verage size is just above minimum Consequences Payload Reduced bandwidth demand Headers less error sensitive (smaller) Reduced delay, since fewer frames can be sent in each packet Compressed header Figure 7.13 Compression of voice packet headers can have positive effects as well 7.5 QOS ENABLERS In order to enable real-time services over IP three protocols will be briefly presented: RTP, RTCP and RTSP. For QoS adaptation of ATM to various traffic types the three adaptation layers AAL 1, AAL 2 and AAL 5 are provided. Differentiated services and integrated services are used for resource allocation, as well as MPLS, which is also a traffic-engineering tool. All these enablers will be briefly treated. See also Figure 7.14. Signalling Voice, video Voice, video Media codec encapsulation H.323 protocols Diffserv IPv4, IPv6 IP/ATM) Encapsu- lation IP/WLAN IP/GPRS IP/WCDMA PLMN/PSTN/ IP/CDMA Ethernet IP/DWDM PDH/SDH/ Radio bearers, SONET PDH/SDH/SONET/xDSL Figure 7.14 QoS adaptation protocols, resource allocation and traffic engineering protocols Understanding Changing Telecommunications - Building a Successful Telecom Business 7.5.1 ADAPTATION-TYPE ENABLERS RTP (RFC 1889) RTP provides end-to-end delivery services for data with real-time characteristics, such as interactive and streaming audio and video, including IP telephony. RTCP (real-time control protocol) is a part of RTP and helps with lip synchronization and QoS management. RTP is used by RTSP, SIP and H.323 for the user data portion of these protocols. The RTP services include payload type identification, sequence numbering, time stamp- ing and delivery monitoring. The time stamp is related to the sampling or the presentation or composition time of the media carried in the payload of the RTP packet. It is used for playing back media at the correct speed, and together with RTCP, it is used for syn- chronizing the presentation of other streaming media. Applications typically run RTP on top of UDP to make use of its multiplexing and checksum services. Both protocols