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From: gsteckel@vergil.East.Sun.COM (Geoff Steckel - Sun BOS Hardware)
Newsgroups: comp.dcom.modems
Subject: Re: Capacity of a Channel (Was: Inexpensive 9600 baud modems)
Keywords: Shannon, 9600, V.32, V.42, bis
Message-ID: <3599@jaytee.East.Sun.COM>
Date: 11 Dec 90 23:51:50 GMT
References: <136548@pyramid.pyramid.com> <16309@cbmvax.commodore.com> <1990Dec10.162413.13959@sparrms.ists.ca> <1990Dec11.185205.13752@wsrcc.com>
Sender: news@East.Sun.COM
Reply-To: gsteckel@east.sun.com (Geoff Steckel - Sun BOS Hardware)
Organization: Omnivore Technology, Newton, Mass. (617)969-3448
Lines: 46

In article <1990Dec11.185205.13752@wsrcc.com> wolfgang@wsrcc.com (Wolfgang S. Rupprecht) writes:
>
>Also, remember that the 3.4 kHz is the MINIMUM bandwidth the phone
>line will present to the modem.  The sky is the limit.  Don't folks
>regularly send 10 Mhz ethernet connections over (admittedly shorter
>sections of) these wires?  I believe 64k baud ISDN links actually go
>from the CO to your jack over these same wires.  The trick here is
>that it is fairly easy to get good freq responses out of short
>sections of twisted pair.

Not quite true.  I refer people to the experts on comp.dcom.telecom,
but the 3.4 KHz is the limit for which the telco people minimize dispersion
and maintain a reasonable frequency response through amplifiers,
multiplexors, and all the other interesting equipment between your modem and
the other one.  With the same twisted pair, but different compensating
networks, amplifiers, etc, at intermediate points in the circuit, you
can get pretty much any response you want.  The telco can also make your
circuit pretty quiet.

A twisted pair, per se, can be used into the megahertz if you know a
LOT about its properties.  ISDN works (at 176 KB/s) because the engineers
worked very hard to overcome two problems: dispersion and attenuation.
The subtler part is dispersion: differential propagation delay versus
frequency.  Messes the heck out of pulses, and really destroys complex
modulation schemes.  Within limits, however, the dispersion and frequency
dependent attenuation are predictable for a copper loop to the Central
Office, and this information is used to `pre-distort' the ISDN loop
waveforms and to recover the data at the far end.

T1 (1.544 MB/s) also uses twisted pair - with sophisticated phase,
amplitude, and frequency compensation on the transmitted and received
signal.

Once at the central office, ISDN uses all-digital transmission to the
terminating central office, which then uses the twisted pair loop to
the user.

These techniques of precompensation and recovery are used by some `analog'
modems as well, but the possibility of FDM or TDM links (with attendant
brick wall anti-aliasing filters) in the transmission path limits their
usefulness.  14400 B/s with V32bis represents a triumph of engineering
over ugly limitations.
	geoff steckel (gwes@wjh12.harvard.EDU)
			(...!husc6!wjh12!omnivore!gws)
Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line.
This posting is entirely the author's responsibility.