On 10.02.2007, at 17:44, Frank Bulk wrote:
Andreas:
On what basis do you say that most modern IP phones use G.729? Is there a certain class of IP phones (PacketCable, Vonage, 8x8, enterprise (Cisco, Avaya, etc), VoFi) that you had in mind?
Frank
G.729 is one of the best codecs when it comes to efficiency and providing excellent quality. For software implementations its a bit problematic because of patent issues. The hardware versions don't have this because you simply buy chips which do G.729 and youre done because the chips vendor has a license for G.729 and they usually do much bigger volumes than a small software company developing a soft phone.
If you compare the different codecs out there:
G.726 ADPCM is also good but doesnt compress so much so uses quite a bit of bandwith. Perfect choice for "a little bit of compression but not too much".
G.711 dont compress and waste a lot of bandwith (80kbps)
GSM codec is popular on soft phones and open source because its free to use (even there are patents about it).
G.728 is not so good in quality and not so efficient than G.729 but less CPU intensive.
G.723 is the one which compresses most (as low as 5.3kbps without IP headers) but its clearly audible and quality is not considered very good. But useful for applications where minimizing the bandwidth is more important than quality.
From the experience with IP phones, especially WiFi IP phones, we learned that those people who care about quality and provide phones which actually work (there's unfortunately a lot of crap out there too), they usually implement G.726, G.729, GSM and of course G.711 a-law and G.711 µ-law for the guys with tons of bandwidth available.
The Cisco IP phones (Desk phones) as far as I know do G.711 and G.726. UTS Starcom does G.711, G726 and G.729. The Hitachi WiFi Phones WIP3000 and WIP5000A do G.711, G.729 and GSM. Asterisk, the open source PABX does G.711, G.729 (not for free), GSM and I think also G.726 and a few other codecs. The Grandstream phone adapters also do G.729
I wont mention any "fake" wireless IP phones like the wireless Skype phones. Those are nothing else than a microphone and a loudspeaker connected to the computer and all the processing is done on the computer. So you get whatever codec the soft phone has and the phone is just dumb. I prefer real WiFi phones like the Hitachi Cable WIP 5000A which you can use on any accesspoint and just work. And I prefer G.729 because its excellent quality.
Just my personal opinion...