tailieunhanh - emerging wireless multimedia services and technologies phần 3

Như đã đề cập, RTP là một khung giao thức cho phép sự hỗ trợ của mã hóa và các tính năng mới. Mỗi ứng dụng dựa trên RTP / RTCP, đặc biệt là kèm theo một hoặc nhiều tài liệu: a proï ¬ le speciï ¬ cation tài liệu, mà dei ¬ nes một tập hợp các mã số loại tải trọng và lập bản đồ của họ sang các định dạng tải trọng | 72 Multimedia Transport Protocols for Wireless Networks RTP Payload Types As already mentioned RTP is a protocol framework that allows the support of new encodings and features. Each particular RTP RTCP-based application is accompanied by one or more documents a profile specification document which defines a set of payload type codes and their mapping to payload formats . media encodings - a profile for audio and video data may be found in the companion RFC3551 27 payload format specification documents which define how a particular payload such as an audio or video encoding is to be carried in RTP. RTP Profiles for Audio and Video Conferences RFC3551 RFC3551 lists a set of audio and video encodings used within audio and video conferences with minimal or no session control. Each audio and video encoding comprises a particular media data compression or representation called payload type plus a payload format for encapsulation within RTP. RFC3551 reserves payload type numbers in the ranges 1-95 and 96-127 for static and dynamic assignment respectively. The set of static payload type PT assignments is provided in Tables and see column PT . Payload type 13 indicates the Comfort Noise CN payload format specified in RFC 3389. Some of the payload formats of the payload types are specified in RFC3551 while others are specified in separate RFCs. RFC3551 also assigns to each encoding a short name see column short encoding name which may be used by higher-level control protocols such as the Session Description Protocol SDP RFC 2327 25 to identify encodings selected for a particular RTP session. Mechanisms for defining dynamic payload type bindings have been specified in the Session Description Protocol SDP and in other protocols such as ITU-T Recommendation . These mechanisms associate the registered name of the encoding payload format along with any additional required parameters such as the RTP timestamp clock rate and number of channels with