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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