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From: kitty!larry@uunet.uu.net (Larry Lippman)
Newsgroups: comp.dcom.telecom
Subject: Splitting Call Transmission Directions
Message-ID: <12220@accuvax.nwu.edu>
Date: 16 Sep 90 14:54:09 GMT
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In article <11532@accuvax.nwu.edu> dolf@idca.tds.philips.nl (Dolf
Grunbauer) writes:

> I always assumed that when making a telephone call the line to the
> otherside is the same the line back from him to me. The other day
> someone told me that this is not the case, especially when making a
> international phone call.  According to him it is possible that for
> example when calling from Europe to the USA one line could use a
> satellite connection while the other could use a transatlantic cable.
> Is this true?

	Unless echo suppressors have become *much* more sophisticated
than those with which I was once familiar, I would be surprised if
such vastly different propagation paths could be used on the E-W and
W-E directions of a given intertoll circuit.

	Control of echo on intertoll circuits can be implemented by
simple attenuation in the trivial case, and voice-switched attenuation
through echo suppressors in the more common case.  The proper design
of intertoll circuits, including configuration of echo suppressors, is
governed by the Via Net Loss (VNL) concept.  VNL design requires
knowledge of propagation delay in milliseconds.  At the time I was
involved in the telephone industry, the VNL design with which I was
familiar imposed a maximum of 22.5 milliseconds propagation delay to a
DDD switching midpoint, with an maximum overall delay of 45
milliseconds on any given DDD circuit.  It was always a "given" that
VNL design required the same propagation delay in each direction.

	While I admit that I have no firsthand experience with
intertoll circuits involving satellites or transoceanic cable, I would
find it difficult to believe that any satisfactory transmission (and
echo) performance could be achieved with the E-W and W-E directions
that have widely *differing* propagation times.

	Obviously, satellite transmission by its very nature imposes
propagation delays which far exceed 45 milliseconds.  However, echo
suppressors and intertoll circuit design can be set up to deal with
such increased propagation delays - *provided* that the delay is equal
in each direction.

	Also, I can think of no valid reason to split E-W and W-E
routing between different transmission facilities.  *ALL* transmission
facilities used for intertoll circuits are, by their very nature,
bi-directional.  While I have seen all sorts of route diversity and
failure protection switching, I have never seen anything that split
transmission directions through different facilities.


Larry Lippman @ Recognition Research Corp.
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