Overview of a Voice Call Setup

As shown in Figure 21, VoIP uses two streams:

• Signaling stream, which handles the agreement to set up calls. The signaling stream can use Session Initiation Protocol (SIP) or other signaling protocols.
• RTP stream for each leg of the voice call.

Figure 21: Establishing a Voice Call

The process of setting up a VoIP call in the network shown in Figure 21 is as follows:

1. Customer A initiates a VoIP call to Customer B.
2. The VoIP phone sends a SIP message to the SIP server, which establishes a session with Customer B.
3. The SIP server directs the PGC to request (using a H.248 message) that the VPG on the packet gateway open a gate.
4. The pgcpd process in the Routing Engine sends IPC messages to the PIC requesting that the PIC open gates for each call leg.
5. The PIC creates a gate with the behaviors specified in the IPC messages, and it sends a reply to the pgcpd process.
6. The pgcpd process sends a response message to the PGC.