When a Wi-Fi subscriber moves between the access points (APs), a CCR-U message is triggered on the node, carrying the Called-Station-Id AVP. The Called-Station-Id AVP carries the MAC IP address of the new AP. This functionality allows the PCRF to make a location-based policy decision.
This functionality is enabled via event trigger USER_LOCATION_CHANGE (13) [3GPP 29.212, ยง5.3.7] sent to the node by a PCRF in a CCA or RAR message.
The same event is reported back from the node to the PCRF in a CCR-U message when the user location changes.