[asterisk-users] How can I improve call quality?

Brian D brian at prevision.bm
Mon Apr 23 04:46:49 MST 2007

Well g.711 (a/u) is essentially the same as PCM (raw) which is used to 
traditional circuit switched voice environments. 

 There are such mechanisms as "packet loss concealment", where a 
"predictive" algorithm is used to determine wave forms.

But outside of this, whether you use g.711 or g.729 you should be 
inspecting your QoS configuration - and using a call admission control 
mechanism.  Ensure that your RTP and signaling streams are being 
classified correctly and being provided with bandwidth gurantees.  You 
should have the RTP stream placed in your low latency queue (LLQ) for 
direct access to the tx-ring.  ONLY the RTP stream should be in the 
LLQ.  The call control signaling does deserve a bandwidth guarantee, but 
does not require to be in the LLQ.  You will have to sit down and 
determine the business requirements by auditing internally used 
applications and their bandwidth requirements, then hash out an 
"internal" SLA and on which to base your QoS scheme.  Whether you have 
2mb of bandwidth or 2^10, a "thirsty" TCP flow can potentially starve 
out your voice.

- b

Gordon Henderson wrote:
> On Mon, 23 Apr 2007, Adrian Marsh wrote:
>> So which is the best quality?
>> Gradwells www site lists g711u and g729a, but we currently use ulaw/alaw
>> with them too..
> ulaw is g711u ...
> g711 (u or a), or ulaw or alaw which are the same things will give you 
> the best audio quality, but it's not better than a perfect 
> copper-wires telephone line because that's what BT (& others) use 
> internally inside their digital networks.
> So if you have the bandwidth, you'll never get better audio quality 
> than g711.
> But if you're bandwidth starved then you may have no choice to use a 
> compressing codec in which case, it's a trade-off between CPU power 
> required to encode/decode the audio data vs. speech quality. You can 
> get some fantastic compressions, but then you end up sounding like a 
> dalek...
> The difficulty is when you have packet loss - in this case, which 
> codec gives the best results when it loses a packet or 2...
> I wish I knew the answer to that last one... I've used g711 over a 
> lossy wifi connection and it's been acceptable to me, but maybe not to 
> others!
> Gordon
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