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From: dricejb@drilex.UUCP (Craig Jackson drilex1)
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: <18902@drilex.UUCP>
Date: 14 Dec 90 02:20:34 GMT
References: <136548@pyramid.pyramid.com> <16309@cbmvax.commodore.com> <1990Dec10.162413.13959@sparrms.ists.ca> <1990Dec11.185205.13752@wsrcc.com>
Organization: DRI/McGraw-Hill, Lexington, MA
Lines: 29

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.  Its just that long runs are killers.  The
>specs are a worst case spec.  In practice bandwidth is (hopefully)
>going to be much better.

A dry local loop is indeed capable of much higher bandwidth--suitable
for ISDN, etc.  However, the nature of twisted pair is that it is
capacitive, which tends to attenuate the highs.  Long local loops
(over 1 km or more) have inductance added, in the form of 'loading coils'.
These basically kill ISDN, etc for that loop.

Also, nearly all connections which are not completed within a single exchange
go through some form of frequency-division multiplexed link (known as
'carrier' of various sorts).  These do indeed go through filters.

>I wouldn't be surprised to hear that the 18k tb+ spec assumed that you
>connected the two modems with a 3ft section of phone patch cord.

Naw.... if that were true, they'd have claimed 10Mbps...
-- 
Craig Jackson
dricejb@drilex.dri.mgh.com
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