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Cisco IP Phone Audio Codecs

Nov. 23, 2009
Doug McKillip

Cisco IP phones support a variety of different audio codecs. In this post, I will explain some of the differences and explain which versions of CUCM and the Cisco IP phones support the various audio codecs.

Audio codecs are responsible for sampling human speech (a sine waveform) and representing human speech on a communications network (IP in our case). The digital signal processor (DSP) on the Cisco IP phone is responsible for sampling voice and the ASIC (application specific integrated circuit) is responsible for packetizing the audio sample with an IP/UDP/RTP header.

All 7900 series Cisco IP phones support the G.711 and G.729 audio codec. G.711 is a 64kbps audio codec that has been used on the public switched telephone network (PSTN) for many decades. G.711 is considered a toll quality audio codec. G.711 utilizes 80kbps of bandwidth on the network due to the 16kbps of overhead incurred when sampling voice every 20 milliseconds (ms). G.711 has a mean opion score (MOS) rating of 4.0.

The G.729 audio codec is a compressed audio codec, normally utilized on wide area network (WAN) links where bandwidth is limited. G.729 calls are sampled at 8kbps. The packetization overhead of 16kbps brings G.729 call bandwidth consumption to 24kbps without factoring layer 2 link overhead. G.729 voice has a MOS rating of 3.8, bringing the user experience very close to that of G.711 voice. G.729 quality degrades fast in unfavorable network conditions (packet loss). Packet loss must be kept under 1% to maintain high quality G.729 phone calls.

Cisco introduced the G.722 and iLBC audio codecs in Cisco Unified Communications Manager (CUCM) 6.0. G.722 and iLBC will work on any Cisco Type B phone (7906, 7970, 79x1, 79x2, 79x5), but will not work on the older Cisco Type A phones (7905, 7910, 7912, 7940, 7960). G.722 is a high definition audio codec that provides much higher audio fidelity than G.711. G.711 has a frequency response range of 8khz, while G.722 has a range of 16khz. A wideband audio handset is required to fully experience the quality of G.722. The wideband audio handset ships with all 79x2 and 79x5 phone models, but can be ordered for the 7970 or 79x1 model phones. G.722 uses compression and has a bandwidth impact of 64kbps before packetization (same as G.711).

iLBC (Internet Low Bandwidth Codec) is an Internet Engineering Task Force (IETF) open standard that has been used by Skype. iLBC is a compressed audio codec that utilizes 16kbps of bandwidth (32kbps after packetization). iLBC uses 8kbps more bandwidth than the G.729 compressed audio codec, but the quality of iLBC does not degrade as fast as G.729 when packet loss occurs.

Author: Dennis Hartmann

Editor's Note: For more on this topic, consider taking the CVOICE: Cisco Voice over IP training class.