ketization period of the codec of a single call in the middle of the stream; for example, one endpoint uses G.729A 20 ms whereas the other endpoint on the same call uses G.729A 30 ms, and a device in the middle of the stream transrates the RTP stream from G.729 2 20 ms to G.729A30 ms packetization) Payload Types All the different voice and video codecs and ancillary traffic such as DTMF relay and voice-band data (for example fax and modem) are indicated in the RTP protocol by a des- ignated payload type. Payload types assigned by the standards are given in RFC-1890 and RFC-3551. The range between 96 to 127 is unassigned and listed as dynamic, which means vendor implementations can use these as they prefer. This is the range where incompatibilities occur and where it becomes important to configure or convert (when configuration is not possible or desired) between a payload type for a specific type of traffic. Example 8-6 shows an example of explicit mappings of payload-types on a Cisco IOS dial-peer. Payload-type 96 is used by default for fax by Cisco IOS code. However, Cisco Telepresence systems also use this by default for the Advanced Audio Coding Low Delay (AAC-LD) codec. The configuration here assigns 96 to the Telepresence AAC-LD codec and maps fax to payload type 110 instead. Example 8-6 Payload Type Configuration Commands dial-peer voice 11 voip description payload type conversion destination pattern 408555. rtp payload type cisco-codec - fax - ind 110 rtp payload type cisco - codec - aacld 96 rtp payload-type cisco codec - video h264 112 session protocol sipv2 session target ipv4:10.10.10.1 dtmf-relay rtp-ntc no vad 152 SIP Trunking Example 8-7 shows the payload types assigned in Cisco IOS in the 96 to 127 range and the keywords that can be used to change the assignment if needed. Example 8-7 Payload Type Options in Cisco IOS c2851 - cube2 (config-dial-peer)#rtp payload-type ? cisco cas payload Cisco CAS RTP payload cisco - clear - channel Cisco clear channel RTP payload cisco codec aacld Cisco codec AACL