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From: goldstein@carafe.enet.dec.com (Fred R. Goldstein)
Newsgroups: comp.dcom.telecom
Subject: Re: TASI Acronym?
Message-ID: <11691@accuvax.nwu.edu>
Date: 4 Sep 90 19:36:08 GMT
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In article <11667@accuvax.nwu.edu>, geek@media-lab.media.mit.edu
(Chris Schmandt) writes...

>A number of conversations can share a significantly smaller number of
>circuits by freeing the circuit associated with a conversation during
>silent periods, and re-assigning one when speech starts.  I recollect
>such a scheme being referred to as "TASI", but can't find this term in
>the online glossaries.

> Can anyone enlighten as to what it stands for?  Is there a standard
>number for how much "circuit compression" it provides?

Time Assignment Speech Interpolation is actually a form of packetized
voice transmission.  It was first used in the early 1970s on undersea
cables.  The idea is to take small chunks of audio, run them through a
level detector, and only send them if they aren't "silent".  Then each
one can be prefaced with a channel header so the receving end knows
which channel is getting which call over the trunk pool.  Typically
you get nearly 2:1 compression with voice.

While AT&T's large TASI systems came first, many private networks used
the Storage Technology COM-II TASI about a decade ago.  (We did here
at Digital.)  But analog TASI is obsolete.  Newer Digital TASI, now
called DSI, systems generally run over T1 carrier.  The StrataCom IPX
is such a box; it sends 192-bit frames with a 24-bit header and
168-bit audio payload. It's nearly toll-quality even with silence
suppression enabled.

Another example is AT&T's Integrated Access Cross-Connect Switch
(IACS), which uses frame relay-based DSI.  It's widely used on AT&T's
international network.  You may have used it without knowing.

Typical 2:1 compression ratios are being impacted by fax.  Since fax
(like dial-up data) sends a constant audio signal, it can't be
compressed.  We had the same problem on our TASI years ago, and
actually routed a lot of dial-up data around it.


Fred R. Goldstein   goldstein@carafe.enet.dec.com 
                 or goldstein@delni.enet.dec.com
                    voice:  +1 508 486 7388 
opinions are mine alone; sharing requires permission