On Wed 07 Nov 2001 10:42, Fergal wrote:
> I don't think there's an official definition of broadband [...]
Well, in classical signal theory, "baseband" would refer to signalling
by modulating the large-signal voltage on a line directly; eg: by
putting 5V down the line to represent a 1 bit, and 0V to represent a 0
bit, or -12V and +12V, or what have you.
Modern POTS modems use an advanced form of baseband signalling to
transmit multiple bits per baud ("baud" is the number of signal
transitions per second on the line) - typically a set of four or eight
frequencies with four phases in each, to transmit 4 or 5 bits per
baud. However, this is still baseband.
"Broadband" usually refers to using the high-frequency capabilities of
a connector to transmit much higher information rates. If a given
signal medium (a phone line, for example), can cleanly transmit a 4MHz
carrier signal, for example, then you can use Frequency Modulation
(slight variations in the frequency of the transmitted signal) to
transmit information much more rapidly than is possible using
baseband. I think the maximum FM data rate is "carrier frequency /
4", but it's been a very long time since I studied this stuff. ADSL
works on this principle; it ignores the "large signal" properties of
the phone line (a simple high-pass filter) and listens only to the FM
data on the (8MHz?) carrier wave.
Very neat, but it does rely on the phone line being able to transmit
the carrier frequency cleanly - certain types of junction will mess
with the signal, and very long wires attenuate it too much - this is
the source of the ADSL distance limitation.
Hope this helps,
Colm Buckley : Systems Architect @ NewWorld Commerce
Business: +353 1 4334334 / colm at nwcgroup.com / http://www.nwcgroup.com/
Personal: +353 87 2469146 / colm at tuatha.org / http://www.tuatha.org/~colm/
Gotta run. Neighbours just sighted Elvis making crop circles.
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