What is SRI Send Routing Information

SRI (Send Routing Information) Explained in Detail

Within the realm of mobile telephony and specifically GSM (Global System for Mobile Communications) networks, SRI stands for Send Routing Information. It's a crucial message used in the Mobile Application Part (MAP) protocol for establishing call connections between mobile subscribers.

The Call Setup Process:

1. Initiating the Call: When a mobile subscriber initiates a call, the Mobile Switching Center (MSC) serving the calling subscriber sends a call setup request message to the Gateway Mobile Switching Center (GMSC).
2. GMSC and HLR Interaction: The GMSC, responsible for routing calls across different networks, needs to determine the serving network of the called subscriber. To achieve this, it sends an SRI message to the Home Location Register (HLR).
3. SRI Message: This message includes the following key information:
   • Mobile Subscriber ISDN Number (MSISDN): The phone number of the called subscriber.
   • Other Optional Information (Depending on Network Configuration): Additional details like Location Area Code (LAC) or Cell Identity might be included to assist the HLR in identifying the serving network.
4. HLR Response: Upon receiving the SRI, the HLR queries its database to find the called subscriber's information. This information typically includes the following:
   • Serving Mobile Switching Center (MSC) Address: This identifies the network where the called subscriber is currently located.
   • Mobile Station Roaming Number (MSRN) (if applicable): If the called subscriber is roaming on a different network, the HLR provides the MSRN assigned for their roaming session.
5. Routing Completion: The HLR sends a response message back to the GMSC containing the retrieved information, specifically the serving MSC address.
6. Call Routing: Armed with the serving MSC address, the GMSC can now route the call setup request to the appropriate network, ultimately reaching the called subscriber's MSC.

Benefits of SRI:

• Efficient Call Routing: SRI facilitates efficient call routing by enabling the network to identify the serving network of the called subscriber.
• Subscriber Location Management: The HLR plays a vital role in managing subscriber location information, and SRI leverages this to establish connections regardless of the subscriber's current network.
• Roaming Support: SRI functions effectively even for roaming subscribers, as the HLR provides the MSRN for proper call routing within the visited network.

Additional Notes:

• The specific format and content of the SRI message might vary slightly depending on the specific version of the MAP protocol implemented by the network.
• Security measures are in place to ensure the authenticity and integrity of the SRI message exchange between the GMSC and HLR.