Path: utzoo!news-server.csri.toronto.edu!rutgers!sun-barr!newstop!sun!amdcad!jetsun!pyramid!lstowell
From: lstowell@pyrnova.pyramid.com (Lon Stowell)
Newsgroups: comp.dcom.modems
Subject: Re: LAP-M frame formatting
Message-ID: <147988@pyramid.pyramid.com>
Date: 12 Mar 91 19:06:43 GMT
Sender: daemon@pyramid.pyramid.com
Reply-To: lstowell@pyrnova.pyramid.com (Lon Stowell)
Organization: Pyramid Technology Corp., Mountain View, CA
Lines: 20

In article <5098@mindlink.UUCP> Mike_Benna@mindlink.UUCP (Mike Benna) writes:
>Based on some of the things Toby has said about LAP-M packet assembling
>I'm led to believe that LAP-M packets have a header, a _variable_ length
>data area, and then a frame end marker/crc.
>
>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.
>
>The only practical solution I can think of is to escape certain
>characters when they occur in the data portion of the frame but if this
>is the case then there must be some characters which take more bandwidth
>than others.  What are these characters (if they exist)?

The frames are delimited by Flag characters.  If these are
in the datastream, they are stuffed (by the hardware level) to
prohibit the receivers detecting an end of frame...just like
HDLC or SDLC.  No escaping is needed, as the hardware layer
stuffs and destuffs zero's to maintain flag dectection.