S6m (application id: 16777310)

The S6m interface is an integral component of the Diameter protocol architecture, specifically designed for Machine-Type Communication (MTC) systems. S6m reference point establishes communication between the Machine-Type Communication Interworking Function (MTC-IWF) and the Home Subscriber Server (HSS). This interface plays a critical role in managing the lifecycle, authentication, authorization, and service requests for MTC devices within the network.

Architecture of the S6m Interface
In the broader MTC architecture, the S6m interface operates alongside other key interfaces:
S6n: Connects the MTC-AAA (Authentication, Authorization, and Accounting) server with the HSS, enabling authentication and mapping of external identifiers.
S6t: Connects the Service Capability Exposure Function (SCEF) with the HSS, facilitating configuration and reporting of monitoring events and communication patterns.
The S6m interface allows the MTC-IWF to perform various operations, including retrieving subscriber information, requesting routing data, and authorizing service capability requests. This is essential for ensuring that MTC devices can effectively communicate and function within the network, even under conditions that differ from traditional human-to-human communication models.

The S6m interface is designed to support the following key functions:

• Retrieving Subscription Information: The MTC-IWF uses the S6m interface to access detailed subscription data for a UE stored in the HSS. This data is necessary for managing the UE's interaction with the network, particularly in scenarios where the device may have limited or specialized communication needs.
• Requesting Routing Information: The interface allows the MTC-IWF to request routing information from the HSS, including the addresses of the serving nodes (such as MSC, MME, or IP-SM-GW) that support SMS delivery to the UE. This ensures that messages and commands are correctly routed through the network.
• Retrieving Identifiers: The S6m interface facilitates the retrieval of the IMSI (International Mobile Subscriber Identity) of the UE, as well as any external identifiers linked to specific applications or services. These identifiers are crucial for mapping various identities associated with a single UE.
• Authorizing Service Capability Requests: When a Service Capability Server (SCS) requests to trigger a specific service for a UE, the MTC-IWF uses the S6m interface to authorize this request with the HSS. The HSS evaluates the request against the UE’s subscription data and either approves or denies the service initiation.

S6m interface workflow:
The workflow of the S6m interface can be described through the following steps:

1. Initiation of Subscriber Information Retrieval:
   • The MTC-IWF initiates a request to the HSS through the S6m interface to retrieve subscriber information. This may include translating an external identifier (e.g., MSISDN) to the IMSI of the UE or fetching routing details for service delivery.
2. Request for Routing Information:
   • Once the subscriber information is retrieved, the MTC-IWF may request routing information to determine the address of the UE's serving node, such as the MSC or MME. This information is crucial for directing SMS or other services to the correct network destination.
3. Retrieval of Identifiers:
   • The MTC-IWF retrieves the IMSI and any associated external identifiers from the HSS, ensuring that the UE is correctly identified within the network and that all associated services are properly managed.
4. Authorization of Service Requests:
   • The MTC-IWF sends a service capability request to the HSS via the S6m interface for authorization. The HSS verifies the request based on the UE’s subscription data and decides whether to grant or deny the requested service. This step ensures that only authorized services are executed for the UE.
5. Response Handling:
   • The HSS processes the request and sends a response back to the MTC-IWF. If approved, the service is initiated; if denied, the MTC-IWF may notify the requesting entity (e.g., SCS) of the denial, potentially triggering alternative actions.

For complete technical specification of S6m interface in Diameter protocol please refer to: [3GPP TS 29.336]