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

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The process of setting up a VoIP call in the network using SIP, as shown in Figure 21, 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 PGC (SIP server) sends an H.248 request for gate allocation from the VPG.
The pgcpd process running on the Routing Engine sends IPC messages to the PIC requesting that the PIC open gates for each call leg.
The PIC 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 VPG sends an H.248 response providing allocated gate information to the PGC.
The SIP server sends the modified SIP signaling (based on the gate info sent by the VPG) to the destination VoIP telephone B.
VoIP telephone B replies to the SIP request to the SIP server.
The PGC updates the VPG with the new information sent by VoIP telephone B.
Steps 4-6 are repeated, where the PIC is updated with the new information provided by the PGC. 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.