Why does slower codec work better?

Posts: 6 · New Forum Member Joined: Jan 12, 2006 Posts: 6

I was just wondering why on testyourvoip, the g729 gives me a consistent MOS score of 3.6-3.8, and the g711 gives 2.9-3.2? Rarely will it return the 4.2-4.3 it should. (It will suprise me sometimes though). The g711 usually results in high packet loss and discards, while the g729 results in high discards, but no so much loss.

Any input is greatly appreciated.

Posts: 350 · Vonage Forum Evangelist Joined: Dec 04, 2005 Posts: 350

G711 typically uses a payload of 20 which G729 typically uses a payload of 30. The payload is the amount of milliseconds of audio in each packet. Fewer (and smaller) packets usually leads to less packet loss.

Posts: 6 · New Forum Member Joined: Jan 12, 2006 Posts: 6

My experience has been the opposite. The G711 has resulted in higher packet loss and lower MOS scores on testyourvoip. I just thought that it was strange.

Posts: 350 · Vonage Forum Evangelist Joined: Dec 04, 2005 Posts: 350

redryder77 wrote:

My experience has been the opposite. The G711 has resulted in higher packet loss and lower MOS scores on testyourvoip. I just thought that it was strange.

??

G711 (in Vonage-speak the "Higest" quality setting in the Bandwidth Saver) uses 64 kilobits per second in both directions. With IP overhead it comes out to about 90 kilobits per second. Using the typical payload of 20, that's 50 packets per second.

G729 (again, in Vonage-speed "Normal" quality) uses approximately 16 kilobits per second. With IP overhead it comes out to about 30 kilobits per second. Using the typical payload of 30, that's 33 packets per second.

Now, if my math is correct (and since its 2:30AM after doing a 10 hour shift)...

G711 with payload 20 will send out 50 packets per second with a size of approx. 1.28 kilobits (1311 bits).

G729 with payload 30 will send out 33 packets per second with a size of approx. 0.48 kilobits (492 bits).

So... G729 sends out fewer, smaller packets than G711.

Posts: 1715 · Posted: Tue Jan 24, 2006 1:37 pm Post subject:

Very simply, G.711 uses more bandwidth than G.729. If your connection isn't able to keep up, which your scores show it isn't, you're going to see this type of behavior. Take a 33kbps dialup connection. It works fine for web browsing, even works for low bitrate web streaming. Try to push a 100kbps video stream through it and you'll find plenty of dropped packets & packet discards. If you're only getting a testmyvoip score of 2.9-3.2 with G.711, you need to contact your ISP and get them to fix your connection. Even the worst broadband connection should be able to get better than that.