September 1, 2010

voip codecs and bps

  • g.711 is raw data, highest quality, but requires highest bandwidth 
  • g.729a is the next best, very good quality, very low datarate, but it requires a $10 license per channel for asterisk. There is a free version for research and education use at
  • iLBC may be the next best, it is free, good quality, and relatively low datarate. Go to this link to find out how to add iLBC back to Asterisk. Remember, you will need to copy the original Makefile in the ilbc folder to the new ilbc folder.
  • GSM quality is acceptable, but not very good.
  • G722 is wide band, hi-def stuff. 

Real bps numbers:
  • GSM: 30kbps
  • g711: 80kbps
  • iLBC: 30kbps
  • g729: 30kbps (why it is this high? bps seen by bwm-ng)
* G.711 has a Mean Opinion Score of 4.3-4.7 and uses 80 kpbs (if you send 50 packets/second with 20 ms of RTP payload per packet) or 74.7 kbps (@ 30 ms, meaning 33.3 packets/second).
* G.729 (NOT G.729a) has a MOS of 3.9-4.2 and uses 24 kbps @ 20 ms or 18.7 kbps @ 30 ms.
* G.729a has a MOS of 3.7-4.2 and uses 24 kbps @ 20 ms or 18.7 kbps @ 30 ms.
* G.723 has a MOS of 3.8-4.0 and uses 17.1 kbps @ 30 ms.

MOS is what nontechnical people think about each codec (5.0 is perfect). All of the above numbers are in EACH direction, so total bandwidth is double the above figures.

As you can see, there is very little quality or bandwidth difference betweeen G.729a and G.723. However, G.729a can send 50 packets/second, each packet containing 20 ms of voice payload. G.723's lowest setting is 30 ms. I think 20 ms sounds better than 30 ms because of smoothing (used to fill in for late packets). That's why I chose G.729a @ 20 ms over G.723 @ 30 ms for my Sipura adapter. (Sipura's default setting is 20 ms (that is, 'RTP Packet Size' = '0.020').)

It would be a very good idea to turn on silence suppression ('Silence Supp Enable' = 'yes'), because it will reduce your bandwidth usage by 65%. (Apparently each person in a two-person conversation only talks about 1/3 of the time.) With my Sipura 2100, I could not hear any difference between silence suppression being on or off.

G729 uses less compression (less latency problems) and has a higher level of voice quality BUT does use more bandwidth. I personally am not fond of g723.1 but usually find g729 to be OK. Some people despise both.

Packet8 started using g723.1 and later switched to g729 most likely due to customer complaints about the poor quality of g723.1 calls.

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