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Explain the role of the Mobile Station Roaming Number (MSRN) in GSM.

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The Mobile Station Roaming Number (MSRN) is a critical element in GSM (Global System for Mobile Communications) networks, particularly in the context of call routing and handling when a mobile station is roaming outside its home network. The MSRN is a temporary number assigned to a roaming subscriber, enabling the network to route calls to the visited network where the subscriber is currently located. Here's a technical explanation of the role of the Mobile Station Roaming Number (MSRN) in GSM:

  1. Home Location Register (HLR):
    • The Home Location Register is a database in the home network that stores subscriber information, including the subscriber's Mobile Subscriber ISDN Number (MSISDN), which is the telephone number associated with the subscriber.
  2. Location Update:
    • When a subscriber roams into a visited network, the Home Location Register (HLR) is updated with the subscriber's current location information, including the location area and the visited Mobile Switching Center (MSC).
  3. Temporary MSRN Assignment:
    • The HLR assigns a temporary Mobile Station Roaming Number (MSRN) to the roaming subscriber. The MSRN is a unique number associated with the subscriber's current location in the visited network.
  4. Routing Incoming Calls:
    • When an incoming call is directed to a roaming subscriber, the call is first routed to the subscriber's home network. The HLR is queried to determine the subscriber's current location, and the HLR returns the temporary MSRN associated with the visited network.
  5. Temporary Routing Number:
    • The MSRN serves as a temporary routing number for the incoming call. It is used to route the call from the home network to the visited network where the subscriber is currently located.
  6. Visited Location Area:
    • The visited MSC uses the MSRN to locate the roaming subscriber within its coverage area. The MSRN is associated with a specific location area and cell, enabling the MSC to route the call to the appropriate radio cell for delivery to the subscriber.
  7. Call Delivery to Mobile Station:
    • With the MSRN, the visited MSC can successfully route the call to the roaming subscriber's current location. The call is then delivered to the mobile station, allowing the subscriber to answer the call.
  8. Dynamic Assignment and Release:
    • The MSRN is dynamically assigned and released as the subscriber moves between location areas within the visited network. When the subscriber moves to a new location area, a new MSRN may be assigned, ensuring accurate call routing.
  9. Optimizing Call Setup:
    • The use of the MSRN optimizes call setup procedures for roaming subscribers. It allows the call to be efficiently routed to the current location of the subscriber without the need for the caller or the home network to have detailed knowledge of the subscriber's exact location.
  10. Support for Mobile-Originated Calls:
    • In addition to supporting incoming calls, the MSRN is also used for mobile-originated calls. When a roaming subscriber initiates an outgoing call, the MSRN is used to route the call through the visited network to the destination.

In summary, the Mobile Station Roaming Number (MSRN) in GSM networks is a temporary routing number assigned to a roaming subscriber, enabling efficient call routing and delivery to the subscriber's current location in the visited network. It plays a crucial role in optimizing the communication process for roaming subscribers while maintaining a seamless and transparent user experience.