IMSG Session Border Control Solution Overview

The IMSG provides the session border controller (SBC) role in the European Telecommunications Standards Institute (ETSI) Telecommunications and Internet Converged Services and Protocols for Advanced Networks (TISPAN) architecture as shown in Figure 19. The SBC functionality encompasses the AF, SPDF, and BGF components.

Figure 19: IMSG in the ETSI-TISPAN Architecture

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IMSG Terms and Abbreviations

Table 14 defines the terms and abbreviations used in this topic.

Table 14: Terms and Abbreviations

Term	Description
B2BUA	Back-to-back user agent. A B2BUA is a SIP user agent to both ends of a SIP call. That is, it both establishes and terminates a call. In the TISPAN model, the B2BUA provides the application functions (AF). In the IMSG, the BSG acts as a B2BUA.
BGF	Border gateway function. Resides in the transport layer and polices and enforces traffic flows based on instructions from the SPDF.
BSG	Border signaling gateway. The BSG controls VoIP media resources on the router and is responsible for all SIP processing. The border signaling gateway provides a full B2BUA implementation and an embedded SPDF.
Dialog	A peer-to-peer SIP relationship between two user agents that persists for some time.
Gq’	Interface between the AF (in the IMSG, this is the B2BUA) and the SPDF that is used to exchange session-based policy setup information between the SPDF and the AF.
Ia	A profile of the interface between an SPDF and the BGF.
SIP	Session Initiation Protocol.
SPDF	Service policy decision function. The BSG contains an embedded SPDF that controls the instructions that the BSG sends to the BGF.