SGd (application id: 16777313)

The SGd interface is used to facilitate the transfer of short messages (SMS) between the Mobility Management Entity (MME) and central SMS functions such as the SMS Interworking Mobile Switching Center (SMS-IWMSC), the SMS Gateway MSC (SMS-GMSC), and the SMS Router. The SGd interface enables these entities to exchange SMS-related signaling, ensuring the efficient delivery and management of SMS services within the network.

The primary purpose of the SGd interface is to handle the signaling required for SMS operations between the MME and other SMS-related components in the network. This includes both mobile-originated (MO) and mobile-terminated (MT) SMS messages, as well as the alerting of the SMS-GMSC by the MME. The SGd interface ensures that SMS messages are correctly routed, delivered, and managed within the mobile network.

The SGd interface connects the following key network elements:

• MME (Mobility Management Entity): Manages signaling between the user equipment (UE) and the core network, including SMS-related functions.
• SMS-IWMSC (Interworking Mobile Switching Center): Serves as a gateway for SMS between different networks.
• SMS-GMSC (Gateway Mobile Switching Center): Routes SMS to the appropriate destination network.
• SMS Router: An optional component that facilitates the routing of SMS messages within the network.

The SGd interface uses the Diameter protocol to support these components, employing specific Diameter commands and applications designed for SMS-related signaling.

SGd interface workflow:

1. Mobile-Originated (MO) Short Message Procedure:
   • When a user sends an SMS, the message is first received by the MME.
   • The MME then forwards the SMS to the SMS-IWMSC or SMS-GMSC via the SGd interface.
   • The SMS-IWMSC or SMS-GMSC routes the message to the intended recipient, whether that be another mobile user or a service center.
2. Mobile-Terminated (MT) Short Message Procedure:
   • When an SMS is destined for a user within the network, it is first received by the SMS-GMSC.
   • The SMS-GMSC then uses the SGd interface to forward the message to the MME.
   • The MME delivers the SMS to the user's device.
3. Alerting Function:
   • The SGd interface also supports alerting functions, where the SMS-GMSC can notify the MME of incoming SMS messages.
   • This ensures that messages are delivered to the user as soon as they are available.

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