Transcoding translates a media stream encoded by using one codec into a media codec encoded by using another codec. Avaya SBC performs transcoding when the inbound and outbound entities have incompatible codecs. The Session Description Protocol (SDP) offer contains information about the codecs that the device sending the message prefers. The device that receives the message responds to the SDP offer by using the set of codecs that the receiving device supports.

Transrating reduces the bit rate of the media while retaining the original media format. Transrating is required where bandwidth is a constraint, for example, on the Wide Area Network (WAN). Enabling transrating results in a lesser number of packets and packet overhead because the packetization period is increased. For example, the packetization period (ptime) is 40 ms on WAN and 10 ms on an internal enterprise network for the same codec. For this example, transcoding is not required, but transrating is required because the packetization period for the same codec is different between inbound and outbound streams.