IMSG VoIP Solution Architecture

Figure 27 shows the main components of the IMSG architecture.

Figure 27: IMSG Architecture

BGF

In the IMSG architecture, the BGF controls VoIP signaling and media based on instructions that it receives from the Service Policy Decision Function (SPDF). The BGF process runs on a data PIC.

BSG

The BSG (Border Signaling Gateway) is the component that controls VoIP media resources on the router and is responsible for all SIP processing.

The BSG acts as a Session Initiation Protocol (SIP) back-to-back user agent (B2BUA) that fully terminates incoming signaling sessions and then starts a new signaling session on its other side.

SPDF

Each BSG instance has an embedded SPDF that is a standard TISPAN component with Gq’ and Ia interfaces as defined by TISPAN (see Figure 26).

The SPDF is responsible for:

• Implementing media resource allocation
• Authorizing media resource requests
• Load balancing between BGFs
• Mapping of media type and service class for:
  • DSCP marking
  • Quality of service

How the SPDF Works

The SPDF configuration uses service classes that classify media sessions and then specify the actions to take on the media session—marking, dropping, or applying QoS parameters.

The SPDF coordinates resource reservation requests that it receives from the application function (in this case the B2BUA in the BSG) as follows:

1. The B2BUA sends to the SPDF a summary of any new media session that is signaled through SIP (for example, a gold-class, 256 Kbps, audio/video call is about to start between IP1 and IP2).
2. The SPDF determines whether the request received from the BSG is consistent with the policies defined in the SPDF.
3. If the request is consistent with defined policies, the SPDF maps the session information into:
   • The BGF to be used.
   • The gates and gate parameters to be installed on the BGF.

IPSec

The IPSec process is located on the same PIC as the BGF.