[asterisk-biz] Low Bandwidth VoIP

[David Knell]

On Mon, 2012-07-16 at 11:11 +0200, Martin Vit wrote: 
>         The short answer is no you can't get 32 *concurrent* channels
>         on a
>         250kbps uplink
>         
>         With G729 as a codec you need around 32kbits per channel
>         including the
>         overheads from the voip protocol and tcp etc, so that would
>         give you
>         around 7 *concurrent* channels. 

This is correct if you're using vanilla RTP and no silence suppression.
The G.729 payload runs at 8kbits/sec during talk and 1.6kbits/sec otherwise;
assuming 50% silence gives an average bitrate of 4.8kbits/sec.
Alternatively,
just dropping silent frames brings that down to 4kbits/sec.

Multiplexing 32 of these over a 250kbits/sec connection should not be a 
problem, provided that they're not individually wrapped with RTP, UDP
and IP headers.

Incidentally, at 50% talk/silence (which is an over-estimate - people listen
more than they talk), the chance of everyone at one end of 32 calls talking
at the same time is 2*10^-10.  So there's not going to be many frames 
dropped as a result.

--Dave