Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: Using High Speed Modems
Message-ID: <3960.283d0e47@hayes.uucp>
Date: 24 May 91 12:59:51 GMT
References: <156460@pyramid.pyramid.com>
Distribution: na
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 48

In article <156460@pyramid.pyramid.com>,
lstowell@pyrnova.pyramid.com (Lon Stowell) writes: 

> In article <1991May23.040723.18039@cynic.wimsey.bc.ca> curt@cynic.wimsey.bc.ca (Curt Sampson) writes:
>>
>>Well, V.42 is an error correcting scheme.  It does introduce a certain
>>amount of "compression" when it strips the start and stop bits,
>>generally giving you about a 15% increase in bps rates no matter what
>>you transfer.
>   
>   Any V.22bis or V.32 modem does this....or any other phase
>   modulated modem.  Async data is NOT transferred end to end as
>   a bit stream.  It is converted to a sync format and xmitted.
> 
>   Any modem that needs to be configured for bits/character,
>   parity and/or async DTE baud rate is converting the data...and
>   recreating it at the far end.  

Not exactly true.  If you use a V.22, V.22bis, V.26ter, V.32, or 
V.32bis modem in its "async" mode (using V.14 async-to-sync 
conversion), then, yes, you are actually still transmitting 
synchronously on the phone line.  But the start and stop bits of 
each character are STILL sent with the data -- you still send 10 
bits per character (in most applications).  Thus, a V.32 modem 
operating in "async" mode at 9600 bps sending user data that is 8 
data bits, no parity, and one stop bit can send at most 960 
characters per second.

Using V.42 error control, however, the start and stop bits are 
actually removed from the individual characters, and, instead, the 
characters are packetized with frame headers and trailers.  Instead 
of the 20% overhead of individual start and stop bits, these 
protocol elements typically add only about 1% overhead to the data.  
Thus, a V.32 modem operating in error control mode at 9600bps, 
sending user data that is 8 data bits, no parity, and one stop bit, 
can send approximately 1150 characters per second.

The difference is that in one case the start and stop bits are 
simply aligned with the synchronous clocks of the underlying modem 
to allow them to be transferred on the link, but in the other case 
the start and stop bits are completely removed on the phone line and 
reconstructed on the other end.

-- 
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