Codecs (Coder-Decoders) convert analog voice signals to digital signals and vice versa. Avaya supports several different codecs that offer varying bandwidth usage and voice quality. The following are some codecs that Avaya supports:

• G.711: This codec produces uncompressed audio at 64 kbps.

• G.729: This codec produces compressed audio at 8 kbps.

• G.723.1: This codec produces compressed audio at 5.3 or 6.3 kbps.

• G.722: This codec produces compressed audio at 64, 56, or 48 kbps.

• G.726: This codec produces compressed audio at 32 kbps.

The following table provides a comparison of voice quality considerations associated with some of the codecs supported by Avaya products.

Toll-quality voice must achieve a mean opinion score (MOS) of 4 or above. The MOS scoring is a long-standing, subjective method of measuring voice quality.

Table 1: Comparison of speech coding standards (without IP/UDP/RTP overhead)

Standard	Coding Type	Bit Rate (kbps)	MOS-LQO (Mean Opinion Score - Listening Quality Objective)
G.711	PCM	64	4.37
G.729	CS-ACELP	8	3.94
G.723.1	ACELPMP-MLQ	6.3 5.3	3.78 3.68
G.726	ADPCM	32	4.30
1 As predicted. Measured according to ITU-IT Recommendation P.862 (PESQ). See draft Recommendation P.862.2, application guide for PESQ. 2 Given MOS-LQO values for American English.

In a properly functioning IP network, the G.711 codec offers the highest level of voice quality as the codec does not use compression. Unfortunately, there is a trade-off with higher bandwidth usage. In situations where bandwidth is limited, such as across WAN links, G.729 provides good audio clarity and consumes less bandwidth.

Codecs with compression use twice as many signal processing resources than the G.711 codec. On the G430 media gateway, there are 120 DSP resources. Therefore, one media gateway supports:

• A maximum of 120 connections that use the G.711 codec

• A maximum of 60 connections that use the G.729 codec with compression

The G450 media gateway supports:

• 320 channels of G.711 (u/a-law)

• 320 channels of G.729A/G.729AB

• 320 channels of G.726 (32 kbps only)

• 320 channels of T.38

• 320 channels of V.32 SPRT

The above channel counts are the same if Advanced Encryption Standard (AES) encryption and SHA-1 authentication are enabled.

The Avaya One-X Deskphones (96xx) support the G.722 codec with 64 kbps and with 20 ms packets.

Usually, G.711 is used on LANs because bandwidth is abundant and inexpensive whereas G.729 is used across bandwidth-limited WAN links. G430 and G450 Branch Gateways can have varying amounts of DSP resources by adding more number of DAR daughter cards installed.