Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ames!uhccux!munnari.oz.au!murtoa.cs.mu.oz.au!ditmela!yarra!chris
From: chris@yarra.oz.au (Chris Jankowski)
Newsgroups: comp.dcom.modems
Subject: Re: Modem control lines
Keywords: flow control, full duplex RS232 (RTS, CTS), modem/modem protocols
Message-ID: <4916@yarra.oz.au>
Date: 20 Oct 89 03:47:15 GMT
References: <1180@srhqla.SR.COM> <2532@cs.yale.edu> <1989Oct17.170047.18468@utzoo.uucp> <36214@lll-winken.LLNL.GOV>
Reply-To: chris@yarra.oz.au (Chris Jankowski)
Organization: Pyramid Technology Corp Pty Ltd, Melbourne
Lines: 63

In article <36214@lll-winken.LLNL.GOV> casey@lll-crg.llnl.gov (Casey Leedom) writes:
> | From: henry@utzoo.uucp (Henry Spencer)
> | 
> | > From: Owens-Christopher@cs.yale.edu (Christopher Owens)
> | > 
> | > ... Now, presumably, the modem's internal buffer is going to fill up
> | > pretty fast, at which point it is going to want to send some kind of
> | > signal to the other modem to tell it to stop sending data down the
> | > communications line...
> | 
> | It wants to.  However, it can't, in most of the standard modem protocols,
> | because there is no such signal.  If you can't accept data as fast as the
> | other end sends it, you lose some of it.  Simple as that. ... I think
> | Telebit's PEP will do it, but that's a very different story from the rest
> | of the modem world.
>
>   This is a question I've been needing to deal with for a bit now and
> just haven't gotten around to it.  I assumed that Telebit's PEP and
> Microcom's MNP would both have flow control provisions in the protocols
> because they're error controlling protocols, but Henry's letter calls
> that assumption into question.
>
>   Does anyone know if two MNP modems (say running MNP5) have any way of
> throttling each other?  It would be most inconvenient if they don't ...
> I'm running a GraphOn Optimax 200 X terminal at home over a pair of
> Telebit T2500s.  The T2500s can overrun the GraphOn fairly easily.  (I've
> been told there are new PROMs coming out for the Optimax 200 that speed
> up character drawing to about 22KCPS - it's currently somewhere around
> 16KCPS.  I don't know if anything else will be faster.)
>
> Casey

I see the problem the following way. What you need is an out-of-band
logical channel in the same physical stream of bits flowing through the line.

1. If you have any protocol using packets with a header there is nothing 
simpler than specify a special packet to do flow control. Both PEP and MNP5
certainly do it and this is even required for their own use due to the fact
they have to handle loss/corruption of packets. Nearly always all devices 
having buffers of significant size will use some form of flow control.

2. If you do not use any packetised protocol you still can use some form of
out-of-band signalling. There is a very well known and elegant technique
called byte stuffing. It may be used to pass out-of-band signals although
it had been invented for another purpose. I am actually suprised that 
no modem I know of uses it for the purpose. The reason is probably that once
you have buffers and a processor to deal with flow control it is not that
difficult to have a real packetised protocol. It is only more software
in ROM.

3. If you do not do binary transfers you can still use standard XON/XOFF
characters for flow control *between* modems. Some modems do it this way.

Correct me if I am wrong. (I am not a comms expert)

      -m-------   Chris Jankowski - Senior Systems Engineer chris@yarra.oz{.au}
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