ANDSF Premier: Access Network Discovery and Selection Function

ANDSF, which stands for access network discovery and selection function, is an entity within the EPC (evolved packet core) with the purpose of assisting UEs in the discovery/selection of access networks, such as WiFi, WiMax, and LTE, in their vicinity; providing them with rules policing the connection to these networks (1).

ANDSF client, which runs on UE, interacts with the ANDSF server using OMA-DM (Open Mobile Alliance Device Management) protocol over the S14 interface. It’s an IP-based interface which supports ‘pull’ as well as ‘push’ mechanisms for communication. The illustration below shows high level interaction between UE and ANDSF Server.

ANDSF can be used to provide the following category of information to UE, based on operator configuration (4):

Access Network Discovery Information
It consists of a list of access networks, such as WiFi, that may be available in the vicinity of a UE. This information is received in response to the UE’s request which contains its location and capability, such as types of supported interfaces, among others. The received information assists the UE to expedite the connection to these networks. ANDSF response may contain the following information:

• Access the technology type, such as WiFi, WiMAX
• Radio access network identifier, such as SSID of WLANs
• Technology-specific information, such as one or more carrier frequencies

The suggested list of access networks may also include networks which are not owned by the same operator, but that have a roaming agreement with that operator.

Inter-System Mobility Policy (ISMP)
ISRP is valid for the UEs which can have more than one active access network connection simultaneously, and can also route IP traffic concurrently over multiple radio access interfaces. The UE can use this information to select the most preferable (or restricted) access technology/access network or APNs to direct a given traffic (per flow). Thus, it defines IFOM (per flow mobility) policies and can be used by UEs implementing seamless mobility protocols such as DSMIPv6.

All of this received information comes along with some validity conditions which indicate when it shall be valid, such as a location area condition, time-of-day condition, etc. For example, based on a UE’s location (home network or roaming), it can either latch on to H-ANDSF (home) or V-ANDSF (visitors).

• Device information – e.g. device ID, manufacturer, model tag, phone language and DM protocol version
• Client credentials – used for authentication purposes (unless GAA is used)
• Session origin – indicate whether the incoming session is client or server initiated

Data (Management) Phase

In the data management phase, the device-initiated messages (inside HTTP POST Request) consist of status information and results that are provided in response to commands from the server. The first message is the response to commands included in the final setup phase message by the server (inside the HTTP ACK response). From there on, the server can issue new commands in next HTTP ACK message to a previous HTTP POST Request message, or it can simply indicate there are no more operations. In that case, the client will stop sending responses in the HTTP POST Request messages and close the session.

The setup and data phases are carried over a TLS session with the UE and server. The session is always started by the UE, but the server may request it with the alert notification SMS. The TLS session is secured according to the used security mode. In GAA (Generic Authentication Architecture), the TLS session is established using a negotiated PSK key while in the generic OMA DM mode, it is a normal HTTPS session.

If GAA is not used, the server responds with its credentials in order to identify itself to the client for authentication and identification purposes. The server also includes the first data command. In OMA-DM it is always the server which issues commands and the client can only follow those.

External References
3GPP. TS 24.302: Access to the 3GPP Evolved Packet Core (EPC) via non-3GPP access networks.
libdmclient. [Online] https://01.org/libdmclient.
Funambol – OMA-DM Server. [Online] https://forge.ow2.org/projects/sync4j/.