est port, always an odd number; No header extensions are used. For each of the profiles listed in Table 7.4, the profile document lists details of the codec, or a reference for the details is provided. Payloads in the Related Protocols Table 7.4 RTP/AVP Audio and Video Payload Types Payload Codec Clock Description ITU G.711 PCM u-Law Audio 64 Kbps CELP Audio 4.8 Kbps ITU G721 ADPCM Audio 32 Kbps European GSM Audio 13 Kbps DVI ADPCM Audio 32 Kbps 16000 DVI ADPCM 64 Kbps Experimental LPC Audio ITU G.711 PCM A-Law Audio 64 Kbps ITU G.722 Audio 44100 Linear 16-bit Audio 705.6 Kbps 44100 Linear 16-bit Stereo Audio 1411.2 Kbps 90000 MPEG-I or MPEG-II Audio Only ITU G.728 Audio 16 Kbps 90000 CelB Video 90000 JBEG Video 90000 nv Video 90000 ITU H.261 Video 90000 MPEG-I and MPEG-II Video 90000 MPEG-II transport stream Video dynamic Internet low bit rate 15 Kbps [6] dynamic Adaptive Multirate Codec [12] range 96-127 can be defined dynamically during a session. The minimum pay- load support is defined as 0 (PCMU) and 5 (DVI4). The document recom- mends dynamically assigned port numbers, although 5004 and 5005 have been registered for use of the profile and can be used instead. The standard also describes the process of registering new payload types with IANA. There are other references for a tutorial description of many of these audio codecs [10] and video codecs [11]. The information in the first three columns of Table 7.4 is also contained in the SDP a=rtpmap field, which is why the attribute is optional. SIP: Understanding the Session Initiation Protocol PSTN Protocols Three types of PSTN signaling protocols are mentioned in this text: Circuit Asso- ciated Signaling (CAS), ISDN (Integrated Services Digital Network), and ISUP (ISDN User Part). They will be briefly introduced and explained. How these pro- tocols work in the PSTN today are covered in other references [10]. 7.4.1 Circuit Associated Signaling This type of signaling is the oldest currently used in the PSTN today. The sig- naling information uses the same audio