Is G729 codec 32kbps or 8 kbps?

Question

I'm building a VOIP app for iphone and android. I'm currently using the GSM codec ( I chose it arbitrarily) on both versions of my app and on my asterisk server.

Now that I'm fine tuning my app, I'd like to try different audio codecs. I'm considering G729. I did a research and wasn't sure why some sites say the G729 codec uses about 32kbps as in this site here

http://voip.about.com/od/voipbandwidth/f/How-Much-Of-My-Mobile-Data-Plan-Does-Voip-Consume.htm

while others say it is 8kpbs like this site here

http://www.javvin.com/protocolG7xx.html

I did some tests and it seems that 1 minute of conversation with the G729 codec uses up 0.5 mb of data. So it seems like the first link is correct. But i've seen other sites list similar stats of 8kbps...why the discrepancy?

Answer 1

If you look towards the bottom of the first link you show, it hints at the reason - the 8kbps is how much is used to encode the speech itself. You then need to send that encoded speech out over the network to the other end of the VoIP call, and hence need to pack it into an IP 'packet', typically using the RTP protocol.

The actual number of bits transmitted will depend on the number of samples taken per second, the number of samples packed into each IP packet, the protocol headers etc. Much of this is influenced by the codec chosen - the following link gives a good overview (see the table in the section titled 'VOIP - Per Call Bandwidth'):

http://www.cisco.com/en/US/tech/tk652/tk698/technologies_tech_note09186a0080094ae2.shtml