Re: Compressed/Encrypted Voice using Modems

[Phiber Optik <phiber@eff.org>]

> 
> 
> > From: Peter Shipley <shipley@tfs.COM>
> 
> > >And why are you limiting this to V.32 (9600bps)?  V.32bis (14.4k bps) modem
> > >chips cost maybe 20% more than v.32 chips in quantity.
> > 
> > at Interopt I heard some voice demos that were at 9600 4800 and 2400 baud
> > the 2400 sounded a phoneme chip but was *very* resionable.
> 
> Quite likely what you heard were 9600/4800/2400bps (bits per second),
> not baud.  The two are different.  2400bps compression of voice is
> quite a bit beyond current GP CPUs.
> 
> Also, note that 9600 baud (V.32 carriers) are based on a 2400bps carrier.
> (And if they're not, I'm sure someone will correct me.)  2400 baud modems
> are based a bit rate something less than 2400bps, though I can't remember
> exactly what it is right now.
> 
> Jim
> 
> 

I'd wanted to say something about this for quite some time.  Everyone gets it
wrong, and let's just say you were CLOSE.  Lemme give a quick run down...
bps, or bits-per-second, is the bit rate of the datastream.  Now the rate
of MODULATION is called baud, named after the transmission engineer 
(telegraphy), Baudot.  Very simply, when you divide the bit-rate by the number
of bits per symbol, you get baud.  Everyone is in the (bad) habit of saying
'baud' for everything, because in FSK systems which code 1 bit per symbol
(only two phases), bps = baud.  But then and ONLY then.  They are not
universally synonymous.  But don't feel bad, the entire modem INDUSTRY gets it
wrong, too.  We're all throw backs to the 300 baud days, when 300 baud was
the same as 300 bps, the Bell 103 standard.  This was also true for 110 baud,
and the Bell 202 standard (1200 baud half-duplex).  Now then, when 1200 bps
FULL duplex was designed by CCITT, it was the V.22 standard.  The Bell system
created the Bell 212A standard.  In either case (they were slightly different),
Differential Phase Shift Keying (DPSK) was the modulation scheme, with four
phases, and TWO bits per symbol to encode each phase.  Hence, the full duplex
1200 bps standard modulates at a rate of 600 bauds.  Yes, in engineering, you
say 1 baud, and 2 or more bauds, but just saying baud is accepted.  Another
principle was introduced, called scrambling (that noise sound you hear when
modems handshake is the scrambler kicking in).  This alleviates the fact that
the higher the rate of modulation, the more likely that multiple identical
symbols in a row will cause phase sync errors.  The scrambler ensures that
a bit (1 or 0) does NOT get modulated to the same phase repeatedly.  This is
similar to digital carrier systems, which use AMI (Alternate Mark Inversion),
and have schemes such as BnZS or HDB3 (Binary N Zero Substitution and High
Density Binary 3).  But lemme not confuse you.  I was just drawing up the
parallel.  The scrambler is just a pseudorandom sequence generator (there's a
descrambler, too).  Phases are measured in degrees (360), and each phase is
encoded depending on the number of bits per symbol.  In V.22 and Bell 212A,
there are four phases, 0, 90, 180, and 270.  And each phase is coded with two
bits, or dibits (two bits per symbol, remember?)
Now, we all use the CCITT standard for 2400 bps.  It's called V.22bis.
It uses a scheme known as QAM, Quadrature Amplitude Modulation.  The rate
of modulation is also 600 bauds, but the use of amplitude as well as phase
come into play, and double the throughput.  There are four levels of phase,
and four levels of amplitude, encoded using 4 bits per symbol (quadbits). 
Isn't this fun?  There are a few 4800 bps systems, none of which really matter.
For 9600 bps, we use recommendation V.32.  QAM again, (as was 4800 and 2400),
with two modes of operation: 16 carrier states, and 32 achieved using trellis
encoding.  16 carrier states equals 4 bits per symbol again (QUADrature), and
simple arithmetic should tell you that the modulation rate is 2400 bauds.
V.32bis is an improved (and clever) scheme, with a bit rate of 14400 bps.
Everything I mentioned are full duplex systems (except Bell 202).  Higher bit
rates are achievable in half duplex systems, these modems are generally used
for transfers (polling for mail/news).
Oh, don't forget our error correction/compression protocols.  V.42 (EC), and
V.42bis (EC and compression) being the most common.  V.42bis can achieve a
maximum of 4:1 compression ratio, increasing throughput.  There's also your
Microcom Networking Protocol (MNP), MNP level 5 being the most common and
usually used at lower data rates.

Whew.  I'm probably giving you guys information overflow, so I'll end it for
now.  Just remember.  You measure the speed of a modem in BITS PER SECOND,
NOT BAUD.

See ya.