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Hey Chief,
As everyone who has already supplied an answer has said, MODEM =
MOdulator/DEModulator.
What nobody has yet said is that a 56K Modem performs Asynchronous Multiple
Phase Shift Keying, based on a Constellation Mapping, which defines variances in
both Amplitude, and Phase.
In the simplest form of PSK, Quadrature PSK, Four 'Phases' are used, to transmit
Two Bits at a time (I'll come back to why Bit Rate and Baud Rate are different
in a bit). Most PSK Implementations use an NRZ Scheme (NRZ = Non-Return to
Zero), so the previous Phase becomes the reference phase for the next encoding.
In QPSK, the following encodings could be used:
45 degrees: 11 135 degrees: 10
225 degrees: 00 315 degrees: 01
There are two reasons that the encoding does not start at 0 degrees - one, the
calculations are based on Cosines, not Sines, and two, because we are using an
NRZ scheme, the reference signal (our previous bit pattern) is taken to be at 90
degrees.
Now of course, QPSK is no where near fast enough for a 56K modem (IIRC, QPSK is
used in 9600 bps modems), but the theory is similar. We still use a reference
signal from the previous encoding pattern, but we add in a couple of other
tricks. We use more points around the map, avoiding 0, 90, 180 and 270 degrees,
since again we are calculating by cosines, and we need 90 degrees for our
reference signal.
We also use changes in the signal Amplitude on some of the phases in order to
increase the number of different bit patterns we can send. It is common to use
the Amplutude shifts only on phases which can be located extremely easily, like
45, 135, 225 and 315 degrees, though 56K modems use AM on more points in the map
than just those four.
It is important to note the difference between the data rate (Bits per Second)
and the Baud (or Symbol) Rate. For Example, a 14,400 Baud Modem actually
transmits at 19,200 bits per second. The Symbol rate (in this example, 14,400
symbols per second) is the rate at which new data is encoded onto the line, and
each symbol is two bits of data. Now, given that information, you may think
'But, doesn't that mean that I should have a bit rate of 28,800 bits per second'
- well, no... The reason that the Data Rate is only 19,200 bits per second is
error correction (Known as ARQ, or Automatic Repeat Request), this is completely
transparent to the computer, since ARQ happens on the DCE to DCE path, not the
DTE to DTE path. Although the ARQ error correction scheme is useful, it also
has a relatively high encoding overhead, hence the observed data rate of 19,200
bits per second.
Oh, I almost forgot, one other trick played by most modems over about 9600 bps,
is operation in 'Full Duplex' mode. Just to explain a little, there are Three
Ways of communicating Data:
Simplex: One Direction Only, say from a remote weather station to it's
base
Half Duplex: Two Way, but only one 'end' of the link can 'talk' at a time,
eg, CB Radio
Full Duplex: Two Way, both 'ends' can 'talk' at the same time, as on most
telephone systems
The reason I point this out, is that here in Europe and the UK, we use the above
three meanings, but the US don't (or didn't used to anyway) make the same
distinction between Half and Full Duplex!!
Anyway, back to the point - by using two 'offset' master carriers, you can get
full duplex communication between two modems (and effectively between two
computers), which allows for faster data rates (not symbol rates) because each
end does not have to wait for the other to finish before it can transmit data.
And the last trick - the Asynchronous part. A 56K modem can only *receive* at
56K, it cannot send any faster than 33.6K. This is because the 56K encoder
needs to send data onto as 'clean' a line as possible, so the equipment must be
either In the Telco's Exchange building, or connected to the Exchange Building
via Digital Grade lines, to ensure there is as little noise on the line as
possible.
Regards,
Mike Insch - IT Engineer, CCA, DipHE
SFS Creative Business Solutions Ltd - http://www.sfs-creative.co.uk
Tel: +44 (0) 1224 624 014 Fax: +44 (0) 1224 218 318
"The Chief"
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Trivia Question --
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The Latest from The Screwdriver List!
It's time once again for The Screwdriver List Trivia Question of the Week!
As usual, there's no million dollars if you answer it right, but I think
that each of these questions will help at least one member on The List.
Here's this week's puzzler:
What does the word "modem" actually stand for, and what conversion does a
standard 56KBps dial-up modem perform?
Know the answer? Then speak up! The first member that posts the right
answers is The List Trivia Guru for the week.
Good luck!
The Chief
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That's all for now from The Screwdriver List
"Red Stripe to Pin 1"
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