🚀 go-pugleaf

A tip of my hat to Bill Dunn for providing the kind of details I was
hoping for when I asked about ISDN. 

I thought I'd pass on something else related to the part of the
network that I have worked close to, purely for novelty value.

In article <3417@ihlpf.ATT.COM> dunn@ihlpf.ATT.COM writes:
>
>ISDN will be (is) a totally digital telephone system, which means that your
>phone will actually be a A/D converter and a D/A converter. All the signals
>leaving and arriving at your phone will be digital. ...

This is certainly true from a high-level perspective and it captures
the essence of ISDN. What is all the more remarkable, at least to me,
is that on a closer examination, the digital communication between
telephone and central office, along that typical 0.5 to 3 km of copper
pair, is accomplished by analog means, in the form of a high-speed
modem (144 or 160 kb/s). In order to move this high-speed bit load
down the wires robustly, it is transmitted as an analog waveform that
is tailored to the characteristics of the medium (as with most other
digital communication over channels not designed originally for same)
and is subject to optimal-digital-communication refinements such as 
echo cancellation (compensating for line reflections, largely at 
junctions in the wire) before finally being converted back to bit-stream
form. This has created a new market for analog telecom circuits for the
peculiar requirements of ISDN (12-bit, 160 ks/s A/D converters; 
switched-capacitor echo cancelers; etc.) and the IC firms I talk to are
excitedly developing parts for this market.

The bottom line here is that not only is the telephone an A/D and D/A
pair for 3 kHz voice; but it then takes the resulting 64 kb/s digital
representation of the voice, interleaves it with the other digital
signals in the service, and converts it back to high-speed analog
waveform (!) of a different kind, for transmission over the wires.
Certainly a civilized thing to do, from a technical standpoint.

Max Hauser / max@eros.berkeley.edu / ...{!decvax}!ucbvax!eros!max