Sy interface implementation

[FerUy]

As specified in in 3GPP TS 29.219:

The Sy reference point is located between the Policy and Charging Rules Function (PCRF) and the Online Charging System (OCS). The Sy reference point enables transfer of information relating to subscriber spending from OCS to PCRF and supports the following functions:

• Request of policy counter status reporting from PCRF to OCS.
• Notification of policy counter status change from OCS to PCRF.
• Cancellation of policy counter status reporting from PCRF to OCS.

Since the Sy interface resides between PCRF and OCS in the HPLMN, roaming with home routed or visited access as well as non-roaming scenarios are supported in the same manner.
The stage 2 level requirements for the Sy reference point are defined in 3GPP TS 23.203.
Signalling flows related to the Sy interface are specified in 3GPP TS 29.213.
Diameter overload control procedures over the Sy interface defined in Annex G of 3GPP TS 29.213.
Diameter message priority mechanism procedures over the Sy interface defined in Annex J of 3GPP TS 29.213.

Diagram below portrays LTE E-UTRAN/EPC/IMS interface, including Sy interface.