Path: utzoo!attcan!uunet!husc6!uwvax!oddjob!mimsy!eneevax!umd5!umbc3!cbw1!brian
From: brian@cbw1.UUCP (Brian Cuthie)
Newsgroups: comp.dcom.modems
Subject: Re: US Robotics / Micom incompatibility
Keywords: US Robotics, Micom
Message-ID: <180@cbw1.UUCP>
Date: 12 May 88 02:41:48 GMT
References: <267@sagpd1.UUCP> <21560@oliveb.olivetti.com>
Reply-To: brian@cbw1.UMD.EDU (Brian Cuthie)
Distribution: na
Organization: CBW, Columbia, MD 21046
Lines: 41

In article <21560@oliveb.olivetti.com> jerry@oliveb.UUCP (Jerry Aguirre) writes:
>
>Eventualy I found an option in the modem labled 'allow overspeed data'
>that basically allowed 1221 BPS instead of limiting it to 1205 BPS.
>This fixed the problem without requiring two stop bits to be set.  I
>only tested this for output (from Micom to modem) as I had no need for
>continuous input in the other direction.

Most high speed modems are actually Synchronous modems with Asynchronous
converters.  This conversion from Async to Sync, and back, is done (usually)
at the bit level.  The reason that there is an overspeed option is that 
Synchronous modems are within .01% of spec'd speed.  Async tolerances,
however, are much more liberal (-2.5% to as high as 3%).   This obviates
a need for more bandwidth on the Sync side of the channel.

To put 3% overspeed Async data on a 0.01% tolerance line, stop bits are
deleted in the Synchronous data stream.  However, in order to have
time to re-insert the stop bits on the receiver side, the clock
for the Asynchronous transmitter of the Sync to Async converter (part of the
receiver in the modem) must run 3% overspeed.  This doesn't always make
the DTE equipment happy.

Note that the amount of overspeed tolerance need not be a full 3%.  Most
modems allow several selections since, in effect, you are choosing the
speed at which you will throw data at the DTE on the receiving end and not
all DTEs can survive too much overspeed.

Also note that modems from different manufacturers may not behave as
expected when selecting options such as overspeed.  It is possible that,
although the modem on the transmitting end is deleting stop bits to allow
for the overspeed data, that the receiving modem does not know that it must
run it's Async converter faster.  Thus it is possible to overrun the
receiving modem at the *synchronous* level.

Hope this hasn't been too confusing...

-brian

-- 
Brian D. Cuthie                                 uunet!umbc3!cbw1!brian
Columbia, MD                                    brian@umbc3.umd.edu