-up and replay, and fast forward streaming media; or to stop streaming and disconnect the session. RTSP is usually used on top of TCP but can also be used on top of UDP. RTP and RTSP are used together in many systems, but either protocol can be used without the other. Quality of Service RTSP establishes and controls either a single or several time-synchronized streams of continuous media such as audio and video. ATM Adaptation See also the parts on ATM in section 7.4. In order to take care of all kinds of services ATM has its own protocol stack. It consists of three layers: 1. The ATM adaptation layer (AAL). It is here that 48-byte data units with AAL and payload are created. 2. In the ATM layer the five-byte header is added. 3. The physical layer takes care of the mapping of the bits onto the physical medium. ATM is designed for all types of media. Different services demand different kinds of adaptation support and therefore different kinds of AAL protocols. AAL1 takes care of constant bit rates, especially voice coded with 64 kbit/s. AAL2 takes care of variable bit rates like compressed voice and video. AAL 2 is a 'small cell' transfer mode of its own with dedicated AAL 2 switching. AAL5 takes care of data. See Figure 7.15. 7.5.2 NETWORK-DRIVEN QOS COMPONENTS 802.1p, Ethernet QoS The idea with 802. 1p is the use of flags in the media access control (MAC) header to estab- lish packet priority (not reservation!) in shared-media 802 networks. 802.1p awareness is requested for devices, hubs and switches. The issue of differentiating between network packets, and perhaps treating them dif- ferently according to the applicable QoS differentiation, is done by means of the MAC Constant Variable bit rate bit rate bursts bursts Uncompressed Compressed voice or video voice or video ex. IP packets AAL 1 AAL 2 AAL 3/4 AAL 5 Physical Figure 7.15 Adaptation protocols in ATM Understanding Changing Telecommunications - Building a Successful Telecom Business header. The MAC header (the lower half of Layer 2 in the ISO OSI Model) 