What is SM RP (Short Message Relay Protocol)

In the intricate world of GSM (Global System for Mobile Communications) networks, especially those supporting SMS (Short Message Service), SM RP (Short Message Relay Protocol) plays a supporting role within the SMS Relay Layer (SM-RL). It's important to differentiate SM-RP from its close companion, SM-CP (Short Message Control Protocol). Here's a breakdown of SM-RP's function and its relationship with SM-CP.

Understanding the SMS Protocol Stack:

The SMS protocol stack comprises several layers working together for SMS communication. Here's a simplified view:

1. SM-AL (Short Message Application Layer): User interface layer for composing and sending messages (explained previously).
2. SM-TL (Short Message Transport Layer): Handles reliable transfer of SMS messages.
3. SM-RL (Short Message Relay Layer): Facilitates message routing and control information exchange (our area of focus).
4. SM-SC (Short Message Service Center): Central entity for storing, forwarding, and delivering SMS messages.

SM-RL and its Entities:

Within SM-RL, two primary entities are involved in message relay and control information exchange:

• Short Message Relay (SMR): Resides on network elements like the Mobile Switching Center (MSC) and is responsible for:
  • Receiving TPDUs (Transfer Protocol Data Units) from the SM-TL.
  • Forwarding TPDUs based on routing logic.
  • Communicating with other SMR entities using SM-CP.
• Short Message Relay Entity (SMRE): Can be present on both the MS (user's mobile phone) and the MSC. Its functions include:
  • Initiating communication with an SMR using SM-CP.
  • Sending TPDUs containing SMS messages.
  • Receiving TPDUs (incoming messages or delivery reports).

SM-CP vs. SM-RP:

While both reside within SM-RL, SM-CP and SM-RP have distinct functionalities:

• SM-CP (Short Message Control Protocol): This peer-to-peer protocol enables communication between SMR entities. It's used to:
  • Establish and manage communication sessions.
  • Exchange control information like delivery reports and error notifications.
  • Manage the flow of TPDUs between SMRs.
• SM-RP (Short Message Relay Protocol): This protocol defines the format and structure of messages exchanged within the SM-RL layer. It essentially specifies how:
  • TPDUs (containing the actual message content) are transferred between SMREs and SMRs.
  • Routing information is embedded within the TPDU for efficient delivery.
  • Reference identifiers are used to track message flow and correlate them with control information exchanged via SM-CP.

SM-RP Message Structure:

SM-RP messages (Relay Protocol Data Units - RPDUs) encapsulate information related to TPDU transmission:

• Header: Contains information like message type (request or response), reference numbers, and routing indicators.
• TPDU: The actual SMS message content along with addressing and header information, previously handed off by the SM-TL.

Communication Flow:

Here's a simplified illustration of how SM-RP works within the larger SMS communication process:

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-RP): The application interacts with SM-TL to prepare the message for transmission.
3. SM-TL triggers SM-RL: SM-TL on the MS hands over the TPDU to the SMRE within SM-RL.
4. SMRE initiates session and sends RPDU: The SMRE on the MS initiates communication with the SMR on the MSC using SM-CP (not SM-RP) to establish a session. Subsequently, the SMRE transmits an RPDU containing the TPDU via SM-RP.
5. SM-RL forwards RPDU: The SMR on the MSC receives the RPDU and extracts the TPDU for further processing:
   • The SMR might use SM-CP again to communicate with another SMR for relaying the message if needed.
   • The TPDU is ultimately delivered to the recipient's MS or the SMSC based on routing logic.

Conclusion:

SM-RP acts as the internal language within the SM-RL, defining how TPDUs (containing the message content) are transferred between SMREs and SMRs. It facilitates efficient routing of SMS messages within the network, working hand-in-hand with SM-CP for control information exchange. Understanding both protocols provides a comprehensive picture of the technical underpinnings of SMS communication.