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Re: VoIP help needed please

 
 
Gordon Henderson
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      09-21-2008
In article <(E-Mail Removed)>,
Spence <spencelayhathotmaildotcodotuk> wrote:

>I've been looking to see if I can find a spec for the bandwidth required
>for VoIP, but have failed so far. Having said that, if 256kBit upload is
>OK, then the maximum rate AFAICT is about 120Mbyte per hour, which on
>current usage allows about 8 hours of calls a month with my spare Gig.
>That'll do.


The "basic" CODEC used is G711 - it goes by many names, a-alw, u-law,
mu-law, PCM, PCM-a, PCM-u, G711a, G711u and probably others... The key
thing is that the data rate is 64,000 bits per second. 8-bit samples,
8000 times a second = 64,000 bits/sec.

However, when you add the "IP" part, you typcially add on an extra
16Kb/sec per call (SIP), so expect to use 80Kb/sec per VoIP call.

And remember this is both ways. 80Kb/sec in *and* 80Kb/sec out.

It's also per call, so 2 calls is using double the numbers. It's not
uncommon for a company to make 100's of hours of calls a day when they
have many staff making many concurrent calls.

Another consideration is packets per second - it's 50 each way per call.

Ways to decrease bandwidth usage would be to use a compressed CODEC. The
best (arguably) is G729. This is encumbered with software patents in
countries and honour such things though, however the basic bit-rate is
an eigth that of G711. 8Kb/sec plus the fixed 16Kb/sec overhead of SIP.

If you have a local asterisk based PBX, and your provider supports it,
you can use IAX to "trunk" the calls rather than SIP - same codecs, but
the 16Kb/sec overhead is only applied once, so you can squeeze a little
bit more in.

However, do'nt think it will work by calculation alone. 256Kb/sec upload
would suggest 3 concurrent G711 SIP calls, but that's really pushing it
to the limit and I'd expect something to "give" at that level, so stick
to no more than 2 unless you're using a compressed CODEC, but then the
other limit might hit you - packets per second. VoIP packets are small
- Typically 160 bytes for G711 (50x160=8000) so 2 calls need a router
to handle 2 x 50 packets per second each way - 200pps. Keep going and
you might find that some routers can't handle the packet-rate, even
though they can handle the data-rate, if the packets were bigger...
or the ISP might flag this as something "odd" (Which I'm not aware of
any doing so so-far, but ... while it's easy to make a network transmit
vast quantities of data using large packets, doing it with small packets
is something else)

Using IAX, I have had 10 concurrent G711 VoIP calls over a busines quality
ADSL line though (Entanet, 830Kb/sec upstream), and 18 concurrent G729
ones though it.

Gordon
 
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