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From: eds@solar.UUCP
Newsgroups: net.dcom
Subject: TASI is real!
Message-ID: <281@solar.UUCP>
Date: Tue, 25-Oct-83 13:27:41 EDT
Article-I.D.: solar.281
Posted: Tue Oct 25 13:27:41 1983
Date-Received: Wed, 26-Oct-83 07:05:14 EDT
Organization: ABI - ED&D, Holmdel, NJ
Lines: 30

AT&T Communications (formerly Long Lines) certainly *does* do statistical
multiplexing on voice trunks.  The system is called TASI-E (Time Assignment
Speech Interpolation), and I worked on its development during 1979-81 at
AT&T Bell Laboratories.  TASI-E is the third generation of TASIs, and the
first with more than two systems installed (I think there are >20 in service
now, with 240 trunks on 120 channels each).  Currently, TASI-E is installed
only on undersea cable circuits, where the cost of installing a new circuit
far exceeds the cost of providing another virtual circuit via TASI.  Better
than a 2:1 compression of talkers to long-distance channels is achieved by
exploiting natural gaps in conversation.  [While I'm talking, you're
listening, and even when I'm talking there are pauses.]

Anyway, when a modem call comes into TASI-E, a circuit detects the
echo-suppressor disable tone (2000 Hz for >194 ms.), and does two things:
it removes the 50 ms. of delay from the modem circuit (delay is there to
reduce the front-end clipping of speech bursts while they are being detected
and switched), and it "pins" the trunk/channel connection as long as there
is energy detected in either direction of transmission.  This effectively
reduces the number of long-distance channels available for connection to
other trunks, so the average/maximum number of data calls present in a pool
of 240 random trunks was an important consideration in the engineering of
TASI-E.

In 1978-79, Long Lines planned to install TASI-E on many domestic routes
of longer than 1000 miles, but the plans were eventually dropped for a
variety of reasons.  I think the first big discussion of TASI was in a
1954 issue of the Bell System Technical Journal, if you're interested further.

	Ed Schulz, AT&T Information Systems Laboratories, Holmdel, N.J.
	(201) 834-3838     hou**!houca!solar!eds