When an IP endpoint calls another IP endpoint, Communication Manager requests that the second endpoint choose the same codec that the first endpoint offered at call setup. However, if the second endpoint cannot match the codec of the first endpoint, the call is set up with the preferred codec for each endpoint. The data streams are converted between the endpoints, often resulting in degraded audio quality because of the different compressions or decompressions or multiple use of the same codec. For more information, see IP CODEC sets.

When a station or trunk initially connects to the server, Communication Manager selects the first codec that is common to both the server and the endpoint. The Inter Network Region Connection Management screen specifies the codec sets to use within an individual region (intra-region) and between or among (inter-region) network regions. If the endpoint and the G4xx Media Gateway or Avaya Aura® Media Server are in the same region, the administered intraregion codec set is chosen. If the endpoint and the G4xx Media Gateway or Avaya Aura® Media Server are in different regions, the administered inter-region codec set is chosen.

For example, a region might have its intranetwork codec administered as G.711 as the first choice, followed by other low bit rate codecs. The Inter Network Region Connection Management screen for the internetwork region might have G.729, a low-bit codec that preserves bandwidth, as the only choice. Initially, when a call is set up between these two interconnected regions, the G4xx Media Gateway or Avaya Aura® Media Server provides the audio stream conversion between G.711 and G.729. When the media stream is shuffled away from a TDM-based connection, the two endpoints can use only the G.729 codec.