This clause describes the main IP-Connectivity Access Network specific concepts that are used for the provisioning of IMS services over a Trusted WLAN (TWAN) access to EPC.
As IMS is accessed over EPC, most of the features defined in Annex E
for the case of EPC (and PDN connections/EPS bearers) apply in the case of a TWAN access, e.g. a P-GW is used as an IP anchor point and IP-Connectivity Access Network bearers are provided by PDN connections and bearers.
Following specific considerations apply to the case of a TWAN access:
The Mobility related procedures for EPS and TWAN access are described in TS 23.402
TWAG/TWAP (refer to clause 16 of TS 23.402) are used to interface the access network and to control relevant PDN connectivity (over S2a) towards a P-GW.
For a TWAN access, the way the notification of the loss of IP-CAN session for an UE is triggered within a TWAN is out of scope of 3GPP specifications.
For a TWAN access, Access Network Information may be reported to the IMS as described in clause E.7
for the case of a GPRS/EPS access, with following exceptions:
A Geographical Identifier may be generated by the P-CSCF or an IMS AS based on the retrieved Access Network Information, as specified in clause E.8
Information related to the location of the user provided by the access network may be required in IMS in order to comply with regulatory requirements for SMS over IP. The P-CSCF applies the above mechanisms upon reception of a MESSAGE including the distribution of received information to other IMS entities.