ple 8-8 shows filtering of the higher bandwidth codecs by dial-peer 1. With this configuration, codecs other than those specified are disallowed by calls matching dial- peer 1 for routing. Example 8-8 Codec Filtering Configuration Commands voice class codec 1 codec preference 1 g729br8 codec preference 2 g723r53 codec preference 3 g723r68 dial-peer voice 1 voip voice-class - codec 1 Transcoding Codec filtering restricts or controls the codec that can be negotiated by the endpoints. The resulting call uses a single codec end-to-end. Another network interworking option transcoding where a larger selection of external codecs can be enabled even if they are incompatible with the codecs you want to use internally on your network. With transcoding you change the codec at the network border SO that two different codecs are used on different legs of the call. The mechanism of transcoding is illustrated in Figure 8-4. One half of the call uses G.711 whereas the other half uses G.729. DSPs are required for transcoding, and you should generally attempt to avoid transcoding between two low-bitrate codecs because the voice 154 SIP Trunking quality might become impaired to the point of causing end user dissatisfaction. Transcoding between a high-bitrate codec (such as G.711 or G.722) and a low-bitrate codec (such as G.729 or iLBC) is common practice and results in acceptable voice quality. However, transcoding a single call multiple times or transcoding between two low-bitrate codecs (such as G.729A and iLBC) usually results in poor voice quality and should be avoided. Transcoding for contact center calls should also generally be avoided because voice quality is of the utmost importance in providing a superior caller experience when interacting with a contact center customer service application. RTP (G.711) RTP (G.729) H.323 Figure 8-4 Codec Transcoding Transrating The codec (such as G.711 or G.729) specifies the speech sampling rate, encoding algo- rithm, and compression technique used to encode the voice. But it does not spe