Praise for Fundamentals of WiMAX

240 Chapter 7 • Networking and Services Aspects of Broadband WirelessApplication LayerSIP ApplicationServersOSAGatewayWeb ServicesParlay ASSessionControl LayerHSSBillingDNSP-CSCFI-CSCFS-CSCFBGCFCommonResourcesMedia and End-pointLayerMRFMGCFIP Core NetworkMGWSS73G CellularAccessNetworkWiMAXNetworkFixed-LineBroadband-AccessNetworksPSTNUEUEUEUEUEUEFigure 7.7 The 3GPP IMS architecture7.2.2 Real-Time Transport ProtocolSIP provides the necessary session-control functions but is not used for transporting the mediastream. RTP, defined in RFC 1889 [57], is the most popular transport protocol used for transferringdata in multimedia sessions. RTP was developed because traditional transport protocols,such as TCP and UDP, are not suitable for multimedia sessions: TCP offers no delay bounds,and UDP does not guarantee delay or packet loss. RTP typically runs over UDP and providesordering and timing information suitable for real-time applications, such as voice and video,over both unicast and multicast network services. The RTP header contains content identification,the audio/video encoding method, sequence numbers, and timing information to ensure thatpackets are played out in the right order and at a constant rate. The timing information facilitatesjitter calculation that allows receivers to adopt appropriate buffering strategies for smooth playout.RTP is implemented along with RTCP (real-time control protocol), which manages the trafficflow. RTCP provides feedback on the quality of the link, which can be used to modifyencoding schemes, if necessary. By using timing information, RTCP also facilitates synchronizationof multiple streams, such as audio and video streams associated with a session. Synchronizationacross multiple sources, however, requires use of the network timing protocol (NTP).RTCP also provides support for real-time conferencing of groups. This support includes sourceidentification and support for audio and video bridges, as well as multicast-to-unicast translators.RTP and RTCP do not reduce the overall delay of the real-time information or make anyguarantees concerning quality of service.