Re: Baud vs. BPS (Re: disk transfer methods)

[dempson@actrix.gen.nz (David Empson)]

Byron Desnoyers Winmill <wbdesnoy@acs2.acs.ucalgary.ca> wrote:

> Brian Hammack (brian.hammack@rook.wa.com) wrote:
> :  WAU| : ProTERM 3.1 is faster than ADT.  On a IIGS, you can transfer
> :     | : at 56k (that's 56,000 baud!) and besides, it has Zmodem for
> 
> : No, that's 57,600 bits per second.  :)
> 
> I have a vauge idea of how baud and bps work with modems, but it is clouded
> by years of disuse.  :-(  If anybody has a moment, I would appreciate what
> (if any) role baud has with serial ports.

Assuming you are talking about an asynchronous serial port (like the
Super Serial Card or the IIc, or the IIgs running in async mode) then
bps = baud rate.

A synchronous serial port (e.g. IIgs serial port running in FM clocked
HDLC mode as with LocalTalk) may have different bps and baud rate.

In all cases, "bps" refers to the actual data throughput, including any
framing overhead for the transmission format.  For asynchronous data,
this includes the start and stop bits (and parity, if any).  For
synchronous it includes any bit stuffing and framing characters, but
doesn't include clocking information.

The term "baud" refers to the maximum number of changes of state in the
transmission line per second.  A change of state might be a voltage
change or current reversal for a DC signal, or a frequency, phase or
amplitude change for an AC signal.

Asynchronous serial data is transmitted using NRZ (non return to zero),
in which the voltage (or current) changes between either of two levels
according to whether a zero or one bit is being transmitted.  There is a
maximum of one change of state per raw data bit, hence one bit per baud,
hence bps = baud rate.

With FM modulation (as used by LocalTalk), there is a clock transition
in each bit cell with the additional possibility of a data transition,
therefore there is a maximum of two changes of state per raw data bit,
hence half a bit per baud, hence baud rate = bps * 2.  (Thus LocalTalk
is 230400 bps or 460800 baud.)  Incidentally, Ethernet is similar - it
requires a maximum of two transitions per raw data bit, thus 10Base-T
actually needs to be able to transmit data at 20M Baud.

Here is a quick summary of the low end modem formats:

V.21 or Bell 103 (300 bps) uses FSK (frequency shift keying): one bit
per baud, thus 300 baud.

V.23 (1200/75) is also FSK: one bit per baud, thus 1200 baud one way and
75 baud the other way.

V.22 or Bell 212 (1200 bps) uses DPSK (dual phase shift keying): two
bits per baud, thus 600 baud.

V.22bis (2400 bps) uses QAM (quadrature amplitude modulation): four bits
per baud, thus 600 baud.

V.32 (9600 bps) uses Trellis modulation.  Not quite sure about this one,
but I think it is six bits per baud, hence 1600 baud.

V.32bis (14400 bps) is also Trellis, possibly with more states.

I haven't looked into the higher speeds.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand