Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!cs.utexas.edu!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: LAP-M frame formatting
Message-ID: <3875.27e73fb4@hayes.uucp>
Date: 20 Mar 91 10:55:48 GMT
References: <5098@mindlink.UUCP> <3853.27de1d8e@hayes.uucp> <1991Mar18.181406.18104@robobar.co.uk>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 43

In article <1991Mar18.181406.18104@robobar.co.uk>,
ronald@robobar.co.uk (Ronald S H Khoo) writes: 

> Toby, the Internet RFC (1171) for their point-to-point protocol describes
> an async addendum to 3309 as PDAD1/1984.  Do you know if this ever
> passed, and if any of your (or anyone else's :-) modems will
> convert sync/async this way, including conversion of the sync
> bit-stuffing <-> the async character stuffing, maybe + data rate conversion
> as well ?  Might be damned useful that, given the expense and non-standardness
> of synchronous serial cards for PCs ...

Yes; the start-stop framing mode for HDLC is defined in ISO 3309 
Addendum 1, which was published last year.  I wrote the original 
draft, by the way; it was adopted with only minor editorial 
modifications.  Hayes does support start-stop HDLC in our V-series 
error control modems, both for point-to-point modem error control (to 
allow error control to be retrofitted to async-only Smartmodem 
1200s), and for "async X.25" (which is now being studied in CCITT 
Study Group VII).  Of course, this requires a compatible device or 
process on both ends of the line -- which may not be what you want.

Hayes has another feature in our modems called AutoSync.  This 
allows the modem to be hook to the async port of a PC, and connected 
to a _synchronous_ host remotely.  The modem does conversion from 
async to sync, by stripping start and stop bits, calculating FCS, 
adding flags, and inserting zero bits for transparency (and the 
opposite on reception).  The DTE rate MUST be higher than the line 
rate to keep up with the async-to-sync conversion process; the modem 
uses buffering and flow control, as you might expect.  Unlike 
start-stop HDLC (which is wholly implemented either in the modem or 
in the PC), AutoSync "splits" the protocol so that the procedural 
aspects are implemented in the PC software, but the low-level 
framing and error detection functions are built into the modem.  
It's great for laptops, most of which don't have sync cards 
available at any price.

	-- Toby

-- 
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