[Asterisk-biz] voip VPN solution requirement

[Jean-Michel Hiver]

>This sounds almost impossible. In standard carrier grade voice transmission
>you can pass 31 calls with 2 Megs (64 kbit/s per call). In your case you
>have 21kbit/s per call. Plus you want this encrypted, which means you will
>have even less quality. You can do it, but you will loose a lot of
>frequency. 
>Loosing frequencys means one can not hear the  temper of the caller. 
>  
>
64kbits/s is the theoretical bandwith for g.711. Using g.729, you need 
8kbps. Of course when you add UDP + IP it comes to about 24kbps - but 
that's when IAX trunking kicks in.

It seems that using trunking + g.729a, you need *734.84 kbps in each 
direction only. [1]
*
Granted, g.729a doesn't sound as good as g.711, but in comparison with 
the bandwith it uses (and especially other low-bandwith codecs) it 
sounds very good with a MOS of around 4. [2]

*Encryption does add some overhead, but it seems that the added ping and 
jitter would be more of a problem than bandwith in this case...
*
Cheers,
Jean-Michel.

References:

[1] http://www.asteriskguru.com/tool2.php
[2] http://www.networkcomputing.com/1202/1202ws32.html

"Algebraic code-excited linear prediction (ACELP) is the most popular 
compression algorithm found in Voice over DSL integrated access devices 
(IAD) and frame relay or LAN gateways because it provides high quality 
over modest bandwidth. To minimize encoding delay, a digital signal 
processor (DSP) usually is employed in the IAD."

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