1. TS 23.402**): Non-Roaming Reference Architecture for non-3GPP Accesses
HSS
Wx*
**) Colouring of non-3GPP accesses and red arrows (incl. text in red) added for this presentation S4
S6a
2G/3G SGSN
PCRF S3
Rx+
MME S7
S11
S1-MME
SGi
Operator’s IP Services
(e.g. IMS, PSS etc.)
S10
Serving SAE Gateway
PDN SAE Gateway
EUTRAN S5
S1-U
S6c
S2b
Wm*
3GPP AAA
Server
ePDG
S2c
S2a
HPLMN
Wn*
Trusted/Untrusted*
Non-3GPP IP Access
or 3GPP Access
Trusted
Non-3GPP IP Access
Non-3GPP Networks
Untrusted
Non-3GPP IP Access
Wa*
Ta* UE
* Untrusted non-3GPP access requires ePDG in the data path
Handovers (Discovery and Selection of non-3GPP Access is needed before)

2. Key Issue NW Discovery & Selection (NW-DS) (Excerpt from TR 23.882)
It is necessary to optimize NW-DS procedures for frequent mobility events.
It is desirable that the concept for NW-DS in SAE supports effective means for minimizing processing.
Looking at the currently defined NW-DS principles for I-WLAN in TS it becomes clear that they cannot be extended for time critical handovers and for other RATs, due to complexity and latency:
per available WLAN access NW a scan/probe has to be performed
if a WLAN AN is connected to more than one 3GPP NW this is only detected after L2 association and trying authentication.
It is therefore necessary to develop or adopt new, more efficient mechanisms, for both idle and active mode (where this differentiation is applicable).
Solutions may be based on concepts developed in other fora (e.g. IEEE u, IEEE , IETF) or based on concepts defined by 3GPP (e.g. provision of specific databases). Further mechanisms are FFS. When selecting solutions, the amount of signalling, size of stored and transferred data, especially over the radio interface, and terminal power consumption shall be taken into account.

3. Example of HO Message Flow (from informative annex of TS 23.402)
Untrusted
non-3GPP
IP Access
2. UE discovers the
untrusted non-3GPP
access system and
initiates HO