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From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: sci.electronics,sci.astro,comp.dcom.modems,comp.misc,rec.ham-radio
Subject: Re: N.B.S. Time Service
Message-ID: <1255@neoucom.UUCP>
Date: 7 Jun 88 03:08:49 GMT
Article-I.D.: neoucom.1255
References: <455@trane.UUCP> <4691@watcgl.waterloo.edu> <585@otto.COM>
Organization: Northeastern Ohio Universities College of Medicine
Lines: 38
Summary: Satellite delay


A geosynchonous orbit, in which the telecom satellites used by Ma
Bell et. al. is 22,400 miles above the equator.  For any place not on
the equator directly below the satellite, the path is somewhat
longer.

That means that if a call is routed by satellite, just the time to
get up to the bird and back down to the ground is about 300 mS.
There is likely to be some delay in the ground based processing
equipment too.

It is my understanding that calls are often routed so that one half
will be land line, while the other half is satellite to keep the
turnaround delay from becomming excessive.  Ocasionally when I call
overseas, I think I get a call where both sides are going via
satellite.  It makes you tend to repeat yourself becuase the other
person's reply is delayed enough that it feels like he/she is not
understanding what you said.  I've also had overseas calls that are
half-duplex (probably for echo cancellation), and it can be very
confusing because you can't say "uh-huh, yup, right, etc" while the
other person is talking.  The carrier tone of the answer modem
is specially designed to supposedly ensure that the call is allowed
to be full duplex, assuming that the modem will do its own echo
cancelling.

For land links, the trasmission delay plus the dealys introduced by
the terminal equipment seems to me via qalitative observation to be
well under 100 mS.

I'd imagine that Rick Adams must have some interesting stories
about satellite delay.  Those calls to Singapore would require
several satellite hops.

Though I am not a guru, it seems feasible to me that at least some
regions of the AT&T network would be smart enough to decide to give
special routing to a call to a given number.

--Bill