Subject: Re: MCI to offer video services over the Internet

From: [email-masked]
Date: Tue, 19 Mar 1996 13:43:33 -0800

> From [email-masked] Tue Mar 19 11:35:24 1996
> Subject: Re: MCI to offer video services over the Internet
> Date: Tue, 19 Mar 96 15:24:06 +0000
> From: Jon Crowcroft <[email-masked]>
> 
>  >satellite for high-speed Internet connectivity, ** videoconferencing over the
>  >Internet **, shared applications, multi-media messaging and advanced
>  >security. 

Hmm. Doing telephony over satellite - seems like one or two such
hops and you're dead, given that the latency will creep over 1/2 second.

High-speed still isn't low latency.

Joe
----------------------------------------------------------------------
Joe Touch - [email-masked]		    http://www.isi.edu/~touch/
ISI / Project Leader, ATOMIC-2, LSAM       http://www.isi.edu/atomic2/
USC / Research Assistant Prof.                http://www.isi.edu/lsam/

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From: [email-masked]
Date: Tue, 19 Mar 1996 23:29:26 -0500

> hops and you're dead, given that the latency will creep over 1/2 second.

Hmm.. I must have blinked here and missed a sarcasm alert.  Wasn't the original
reason for geosyncronous satellites to provide long distance telephone service?

Remember.. 0.5 seconds latency may be an eternity for silicon-based life, but
for us carbon-based critters, it's still a blink of an eye...
-- 
				Valdis Kletnieks
				Computer Systems Engineer
				Virginia Tech

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From: Ahmed A-G Helmy <[email-masked]>
Date: Tue, 19 Mar 1996 22:56:04 -0800 (PST)

> for us carbon-based critters, it's still a blink of an eye...

Jitter (variance of delay), however, may render voice unintelligible.. !

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From: [email-masked]
Date: Wed, 20 Mar 1996 02:01:59 -0500

> Jitter (variance of delay), however, may render voice unintelligible.. !

Is *that* why I'm incoherent unless I've had the proper dosage of caffeine? ;)

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From: ken emery <[email-masked]>
Date: Tue, 19 Mar 1996 23:12:10 -0800 (PST)

On Tue, 19 Mar 1996 [email-masked] wrote:

> Hmm.. I must have blinked here and missed a sarcasm alert.  Wasn't the 
> original reason for geosyncronous satellites to provide long distance 
> telephone service? 
> 

Actually 0.5 seconds is about the point where us "carbon-based critters" 
can start to detect the echo/delay inherent in satalite hops.  Basically 
Bell labs found that if it was below 0.5 seconds the human ear could not 
detect the delay, over that and it was quite annoying (a friend of mine 
actually worked on this project at Bell Labs).  They also found that is 
satalite was used one way and land lines the other that it was more 
difficult to detect the echo (by the human :-)

bye,
ken emery

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From: Petri Helenius <[email-masked]>
Date: Wed, 20 Mar 1996 10:25:26 +0200

> for us carbon-based critters, it's still a blink of an eye...

And for many of the 'multimedia'-applications jitter (delay variance)
and bandwidth are actually more important than the actual latency. Usually
low jitter comes with either intelligent queueing or high bandwidth. I would
also count low loss as one of the factors since I've yet to see data-
interleaving audio application ...

Pete

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From: Jon Crowcroft <[email-masked]>
Date: Wed, 20 Mar 1996 08:49:23 +0000

>can start to detect the echo/delay inherent in satalite hops.  

this confuses 2 things
1/ there is a end to end delay 
limit (around 200msec) for the turnaroudn that makes
"natural conversation possible

2/ there is a delay loop limit (also around 200ms) around which it is impossible
to speak whilst hering one's OWN voice fed back 

 >difficult to detect the echo (by the human :-)

obviously a round trip time (for case 2) where the outbound is
satellite, and the return path terrestrial removes one of the two
1/4 sec  satellite hops - no mystery....and gets you down close (but
not quite close enough) to the 200 msec....it is still unpleasant to
use

on the subject of jitter....

so long as the jitter is a second order effect (as is likely with
satellite hops, unless your satellite is seriously out of geosynch -
e.g. LEO), then you can accomodate it easily with playout
buffers.....if the _mean_ delay varies (e.g. satellite slowly
drifts:-), you need an adaptive playout buffer....technology exists
for this, doesn't it :-) [this is the mbone list, right - have a look
at the vat code.....or half a dozen other pieces of internet phone
technology)

a lot of telephone engineers get confused coz they think in terms of
nets with non-work conserving switches, and "terminals" with a few
bits bufferingh (i.e. ISDN telephones) whereas WE (the real world:-)
use computers which tend to have enough memory to have a few minuites
of audio playout and enough processing to do things like varying the
silence gap lenghts  between talk spurts in real time, or even adding
pink noise between missing samples of talk spurt.....or even
interpolating the speech (given half of the compression algorithms are
based on a speech model, this is not really any more than an obvious
extrapolation of the compression scheme to do either
a) make up for loss
b) make up for delayed samples

going the other way (if the delay decreases rapidly - e.g. your
satellite is coming down to earth:-), can be tricky for heavily
comprewssed speech, but its doable....and easy for 64kbps PCM...

sorry to repeat all this gubbins again....

 jon

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From: Petri Helenius <[email-masked]>
Date: Wed, 20 Mar 1996 11:09:50 +0200

> on the subject of jitter....
Try mixing digital cellular phones with intercontinental satellite
telephone and then you are talking about delay loops and transmission
latency!

Pete