u-law, a-law verses G.723, G.729, GSM etc

James Knott james.knott-bJEeYj9oJeDQT0dZR+AlfA at public.gmane.org
Tue Dec 16 20:07:04 UTC 2008

William Muriithi wrote:
> Hi pals,
> I am not sure if what I have in mind is correct and nothing picked up
> by google seriously satisfied me and that is why I am posting this.
> Its on VOIP. There is these compounding algorithms - according to
> wikipedia, u-law and a-law. Then there is the voice compression
> algorithms or codecs? I mean G.723, G.729, GSM to name a few.
> Now I kind of know how they work but a little mixed up as to whether
> they are inclusive, exclusive or just one thing with different names.
> For example, G.711 is in some on line article also called u-law. This
> is not a single incident and the whole business has made me feel
> confused.
> My take of it is this, when a call is received, its taken through
> either u-law or a-law depending on geographical location. The
> resulting audio signal is then taken again through the later group -
> G.723 or GSM. The result is then packed as an IP packet and delivered
> to destination where the reverse happen. Now, is this the correct or I
> could as well be smoking pot?
> audio <-> a-law <-> G.723 <-----------------> G.723 <-> a-law <->
> Ah, now I even noticed something else. Someone could call from Canada
> - read a-law to UK where u-law is used. Now that would result to using
> one algorithm for compressing and another for expanding and I do not
> believe the result would be intelligible. Someone please educate me
> here.

A-law & u-law refer to the analog/digital encoding method, used to 
reduce quantizing noise.  u-law is used in North America, a-law in 
Europe etc.  It doesn't really matter which you use, so long as you use 
the same on both ends.  Some systems can work either and some only one. 
  This is more of a concern for carriers, providing trunks across the 
pond.  On an individual call basis, pick which ever one is convenient, 
though I'd recommend using u-law only within North America & Japan. 
G.729, G.711 etc., refer to the compression method used to reduce 
bandwidth.  G.711 is no compression, giving the full 64 Kb/s bandwidth. 
  G.729 gives the greatest compression, and worst call quality, 
squeezing a voice call into 8 Kb/s.  G.721 & G.723 have been superseded 
by G.726, which provides a few different compression rates.  G.726 falls 
between G.729 and G.711, in both bandwidth required and call quality. 
GSM compression is for use with GSM cell phones.  You'd normally not see 
it off the GSM cell phone network, as it gets converted to G.711 when it 
hits the wired phone network.  The compressed systems are generally used 
on private trunks or some long distance resellers.  I've never had a 
call running u-law on one end and A-law on the other.  I'd suspect it'd 
still be intelligible, though distorted, as the encoding curves aren't 
that different from each other.  The main difference is u-law can handle 
a greater dynamic range and A-law has less noise at low audio levels.

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