Aucbvax.2834

Aucbvax.2834
fa.telecom
utzoo!decvax!ucbvax!telecom
Thu Aug 27 08:58:51 1981
TELECOM Digest V1 #3
>From JSol@RUTGERS Thu Aug 27 08:53:59 1981
TELECOM AM Digest Thursday, 27 Aug 1981 Volume 1 : Issue 3

Today's Topics: Modem Vs. Codec - a Clarification
Direct Dialed Credit Card Calls - Fact or Fiction
IDDD & ESS - The "#" is a return key
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Date: 26 August 1981 01:21 edt
From: JSLove at MIT-Multics (J. Spencer Love)
Subject: Modem vs Codec
Sender: JSLove.PDO at MIT-Multics
To: DJC at MIT-XX

I have been asked to clarify this distinction since I mentioned the
use of CODEC's briefly in TELECOM V1 #1. I will try to avoid the use
of technical terms.

MODEM is short for MOdulator/DEModulator. The purpose of a modem is
to send a bit stream over a voice grade (or other frequency domain)
channel. The primary thing is the bit stream, we don't care what it
sounds like as long as it fits in the bandwidth of the voice channel.

CODEC is short for enCOder/DECoder. The purpose of a codec is to send
a voice over a bit stream channel. The primary thing is the voice, we
don't care what the bit stream looks like as long as the bit rate is
within the capacity of our digital network.

I have tried to emphasize the mirror symmetry of the concepts. The
reason that codec's are interesting is that we can do a much better
job with digital transmission than with analog transmission. Modern
information theory lets us engineer the necessary redundancy into bit
streams and we can even use error correcting codes so we know that the
voice will be "perfectly" reconstituted at the other end. Anyone who
has consumed frozen orange juice will understand that even though the
reconstitution is less than perfect, it can be done with a uniform and
fairly high quality, and it can be delivered more cheaply.

The contract of a codec is to reproduce the voice as faithfully as
possible with as few bits per second as possible, like the modem's
contract is to get as many bits per second as possible through the
crummy analog channel.
-- Spencer

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Date: 26 Aug 1981 0530-PDT
Sender: LEAVITT at USC-ISI
Subject: Direct Dial charging of calls
From: Mike Leavitt <LEAVITT USC-ISI AT>

As a regular business traveler, I often make
operator-assisted calls instead of direct calls just to charge
those calls to a credit card or to my home number. Are there any
mechanisms or plans for a system where I can dial the number I am
calling, followed by a *, followed by the number I want to charge
the call to? It would surely make things easier.

Mike

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Date: 26 Aug 1981 0926-PDT
From: Ian H. Merritt <MERRITT USC-ISIB AT>
Subject: IDDD

Lauren:
Why do you say 'Usually TSPS'? The TSPS is not usually consulted
unless the caller is on a STEP line, of which there are a rapidly
decreasing number. Any such call from ESS is sent directly to the 4E,
which takes care of getting the call to the overseas sender. STEP
doesn't have the intelligence to parse the variable length sequences,
so anything starting with a '0' is immediately connected to the TSPS
which then receives the remaining digits as the customer dials them.

By the way, for those of you who don't already understand (there must
be some) the trailing '#' character is used as a carriage return on
any call from ESS in which there is any ambiguity in length. In the
case of the IDDD calls, there is *ALWAYS* ambiguous length unless the
maximum number of digits (I think it's about 17) is dialed.

IHM

[Of course if you don't type a carriage return to the phone,
the timeout deamon will insert one for you in 4 seconds -JSOL]

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End of TELECOM Digest
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The Usenet Oldnews Archive: Compilation Copyright (C) 1981, 1996
Bruce Jones, Henry Spencer, David Wiseman.