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From: motcid!crocker@uunet.uu.net (Ronald T. Crocker)
Newsgroups: comp.dcom.telecom
Subject: Re: Data Lines vs. Voice Lines
Message-ID: <12545@accuvax.nwu.edu>
Date: 24 Sep 90 16:13:30 GMT
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X-Telecom-Digest: Volume 10, Issue 675, Message 2 of 9


In article <12490@accuvax.nwu.edu> stox@balr.com (Ken Stox) writes:

>	3) Human speech contains a great deal of dead air/silence.
>When you are pausing in a word/sentence/etc., you are no longer
>sending data. The phone company can now send more packets of data over
>that trunk line while you are pausing between word/sentences/etc.

>	4) Modems don't pause, they will use every available packet
>for that data path. In other words, a modem conversation will not
>allow any other packets through.

 From my experience (former Bell Labs), the type of multiplexing that
you describe above (item 3) is not typical of any switches (digital or
analog) that I am familiar with.  Most telephony connections are
"circuit-switched", i.e. equivalent to hooking a pair of wires between
the two parties.  The only "packet-switched" connections that I know
of are those for ISDN packet data (B or D channel), and these are
handled as "special cases," at least in the 5E.  

Voice is not packet data.  It is not treated in a packet manner.
Whatever happens to be on the voice channel is digitized (PCM),
transmitted across digital carrier facilities (T1) to another switch,
decoded to the equivalent analog signal, and played out of the
receiver in the handset.  No where in this loop is anything trying to
figure out if the digitized voice signal represents "quiet".  T1 is
simply a multiplexed digital version of 24 analog trunks.  Voice-grade
lines are 64Kbps, T1 channels are [nominally] 64Kbps.  Maybe if there
were some compression done the case would be different, but I don't
know of any of that either.


Ron Crocker

Motorola Radio-Telephone Systems Group, Cellular Infrastructure Division
(708) 632-4752 [FAX: (708) 632-4430]
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