When making a call over the Internet, the software (soft-phone) or hardware needs to use a codec to send/receive information in a certain format and convert it to the sounds you hear. Codec stands for code and decode, and is the software that converts the signal from your handset into packets of data that can be sent through the internet. For more details, have a look at http://computer.howstuffworks.com/ip-telephony.htm
Generally, a codec with a higher bandwidth requirements provides better voice quality (If your Internet connection is fast enough to support the codec). Most VoIP providers/hardware/licensed software will support G.711 and G.729 (However be sure to check this before purchasing hardware, or signing up with a VoIP provider!). This is because the G.729 codec is licensed and the VSP has to pay to use it. This charge is obviously passed to you in the form of usage fees.
The G.711 codec requires a connection almost 3 times faster than that required by the G.729 codec so you should not use G.711 unless you have lots of bandwidth.
If you are using a free soft-phone, then G.729 will not be available to you; however, the GSM codec should be, and will give you similar call quality to that of a mobile phone.
The Difference Between Codecs
The g711 and g729 codecs are very different to each other. g711 basically sends all data without compression – very high quality. It requires a relatively high bandwidth and a good service provider. Because there is no compression, there is a greater chance of packets of information being heard out of order. u/a are very similar, but one is used in the US more (g711u) and the other in Australia (g711a). BTW u and a do not stand for the country.
g729 is the highest quality compressed codec available. because it is compressed, sent, then decompressed at the other end, the sound ‘picks up some time’ in transit and hence there is a slight buffering effect giving the codec a greater chance of correcting ‘out of order packets’, and delays. Given telephone conversations happen in real time, the buffering effect has to be in the order of milliseconds otherwise you wouldn’t be able to hold a normal conversation.
A common question from new users is “how much bandwidth will my VoIP service use?”. The short answer is not very much. It
does depend on which codec you use. The following table lists some common codecs and the bandwidth usage.
Codec……………..Bandwidth Usage (Up/Down
G.711u (64 Kbps)…….87.2 Kbps (ulaw – preferred by US and Japan)
G.711a (64 Kbps)…….87.2 Kbps (alaw – preferred by rest of the world)
G.729 (8 Kbps)……..31.2 Kbps
G.723.1 (6.3 Kbps)….21.9 Kbps
G.723.1 (5.3 Kbps)….20.8 Kbps
G.726 (32 Kbps)…….55.2 Kbps
G.726 (24 Kbps)…….47.2 Kbps
G.728 (16 Kbps)…….31.5 Kbps
GSM (7 or kbps)…….low
There is a useful tool at http://www.faktortel.com.au/bandwidthcalc.shtml that you can use to work out how much data you will use. But to give you an idea, a 60 min conversation using the common G729 codec will use just 13.7 MB.