Policy decision function

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The policy decision function is a standalone entity responsible for implementing the service-based local policy (SBLP) control for the IMS operator. These decisions are based on session and media-related information that is forwarded by the P-CSCF.

Basic functions of the PDF are listed below.

  • Store session and media-related information: IP addresses, port numbers, and bandwidth requirements are examples of the types of stored information.
  • Provide/update/revoke authorization decisions: Provide an authorization decision when requested by the GGSN as part of a bearer authorization request. The decision is based on the stored session and media-related information. Similarly, whenever the stored information changes, the PDF will need to update the authorization decision. Finally, the PDF can revoke an authorization decision at any time.
  • Enable/prevent usage of an authorized bearer: The PDF enables the use of an authorized bearer such as the Packet Data Protocol (PDP) context. The PDF can also prevent the use of the authorized bearer while still maintaining the authorization.
  • Inform P-CSCF when a bearer is lost/modified: The PDF informs the P-CSCF when an authorized bearer (e.g., PDP context) is lost or modified. A modification indication is only sent when the bearer is upgraded from or downgraded to 0 kbps.
  • Process charges between GGSN and P-CSCF: The PDF sends an IMS-charging identifier to the GGSN and passes a GGSN-charging identifier to the P-CSCF.