an extension to the N(S) field, and the other half is used as an extension to the N(R) field. In this case, the modulus is extended from 2/7/ to (2/7/-1)2/4/.

When the four extension bits of this third byte are again all set to "1", this is a...