[asterisk-biz] Low Bandwidth VoIP

[Stelios Koroneos]

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. 
> 
> 
> You are slightly wrong - G729 is 8kbit stream and the UDP overhead for
> 20ms packetization is alltogether 25Kbit stream. In theory you can do
> 30 channels over 250 kbit with IAX2 trunking feature which puts all 30
> channels to one big UDP packet which minimizes headers and UDP
> overhead. There is also G729 variant using 6.4kbit stream. 
> 

The 25kbit is as you said theoretical, the real values i see are close
to 30kbits for a single channel.

The IAX overhead i have seen during tests is around 20kbits in trunk
mode and then an additional 9.5 to 10kbits per g729 channel you add to
the trunk
So even with that, reaching the 32 channels is not possible and i have
not accounted the fact they want also VPN.

Had the same issue with a sat link with limited bw so i had to do this
"exercise" before, that's why i speak with some confidence.

Cisco also offers g729 header compression as they call it which is also
rated at 24kbit per channel, i assume that there must be other multiplex
solutions out there, but given the numbers i still believe the 32
channels on 250kbit is a no-go.