1. A message body in this response can be used to carry music on hold or other media. SIP: Understanding the Session Initiation Protocol Caller Call center INVITE 1823 3 Ahead of you 182 2 Ahead of you 182 1 Ahead of you 200 OK Media session Figure 5.1 Call queuing example with call processing center. 5.1.5 183 Session Progress The 183 Session Progress response indicates that information about the progress of the session (call state) may be present in a message body or media stream. Unlike a 100 Trying response, a 183 is an end-to-end response and does establish a dialog (must contain a To tag and Contact). Unlike a 180, 181, or 182 response, however, it does not convey any specific information about the status of the INVITE. A typical use of this response is to allow a UAC to hear ring tone, busy tone, or a recorded announcement in calls through a gateway into the PSTN. This is because call progress information is carried in the media stream in the PSTN. A one-way media connection or trunk is estab- lished from the calling party's telephone switch to the called party's telephone switch in the PSTN prior to the call being answered. In SIP, the media session is established after the call is answered-after a 200 OK and ACK have been exchanged between the UAC and UAS. If a gateway used a 180 Ringing response instead, no media path would be established between the UAC and the gateway, and the caller would never hear ring tone, busy tone, or a recorded announcement (e.g., "The number you have dialed has changed, the new number is ") since these are all heard in the media path prior to the call being answered. Figure 5.2 shows an example where a SIP caller does not hear a recorded announcement coming from the PSTN. Figure 5.3 shows the use of the 183 Session Progress allowing an early media session to be estab- lished prior to the call being answered. The PSTN interworking scenarios in Chapter 10 show this in detail. SIP Response Messages Caller Gateway Telephone switch INVITE sip:555-1111@gateway.com;user=phone IAM Cd