This sounds almost impossible. In standard carrier grade voice transmission64kbits/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.
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.
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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