STa (application id: 16777250)

The STa interface is a reference point within the Diameter protocol architecture that facilitates communication between non-3GPP access networks and the 3GPP AAA server or 3GPP AAA proxy. It is primarily used for authenticating and authorizing User Equipment (UE) accessing the Evolved Packet Core (EPC) through non-3GPP networks, whether these networks are trusted or untrusted.

The primary function of the STa interface is to authenticate and authorize a UE as it attempts to connect to the EPC via a non-3GPP network. The interface is essential in determining whether the non-3GPP network is trusted or untrusted. This decision is made by the 3GPP AAA server during the initial authentication process and is then communicated to the non-3GPP access network.
The STa interface also supports the transport of various mobility-related parameters, including those related to PMIPv6, GTPv2, and MIPv4 FA-CoA modes. This is especially relevant when the UE attaches to the EPC using the S2a reference point. Additionally, the interface can convey charging-related information and optionally include IP Mobility Mode Selection details.

STa interface workflow:

1. Authentication and Authorization Initiation: When a UE attempts to access the EPC via a non-3GPP network, it sends an authentication and authorization request through the STa interface to the 3GPP AAA server. The server processes this request, authenticating the user and determining whether the non-3GPP network is trusted or untrusted.
2. Decision Communication: The decision regarding whether the network is trusted or untrusted is communicated back to the non-3GPP access network. This determines the subsequent handling of the UE’s session.
3. Mobility Management: Upon successful authentication, the STa interface is used to exchange mobility-related parameters such as PMIPv6, GTPv2, or MIPv4 FA-CoA mode information. These parameters are crucial for maintaining seamless UE mobility across different access networks.
4. Transport of Additional Information: The STa interface can also transport charging-related information and, if required, details about IP Mobility Mode Selection. This ensures accurate billing and appropriate service management for the UE's session.
5. Session Establishment: Following the exchange of necessary parameters, a session is established between the UE and the EPC, enabling the UE to access services through the non-3GPP network. The STa interface may continue to be involved in managing session-related updates or modifications as needed.

For complete technical specification of St interface in Diameter protocol please refer to: [3GPP TS 29.273]