T4 (application id: 16777311)

The T4 interface is a crucial component within the Diameter protocol, specifically designed for communication between the Machine Type Communication Interworking Function (MTC-IWF) and the Short Message Service Service Center (SMS-SC) within a Home Public Land Mobile Network (HPLMN). T4 interface is instrumental in managing device triggers in IoT (Internet of Things) environments.

The primary purpose of the T4 interface is to facilitate the transfer of device trigger messages from the MTC-IWF to the SMS-SC. This interface ensures that critical information, such as the identities of serving network elements (e.g., SGSN, MME, MSC, SMSF), is conveyed accurately to the SMS-SC. The SMS-SC then processes these triggers and attempts to deliver them to the intended user equipment (UE). Additionally, the T4 interface allows the SMS-SC to report back to the MTC-IWF on the success or failure of the trigger delivery.

T4 interface workflow:

1. Trigger Generation: The process begins with the MTC-IWF generating a device trigger. This trigger could be used to wake up a device, initiate a data session, or perform a specific action on the device.
2. Message Transmission: The MTC-IWF sends the device trigger message to the SMS-SC via the T4 interface. The message includes important identifiers such as the serving SGSN, MME, MSC, or SMSF, depending on the context of the communication.
3. Processing by SMS-SC: Upon receiving the trigger, the SMS-SC processes the message and routes it to the appropriate UE. This may involve interacting with other network elements to ensure the correct delivery path.
4. Outcome Reporting: Once the SMS-SC has attempted to deliver the trigger, it reports the outcome (success or failure) back to the MTC-IWF. This feedback is crucial for the MTC-IWF to determine any further actions, such as retrying the trigger or logging the outcome for future reference.

For complete technical specification of T4 interface in Diameter protocol please refer to: [3GPP TS 29.337]