BGF VoIP Call Setup Overview

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.
• Media (RTP/RTCP) stream for each leg of the voice call.

Figure 21: Establishing a VoiP Call

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

1. VoIP telephone A initiates a VoIP call to VoIP telephone B.
2. VoIP telephone A sends a SIP message to the SIP server.
3. The gateway controller (SIP server) sends an H.248 request for gate allocation from the virtual BGF.
4. The pgcpd process running on the Routing Engine sends IPC messages to the MultiServices PIC or MS-DPC requesting that the PIC or DPC open gates for each call leg.
5. The PIC or DPC creates the gates with the behaviors specified in the IPC messages, and it sends a reply to the pgcpd process. Gates are allocated in a Drop state.
6. The virtual BGF sends an H.248 response providing allocated gate information to the gateway controller.
7. The SIP server sends the modified SIP signaling (based on the gate information sent by the virtual BGF) to the destination VoIP telephone B.
8. VoIP telephone B replies to the SIP request to the SIP server.
9. The gateway controller updates the virtual BGF with the new information sent by VoIP telephone B.
10. Steps 4-6 are repeated, where the PIC or DPC is updated with the new information provided by the gateway controller. Gates are transitioned into a Forward state.
11. The SIP server sends the modified reply to VoIP telephone A.
12. The call is established. Media streams can now flow through the routers’ open gates.