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From: lfd@lcuxlq.att.com (Leland F Derbenwick)
Newsgroups: comp.dcom.telecom
Subject: Re: Phone Calls at the Speed of Light
Message-ID: <3543@accuvax.nwu.edu>
Date: 6 Feb 90 01:44:08 GMT
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X-Telecom-Digest: Volume 10, Issue 77, message 5 of 5


In article <3396@accuvax.nwu.edu>, boomer@athena.princeton.edu (Don Alvarez) 
writes:

> In article <3335@accuvax.nwu.edu> eli@pws.bull.com writes:
> >The speed of light in fiber is actually slower than the speed of light
> >in coax cable...  (.72 to .76, or some such).  Does anyone know the
> >propagation speed for light in copper phone wire, or whatever else is
> >used for long lines??
 
> [reference data for a bunch of open-wire lines,
> showing velocity factors about .94 or better]
 
> Since the slowest speed listed here is about 94% C, and one can only
> assume wires have gotten better, not worse in the last 47 years, we
> clearly should all drop our fiber optic lines and go back to copper.

This is hardly news: open-wire lines always had much better velocity
factors than coax or anything else with a non-air dielectric.  (Basic
EE, or physics if you prefer.)  Of course, open-wire lines have noise
problems and they tend to take up a little more space than you'd
like.

And since the time difference for a 3000 mile span is something less
than 8 ms, who's going to notice?

But the real question is, why does anyone think this is relevant to
current long-distance service?  Yes, once upon a time, when you asked
the operator for a long distance connection, you got a pair of wires
(two pairs?) just for your call.  But it's 1990 now...  Trunk lines
have been _at least_ T1 running over twisted pair for years.  A "long
lines" connection involves a route via some or all of fiber, digital
radio, satellite, or DS3 over coax.  And most of the delay is in the
conversions and regeneration, not in the transmission.  [Except of
course for satellite, which is just a tad more distant.  :-) ]

 -- Speaking strictly for myself,
 --   Lee Derbenwick, AT&T Bell Laboratories, Warren, NJ
 --   lfd@cbnewsm.ATT.COM  or  <wherever>!att!cbnewsm!lfd
 -- (and no, I have nothing to do with "long lines", except to use them!)