G.711 a/u-law Vs G.729 a/ab

but most carrier does not support g711 as their primary codec is that true?

I would have expected most to support it.

However if you are sending the calls over the internet then a higher compression codec would usually be better unless you have lots of bandwidth available or will only have a small number of simultaneous calls.

Technically G711ulaw is compressed. Normal voice is 8khz 12 bit. ulaw uses logrithmic (spelling?) compression of the amplitude down to 8 bits.
This reduces the amount of data required as catranis said above and means it can normally still represent DTMF tones and low speed modem data calls (faxes etc...).

Per Wikipedia

"The basic digital circuit in the PSTN is a 64-kilobits-per-second channel, originally designed by Bell Labs, called a "DS0" or Digital Signal 0. To carry a typical phone call from a calling party to a called party, the audio sound is digitized at an 8 kHz sample rate using 8-bit pulse code modulation. The call is then transmitted from one end to another via telephone exchanges. The call is switched using a signalling protocol (SS7) between the telephone exchanges under an overall routing strategy."


grblades is probably referncing POTS characteristics.

Yes I was referencing more as to how the digital handsets work rather than an official standard as what quality the handset manufacturer digitises the voice before encoding is purely up to them.
The main point is that although DS0 and ulaw are both 8bit 8khz sampling with a 64kbit bandwidth, DS0 uses standard digital encoding while ulaw encodes the ampliture in a logrithmic way. This improves the quality of quiet sounds but reduces the accuracy of loud ones so the overall quality is slightly better. It does however mean that it stops modems and faxes working properly.

It may seem like a minor point but I thought I would mention it as otherwise some people buy an analogue phone adapter and configure it to use ulaw and think that they will be able to use it  to link their modems to the new phone system. Even faxes running at 14.4 with ECM enabled can have problems.

Comment from mysticaljoey
Date: 02/09/2006 06:33PM PST
 Author Comment  

but most carrier does not support g711 as their primary codec is that true?


The gateway determines the what codec will be used.  Most carriers do not terminate thier own calls.  Most have multible agreements.  and each gateway can define which codecs it will accept/prefer.  Level 3 supports multible codecs but sends them out in a specific order.:  G.711a, G.729a, G.723.1.  The FXS depending on make will ahndle this differently.  You can always make a call in your prefered codec but, only cisco as far as i know can control how it recieves the codec.  Also remember if you limit yourself to one codec and call on NET and the other PBX uses a different codec "only" you will get a fast busy.