[asterisk-users] Best Codec

[Gordon Henderson]

On Wed, 6 Jun 2007, Davis Sylvester III wrote:

> We are evaluating starting a small VoIP consumer based platform.
> What is the best codec to use with customers using primarily DSL as internet 
> connectivity?
>
> I know that g729 is the king-all, but I want to know what the rest of 
> the professional are using out there.  g729 has a cost involved, so does 
> the cost really offset the performance?  Or is it better to go with g711 
> to start off?

It has 2 costs - one the financial one to license it - which you'll need 
if you need to transcode it for any reason, eg. to connect to your 
upstream DID provider (or to any sort of local PSTN connection), and the
other is the CPU overhead of transcoding...

So if you can afford the CPU overhead of transcoding, then off you go!

I read an excellent paper off the Xorcom site recently about CPU usage, 
etc. on a 1GHz CPU. The system managed just over 100 calls using G711, but 
only just over 20 for GSM, but barely 15 for G729. Scaling with faster 
CPUs (with bigger caches) is probably better than linear (to a point - see 
the recent scaling thread!), but it's certainly something worth 
considering.

I'm using G711 for ADSL end-users here in the UK, and I think many others 
are doing the same. I use G726 as a fall-back (half the bandwidth of G711, 
and the transcoding cost is very low) in some lower bandwidth situations, 
or when the client wants to place more calls that can be supported by the 
upstream of their ADSL line (typically 256 or 488 or 832Kbps, in the UK, 
depending on the ADSL product they have, the ISP they connect to, and 
distance from the exchange) I use GSM as a very last resort, or for 
inter-office trunks where quality isn't as important.

If you're the ISP with direct control of the DSL end-points then you'll 
have the best knowledge of what your punters are doing and line speeds, 
suitability etc. I've found some ISPs are better at losing UDP packets 
than others (so I try to migrate my clients off those ISPs and onto good 
ISPs)

Gordon