BGF VoIP Call Setup Overview
This topic describes how a call is set up using the BGF voice solution.
As shown in Figure 1, 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 1: Establishing a VoiP Call
The process of setting up a VoIP call in the network using SIP, as shown in Figure 1, is as follows:
- VoIP telephone A initiates a VoIP call to VoIP telephone B.
- VoIP telephone A sends a SIP message to the SIP server.
- The gateway controller (SIP server) sends an H.248 request for gate allocation from the virtual BGF.
- 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.
- 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.
- The virtual BGF sends an H.248 response providing allocated gate information to the gateway controller.
- The SIP server sends the modified SIP signaling (based on the gate information sent by the virtual BGF) to the destination VoIP telephone B.
- VoIP telephone B replies to the SIP request to the SIP server.
- The gateway controller updates the virtual BGF with the new information sent by VoIP telephone B.
- Steps 4 through 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.
- The SIP server sends the modified reply to VoIP telephone A.
- The call is established. Media streams can now flow through the routers’ open gates.
