Define "unacceptable".
Old-style telcos have delay budgets for their designs; if the
round-trip time is too long, people find the call unpleasant. While
VoIP does have its own delay issues (see below), the big problem here
is the satellite link. VoIP doesn't give you an exemption from the
speed of light laws; if you find satellite phone calls unpleasant -- I
do -- you're not going to like satellite VoIP calls, for reasons that
have little to do with the IP. This is one reason why companies have
spent fortunes putting in transoceanic fibers instead of launching more
satellites -- the customers prefer the quality. Satellite calls are
cheaper, but they're noticeably -- and for many people, unacceptably --
worse in quality.
I should note that many VoIP systems make this noticeably worse, though
(of course) less so as a percentage of the total delay than for
domestic US VoIP calls. The problem is the tradeoff between delay and
efficiency. Suppose you're sending 56 Kbps, uncompressed -- the
equivalent of so-called "toll quality" voice. That's 7 kilobytes/sec.
If you want nice, big UDP packets with 1K payloads, you've just
incurred about 143 ms of buffering delay, independent of transmission
time. (And on a DS1 line, transmission time for that packet is
non-trivial.) The total delay budget is, as I recall, about 150 ms.
You can go to nice, short packets -- say, 100 bytes -- but then your
IP, UDP, and RTP headers add a substantial amount of bandwidth
overhead. Apart from line efficiency, on relatively slow lines that's
a lot of serialization time going out over the wire. Compression makes
the packets nice and short; depending on what you do, it may or may not
help with the headers, but you have to spend a chunk of CPU time after
you've collected a large enough voice sample to be worth compressing,
and that means more delay time.
Bottom line: VoIP is inherently costly, either in delay time,
bandwidth, or both. It doesn't mean it's unacceptably bad, but that
150 ms delay budget came from many years of psychoacoustic studies.
Cisco has a good web page on this, with lots of numbers. See
http://www.cisco.com/warp/public/788/voip/delay-details.html
--Steve Bellovin, http://www.research.att.com/~smb
It means, that satellite (with it's 1 second delay and unavoidable echo)
should be better accepted for _profeccionsl_ phone connection (such as, for
example, connection between remote oil wells and central office), because
you can always stay with delay, if follow some talking discippline (and it
is better than trunking radio, anyway), while it will be less acceptable for
home / residential users.
Delay (by itself) does not influence voice quaility, but it required strick
talking discipline / policy to avoid misunderstanding, echo and so on. We
can expect such discipline from professional workers, but you never expect
it from your 5 y.old kid.
Alex
>
>Thanks for the answers about Voip usage over satellite (I did not know,
that
## On 2004-03-27 19:30 -0800 Alexei Roudnev typed:
It means, that satellite (with it's 1 second delay and unavoidable echo)
Geosynchronous satellite IP link RTT can be just over 500 mill-sec
(real life experience) IMHO thats a rather significant difference
500 RTT, + 150 jitter buffer, + something else... it will be 700 - 800 msec,
more likely. When we worked with a few sattelliite lines (5 years ago), I
never saw ping rtt less than 800 msec. Of course, it does not mean that you
can not see RTT = 500 msec (but I never saw it).
But I was talking aboutt other thing - ~1second delay != bad quality, it is
just a delay, which means that, if you have good echo cancellers (which is
interesting question) and follow talking discipline, you can talk without
any problems. It explains, why satellite links + VoIP can be a good
combination (moreover; after satellite delay, which is 500 - 600 msec, VoIP
additional delay ,which is 50 - 150 msec, does not change overall delay so
much, as in case of VoIP over bad link _vs_ traditional telephony (200 msec
vs 20 msec = 10 times; 800 msec vs. 600 msec = 30%).
## On 2004-03-27 19:30 -0800 Alexei Roudnev typed:
>
> It means, that satellite (with it's 1 second delay and unavoidable
echo)
None of the satellite circuit I have worked on during the last five years
has been more than 550 ms RTT. They are all C-band VSAT type systems in
North America and Latin America. The economy (both $$ and quality) of
satellite is such that it is only considered ...
1) No reasonably priced or reliable terrestial alternative is available.
This has generally been the case for mineral exploration type operations.
Political motivations have also come into play in some cases, e.g. when an
unfriendly jurisdiction/neighbour exists between A and B.
2) When you don't have 2-4 weeks for all the xLEC/IXC/PTT to agree on things
so that you can come up with an end-to-end build design, then spend another
6-8 weeks coordinating the build out, and it is not unusual to spend yet
another week or two for the said xLEC/IXC/PTT to blame each other when the
circuit won't turn up because someone left a piece of tone generating test
equipment plugged in. I can turn up a sat circuit in as little as 24-hours
once the teleport is in place (typically 2-3 weeks for remote locations).
The same also goes for increasing/descreasing bandwidth on demand (can be
very expensive).
3) When you need to reduce the number of points of failures to that single
(30,000 km * 2) hop.
4) When you need a reliable/cheaper backup/overflow to the primary
terrestial circuit (my original question for starting this thread 