nt exchanges may be addressed directly to the user agent, without traversing any server. The normal exchange of SIP messages is illustrated in Figure 14.9. Translation between telephone number and SIP address is performed in a gateway at the PSTN-IP border. SIP uses other protocols for specific tasks, for example DNS for user-domain location and SDP for media selection. 3GPP is dedicated to using SIP for call control. 14.4.2 SESSION DESCRIPTION PROTOCOL (SDP) Session descriptions have a list format containing information about the session (see RFC 2327). Message headers are in plain text and look similar to e-mail headers. SDP is designed to convey session information to recipients and media information that pertain to the session. It allows more than one media stream to be associated with a session. SDP inform the receivers of its messages the existence of a session. It also conveys sufficient information to enable joining and participating in the session. See also the signalling plan in Chapter 8. Understanding Changing Telecommunications - Building a Successful Telecom Business 14.5 CIRCUIT MODE DOMAIN The circuit domain embraces mobile and fixed networks mainly intended for voice. The typical circuit mode feature is the functions of the servers, whereas the traffic path normally migrates to become packet based. Data traffic can manage with the connectivity layer. See Figure 14.10. Let us start with the mobile servers, which are called MSC and TSC/GMSC (TSC = transit switching centre). 14.5.1 MSC SERVER The MSC server remotely controls the circuit-switched part of the MGW and initiates set-up/release via the gateway for the user plane. The control is supported by the gateway control protocol (GCP, H 248). When needed, the MSC server connects devices to the payload path (e.g. codecs, tone senders). Figure 14.11 gives an overview. The MSC server also supports the radio access network application part (RANAP) for control signalling over the RNC-MSC interface, SO that radio access bearers can be established and re