What is SM RL (Short Message Relay Layer)

In the realm of GSM (Global System for Mobile Communications) networks, specifically those supporting SMS (Short Message Service), SM-RL (Short Message Relay Layer) plays a critical role in the message delivery process. It acts as an intermediary, facilitating the exchange of SMS messages and control information between various network entities.

Understanding the SMS Protocol Stack:

The SMS protocol stack is a layered architecture that ensures efficient and reliable SMS communication. Here's a simplified breakdown:

1. SM-AL (Short Message Transfer Layer): The user interface layer for composing and sending messages (explained previously).
2. SM-TL (Short Message Transport Layer): Handles reliable transfer of SMS messages between network entities.
3. SM-RL (Short Message Relay Layer) - Our focus here!
4. SM-SC (Short Message Service Center): Central entity responsible for storing, forwarding, and delivering SMS messages.

Core Function of SM-RL:

SM-RL resides within the SMS protocol stack and provides two key services:

1. Transferring SMS Messages (TPDUs):
   • SM-RL acts as a relay point for Transfer Protocol Data Units (TPDUs), which encapsulate the actual SMS message content along with addressing and header information.
   • It receives TPDUs from the SM-TL layer and forwards them to the appropriate destination within the network based on the recipient's address.
   • This forwarding can involve routing messages to the SMSC (Short Message Service Center) for further processing or directly to another mobile station (MS) if both sender and receiver are within the same network coverage area.
2. Exchanging Control Information:
   • SM-RL facilitates the exchange of control information related to SMS delivery using a dedicated protocol called Short Message Control Protocol (SM-CP). (We'll delve into SM-CP in a separate explanation).
   • This control information includes:
     • Session management for establishing and terminating communication between network elements.
     • Delivery reports indicating the success or failure of message delivery.
     • Error notifications for handling issues encountered during message transmission.

SM-RL Entities and Communication:

Within SM-RL, two primary entities handle message relay and control information exchange:

• Short Message Relay (SMR): This entity resides on network elements like the Mobile Switching Center (MSC) and is responsible for:
  • Receiving TPDUs from the SM-TL on the MSC.
  • Forwarding TPDUs to appropriate destinations based on routing logic.
  • Communicating with other SMR entities using SM-CP for control information exchange.
• Short Message Relay Entity (SMRE): This entity can be present on both the MS (user's mobile phone) and the MSC. Its functions include:
  • Initiating communication with an SMR on the network (e.g., MSC) using SM-CP to establish sessions and exchange control information.
  • Sending TPDUs containing SMS messages to the network.
  • Receiving TPDUs from the network, potentially containing incoming SMS messages or delivery reports.

Communication Flow:

Here's a simplified illustration of how SM-RL facilitates SMS communication:

1. User composes and submits a message (SM-AL): The user creates a message using a messaging application.
2. SM-AL interacts with SM-TL (not SM-RL): The application interacts with SM-TL to prepare the message for network transmission.
3. SM-TL triggers SM-RL: SM-TL on the MS hands over the TPDU containing the message to the SMRE (Short Message Relay Entity) within SM-RL.
4. SMRE establishes session and sends TPDU: The SMRE on the MS initiates communication with the SMR on the MSC using SM-CP to establish a session. Subsequently, it transmits the TPDU containing the message.
5. SM-RL forwards TPDU: The SMR on the MSC receives the TPDU and forwards it based on the recipient's address:
   • To the SMSC for further processing if the recipient is outside the coverage area.
   • Directly to the recipient's MS (another SMRE) if within the same coverage area.
6. Delivery and notifications: The SMSC or recipient's MS processes the message and attempts delivery. Delivery reports might be sent back via SM-RL and SM-CP.

Conclusion:

SM-RL serves as a crucial intermediary within the SMS protocol stack, ensuring efficient routing of SMS messages (TPDUs) and facilitating the exchange of control information through SM-CP. Understanding its role sheds light on the technical foundation of how SMS messages traverse the network and reach their intended recipients.