what is the difference between G711a and G711u ?
which is better

im using

internode 512/128
spantalk, sipme, brown bear
billion 7401vp

reubens writes...

what is the difference between G711a and G711u ?

In the traditional digital voice world there are two main methods of encoding analogue voice onto an 8khz/8bit (ie. 64kbps) voice channel that are used in the world. These are ulaw and alaw. G711a is alaw and G711u is ulaw.

Basically, the one you use depends where in the world you are. In Australia (and basically most of the world outside the USA) we use alaw. So, a call made with G711a can be more easily turned into a TDM voice call when it hits the PSTN.

In reality - it's a minor difference for voice and since a lot of equipment is made in the USA that providers use, often G711u for VOIP is prefered - the equipment reencodes before putting it onto the PSTN.

Matthew Moyle-Croft writes...

a lot of equipment is made in the USA that providers use, often G711u for VOIP is preferred

For exactly this reason I find it a good idea when signing up with a VSP to not force any codec but to let the VSP select the codec and then see which one their equipment prefers.

For this purpose I use X-Pro Softphone. In the settings set it to 'let provider decide on codec'. On the main screen however disable g729 as most VSP's prefer this as it uses much less bandwith.

Using Pennytel and Spantalk at the moment. Pennytel prefers g771u where as Spantalk prefers g711a and the best quality connections have always been made with the VSP provider preferred codecs although the difference is not that big. Still, reducing the amount of transcoding can only be a good thing IMHO.