Path: utzoo!news-server.csri.toronto.edu!rutgers!mcnc!decwrl!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: LAP-M frame formatting
Message-ID: <3853.27de1d8e@hayes.uucp>
Date: 13 Mar 91 12:39:42 GMT
References: <5098@mindlink.UUCP>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 26

In article <5098@mindlink.UUCP>, Mike_Benna@mindlink.UUCP (Mike
Benna) writes: 

> What I'd like to know is how the data stream can be formatted so that
> the header doesn't need to contain the length of the subsequent data but
> that the receiving modem can somehow determine the length of the data
> portion of the frame.

LAPM, like MNP3, LAPB, LAPD, and many other protocols, is based on 
the High-level data link control (HDLC) frame format specified in 
ISO 3309.  This uses a particular sequence of bits (01111110), known 
as a "flag", to delimit frames.  Whenever five 1 bits appear in 
sequence in the frame contents, a 0 bit is inserted for 
transparency, so a flag sequence never appears in the frame 
contents.  Thus, there's no need for the receiver to know in advance 
the length of the information part of the frame; all it does it look 
for the closing flag.  It then knows that the preceeding two or four 
bytes (depending on whether you're using 16- or 32-bit frame check 
sequences) is the FCS, and everything before that and after the 
frame header is the data.

-- 
Toby Nixon, Principal Engineer    | Voice   +1-404-840-9200  Telex 151243420
Hayes Microcomputer Products Inc. | Fax     +1-404-447-0178  CIS   70271,404
P.O. Box 105203                   | UUCP uunet!hayes!tnixon  AT&T    !tnixon
Atlanta, Georgia  30348  USA      | Internet       hayes!tnixon@uunet.uu.net