8.5 Handover with Access Network Discovery and Selection

23.4023GPPArchitecture enhancements for non-3GPP accessesRelease 18TS

8.5.1 Handover between 3GPP Access and Trusted / Untrusted Non-3GPP IP Access with access network discovery and selection

The figure below shows the main steps involved in a handover between a 3GPP access and a non-3GPP IP access (also called an inter-system handover) when network discovery and selection information is provided by the network (see clause 4.8). This information is provided in order to control the UE’s inter-system handover decisions and in order to reduce the battery consumption for inter-system mobility.

Figure 8.5.1-1: Handover between 3GPP Access and trusted / untrusted non-3GPP IP Access with Access Network Discovery and Selection

1. The UE is connected with a source access network (either a 3GPP access or a trusted / untrusted non-3GPP IP access). Its radio interfaces not connected to any access network may be in power saving or powered down mode.

2. If the inter-system mobility policies (see clause 4.8) in the UE indicate that inter-system mobility is allowed with at least one access technology type, then the UE may decide to discover neighbour access networks with assistance by the network. In this case, the UE discovers the address of ANDSF (if needed) as specified in clause 4.8, establishes secure communication with the ANDSF as specified in TS 33.402 [45] and requests access network info from the ANDSF as specified in the steps below.

3. The UE sends an Access Network Info Request (UE Capabilities, UE Location) message to the H‑ANDSF (in the non-roaming and roaming case) and the V‑ANDSF (in the roaming case) to retrieve network discovery and selection information. The UE Capabilities indicate the capabilities of the UE pertaining to access network discovery, such as the access technology types that can be supported by the UE. If the UE Location is available in the UE, it should be included in the message to indicate the UE’s current location, e.g. for the 3GPP access, Cell ID, TAI, and/or GPS (if available). If the UE Location is not included then other mechanisms may be used by ANDSF to identify the UE’s current location.

NOTE 1: In this Release of the specification, no mechanisms are specified for the ANDSF to identify the UE’s current location, if this information is not provided by the UE.

4. The ANDSF responds with an Access Network Info Response (Available Access Network Info, Updated Inter-system Mobility Policies) message to the UE. The Available Access Networks Info contains a list of access networks that are available in the vicinity of UE. If the UE included one or more access technology types in the Access Network Info Request, then information about neighbour access network with the requested access technology types is included. The Updated Inter-system Mobility Policies may be included in order to update / install operator defined rules / preferences in the UE. These rules / preferences may indicate a preference value for an available access network and help the UE select an available access network that is more preferable to the current access network.

5. The UE powers up its appropriate radio interface(s) (if needed) and measures the available access networks for which inter-system mobility is allowed, as indicated by the updated / current inter-system mobility policies. The UE selects the most preferable available access network for inter-system mobility based on the inter-system mobility policies and user preferences.

6. If the UE selects a preferable access network for handover, then the UE initiates handover to the selected access network as described in clause 8.

NOTE 2: Steps 2, 3 and 4 in the above procedure may not immediately result in an inter-system handover (steps 5 and 6).