NetNews Usenet Archive 1992 #16/NN_1992_16.iso/spool/comp/dcom/cellrel/245

Text File | 1992-07-25 | 2.0 KB | 43 lines

Newsgroups: comp.dcom.cell-relay
Path: sparky!uunet!decwrl!pa.dec.com!e2big.mko.dec.com!cvg.enet.dec.com!pettengill
From: [e-mail] ()
Subject: Re: Cell SIze
Message-ID: <[e-mail]>
Sender: [e-mail] (Guest (DECnet))
Reply-To: [e-mail] ()
Organization: Digital Equipment Corporation
References: <[e-mail]> <[e-mail]>
Date: Sat, 25 Jul 92 08:47:26 GMT
Lines: 30

In article <[e-mail]>, [e-mail] (Mark I. Williams) writes:
|>
|>Yes, but...but.... This analysis considers data traffic only! Remember
|>that ATM will be expected to provide other services as well. With a cell
|>payload of 128 bytes echo cancellation will be even more difficult than
|>with 64 bytes.
|>
|>Consider also a telephone-quality voice channel: the data rate is 16
|>kbps. If you have a cell size of 128 bytes, the "lumpiness" is
|>introducing a delay of 62.5 milliseconds before the bits have travelled
|>an inch. Add a bit of buffering to smooth out delay jitter (which is
|>also somewhat dependent on the cell size) and it quickly becomes
|>apparent that 128 octets is "A Power too Far". Of course you could
|>simply only send 32 octets per cell when you had a long-distance
|>call....:-)

A cell size of 64 or 128 bytes poses no problem for voice.  Echo cancellation
is required for virtually every long distance circuit in the US even for
analog circuits (if any still exist) due to the delay in the wires and the
repeaters.  Even a country as small as France is only barely able to avoid
echo cancellation, and they wanted 32 byte cells so they could continue to
avoid having it within France.  The delays in the intermediate switches can
easily exceed the delay caused by packetizing the data into cells.

The problem with 128 byte cells is that it is larger than needed to carry
the fragments of voice payload with good efficiency.  Since standards always
represent the needs of the past, ATM is well suited to the past, not the
future.

mulp