S6c (application id: 16777312)

The S6c interface is a crucial component of the Diameter protocol, specifically designed to facilitate the communication between the Home Subscriber Server (HSS) and central Short Message Service (SMS) functions such as the SMS Gateway Mobile Switching Center (SMS-GMSC) and the IP Short Message Gateway (IP-SM-GW). S6c interface plays a vital role in managing the routing of SMS messages within both traditional and IP-based mobile networks.

The primary function of the S6c interface is to enable the retrieval and exchange of routing information necessary for the successful delivery of SMS messages. This interface ensures that SMS messages are correctly routed from the originating SMS functions (SMS-GMSC or IP-SM-GW) to the appropriate serving network entity, such as the MSC (Mobile Switching Center), MME (Mobility Management Entity), or SGSN (Serving GPRS Support Node), where the target subscriber is currently registered.

Architecture of the S6c Interface
The architecture of the S6c interface involves the following key components:
HSS (Home Subscriber Server): The HSS maintains comprehensive subscriber information, including the IMSI (International Mobile Subscriber Identity) and the current network location of the subscriber. It is responsible for providing the necessary routing information to the SMS-GMSC or IP-SM-GW upon request.
SMS-GMSC (Short Message Service Gateway Mobile Switching Center): This gateway handles incoming SMS messages from external networks and relies on the routing information provided by the HSS to forward the messages to the correct serving MSC, MME, or SGSN.
IP-SM-GW (IP Short Message Gateway): The IP-SM-GW manages SMS traffic within IP-based networks, particularly in the context of the IP Multimedia Subsystem (IMS). It uses the S6c interface to query the HSS for routing information to ensure that SMS messages are delivered to subscribers within the IMS environment.

S6c interface workflow:
The S6c interface workflow can be described in the following steps:

1. Request for Routing Information:
   • When an SMS message needs to be delivered to a subscriber, the SMS-GMSC or IP-SM-GW initiates a request to the HSS via the S6c interface.
   • The request typically includes the subscriber’s IMSI or MSISDN (Mobile Station International Subscriber Directory Number).
2. Processing the Request by HSS:
   • Upon receiving the request, the HSS retrieves the subscriber's current network location and other necessary routing information.
   • The HSS then sends this information back to the requesting SMS-GMSC or IP-SM-GW over the S6c interface.
3. Routing the SMS Message:
   • Using the routing information provided by the HSS, the SMS-GMSC or IP-SM-GW forwards the SMS message to the appropriate serving entity (MSC, MME, or SGSN) that is currently managing the subscriber.
4. SMS Delivery:
   • The serving entity (MSC/MME/SGSN) is responsible for the final delivery of the SMS message to the subscriber's mobile device.

For complete technical specification of S6c interface in Diameter protocol please refer to: [3GPP TS 29.338]