Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!elroy.jpl.nasa.gov!usc!rpi!uupsi!cmcl2!prism.poly.edu!drubin
From: drubin@prism.poly.edu (Dave Rubin)
Newsgroups: comp.dcom.modems
Subject: Re: V.42bis vs. MNP5
Message-ID: <1991May9.174156.27740@prism.poly.edu>
Date: 9 May 91 17:41:56 GMT
References: <30.28280172@zswamp.uucp>
Reply-To: drubin@prism.poly.edu (Dave Rubin)
Organization: Polytechnic University, New York
Lines: 48


>>I am in the process of upgrading our dialup modems, most 
>>likely with some combination of 9600bps and 2400bps modems.
>>Our telephone switch cannot handle async speeds over 19.2kbps.
>
>   I'm not sure that this has any effect on the choice of modems, unless the 
>modems will be used to transmit data generated by the telephone switch itself, 
>which is unusual or possible.
>
>   If the telephone switch is just a PBX and the modem's serial port is being 
>plugged into another computer, then this isn't an issue at all.
>
>   If the switch is digital and has a low sampling rate, you may have a 
>problem with higher speed modems; only experimentation (or the voice of 
>experience) will be able to tell what the maximum reliable speed will be.
Well, I guess I should elaborate.  Our telephone switch (Intecom IBX) is
digital, and supports async data on an RS-232 box attached to the telephone.
The modems are used in a pool, and are allocated by the switch as needed,
only for outside calls.  Data transmission within the PBX is entirely digital.
Modems are not attached directly to the telephone, in fact, this is impossible
since the phones are digital.  Therefore it is up to the PBX to transmit
data between the modems and the async devices attached to a user's computer,
and this is limited to 19.2k.

>In any case, the maximum physical bit rate of V.32 modems is 9600, and of 
>V.32bis modems it's 14400.  V.42bis doesn't increase the speed at which the 
>two modems communicate with each other, and its presence or absence should 
>have no effect on whether a line will support a certain speed.
The speed limitation I refer to is on the DTE side, not the analog side.
Since V.32/V.42bis could potentially achieve 4:1 compression, I would assume
that to make full use of this potential the DTE would need to be set to
38.4k, which is not possible on our PBX.

>>Therefore, the potential 38.4kbps throughput of V.32/V.42bis
>>modems would probably be wasted.

>   Keep in mind that this "potential" is the result of data compression, which 
>does not in any way affect the rate the real data bits are passed through the 
>phone line!
But it does affect the speed that the DTE port needs to run at, correct?

I am pretty new to these new standards, so please let me know if I
am missing something.

--
Dave Rubin
Polytechnic University
drubin@prism.poly.edu