Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!spool2.mu.edu!uunet!hayes!tnixon
From: tnixon@hayes.uucp
Newsgroups: comp.dcom.modems
Subject: Re: V32bis
Message-ID: <3713.27832d53@hayes.uucp>
Date: 3 Jan 91 13:10:43 GMT
References: <19700002@inmet>
Organization: Hayes Microcomputer Products, Norcross, GA
Lines: 64

In article <19700002@inmet>, gregh@inmet.inmet.com writes: 

>   A friend of mine wanted to know how V32bis achieves 14400 bps. 
>   Not knowing myself, I thought I would turn to a source of higher 
>   knowledge.
> 	
>   What in short does V32bis do differently from V32 to achieve the faster
>   speed?

V.32 and V.32bis use Quadrature Amplitude Modulation.  In both 
standards, the modem's "carrier" is at 1800Hz, and it indicates data
by modulating the phase and amplitude of the signal according to
certain rules.  The phase and amplitude is changed 2400 times per
second, and the value of the bits is determined by the phase an 
amplitude sent during each 1/2400 second period.  

In V.32, you can transmit at either 4800 or 9600 bps.  At 4800, 
there are 4 possible combinations of phase and amplitude.  These 
four combinations can represent all combinations of two bits (00, 
01, 10, 11).  Thus, two bits are signalled in each 1/2400 second 
period, for 4800 bits per second.  

At 9600, there are 32 possible combinations of phase and amplitude
in each period, which means five bits are signalled.  However, one 
of these bits is for redundant encoding of the other four, which 
helps the receiving modem decide which of the 32 possible 
combinations is actually being received.  Therefore, four user data 
bits are being transmitted in each 1/2400 sec period, for 9600bps.

Since V.32 was standardized, modem technology has progressed so that 
it's possible to transmit and received more complex signals with 
greater resolving capability.  In V.32bis, all that was done is 
permit more combinations of phase and amplitude.  At 12000bps, there 
are 64 possible conbinations, and at 14400bps, there are 128 
possible combinations.  V.32bis also permits 7200bps transmission, 
with 16 possible combinations.  In all three of these cases, trellis 
coding is used as at 9600bps; thus, the number of user bits per 
symbol period sent at 7200 is 3, 12000 is 5, and 14400 is 6.

The total signal power that a modem is allowed to transmit is 
governed by national regulations (and the reality of the 
capabilities of the telephone network!).  The average power is 
determined by the combined amplitude of all of the combinations of 
phase and amplitude that can be sent in a particular modulation 
scheme (the modems use scramblers to insure that the transmission is 
actually spread out over all of the possible combinations).  What 
this means is that the total power of all combinations in the 14400 
modulation is limited to the same total power of all combinations in
the 9600 modulation.  You have 128 combinations of phase and 
amplitude, whose amplitudes must add up to the same total as the 32 
combinations at 9600!  This means that these "points" must be closer 
together.  It is therefore more likely that line noise will cause 
one of the combinations to be misinterpreted by the receiving modem 
as being a different combination.  This means that a V.32bis modem 
will require a cleaner phone line to transmit 14400 than 9600.  You 
won't necessarily be able to connect at 14400 on all lines, but it 
should be possible on a majority of circuits -- particularly local 
lines in the USA.  

-- 
Toby Nixon, Principal Engineer    | Voice   +1-404-449-8791  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