ard: there are many vendor selectable options which makes interoper- ability hard to reach. SIP (Session Initiation Protocol) Some short facts about SIP: A simple, flexible, and generic, signalling protocol. HTTP-style, text-based. SIP handles call signalling, see below. Handles all types of conversations. Scales better than H.323. SDP (Session Description Protocol) First, you need to tell the other party what kind of media you want to receive: audio, video, or both. The second thing is how you want the media to be coded SO that you can understand what is being sent. Understanding Changing Telecommunications - Building a Successful Telecom Business The third thing you need to inform the other party about is the address and port you want the media to be delivered to. SDP communicates several session descriptors: Session name and purpose Media type (e.g. audio, video etc.) Media transport protocol (RTP/UDP/IP, H.320 etc.) Media format (e.g. for video, H.261 or MPEG, etc.) Information on how to receive media (e.g. address, port, format etc.) Period of time the session is active 8.8.3 QOS PLAN The subject of QoS is treated in Chapter 7. In this chapter a summary of the connection between services and QoS is given. There are in particular two groups of services that depend heavily on the QoS plan: streaming class and conversational class. Let us start with a simple comparison between streaming and download techniques and requirements. Adaptations are necessary when using packet networks for 'ear or eye' sensitive ser- vices. When information is streamed you look at it with a short delay. When downloading you don't look or listen during the process. Figure 8.29 shows streaming and downloading over IP. The differences between the transfer methods are obvious: three dedicated adaptations are or can be used for streaming: Streaming application Download application Synchronization Synchronization and layout and layout Media Scene Session Media Scene Download Playback decoders description control decoders control descriptio