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How does a Gateway Mobile Switching Center (GMSC) function in GSM networks?

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The Gateway Mobile Switching Center (GMSC) is a key component in GSM (Global System for Mobile Communications) networks, responsible for handling the routing of calls between the mobile network and external networks, such as the Public Switched Telephone Network (PSTN) or other mobile networks. Here's a detailed technical explanation of how a Gateway Mobile Switching Center functions in GSM networks:

  1. Call Origination Request:
    • When a mobile subscriber initiates a call, the request is initially sent to the Mobile Switching Center (MSC) in the subscriber's home network. The MSC processes the call origination request and determines whether the call is destined for a subscriber within the same network or an external network.
  2. Determination of External Network Destination:
    • If the call is destined for an external network, such as a landline or mobile number outside the subscriber's home network, the MSC identifies that the call needs to be routed through the GMSC. The GMSC is the entity responsible for interfacing with external networks.
  3. Subscriber Location Information Retrieval:
    • The GMSC queries the Home Location Register (HLR) to retrieve the current location information of the called subscriber. The HLR stores subscriber-related information, including the current MSC where the subscriber is registered.
  4. Routing Decision:
    • Based on the information obtained from the HLR, the GMSC makes a routing decision on how to forward the call. This decision includes determining the MSC or Visited MSC (VMSC) where the called subscriber is currently located.
  5. Call Routing to the Visited MSC (VMSC):
    • The GMSC routes the call to the appropriate Visited MSC (VMSC) based on the subscriber's current location. The VMSC is responsible for handling calls for subscribers visiting from other networks.
  6. International Gateway Exchange (IGW):
    • For international calls, the GMSC may interface with an International Gateway Exchange (IGW). The IGW facilitates the routing of calls between different countries and networks, managing the interconnection and handoff of call traffic.
  7. Signaling Network Interaction:
    • The GMSC communicates with the Signaling System No. 7 (SS7) signaling network to exchange signaling messages with other network elements. SS7 is used for call setup, teardown, and various signaling functions in telecommunication networks.
  8. Connection Establishment:
    • The GMSC works to establish a connection path for the call, negotiating with the destination network to set up the necessary circuit or packet-switched connections for voice transmission.
  9. Billing and Charging Information:
    • The GMSC is responsible for collecting billing and charging information related to the call. This includes tracking call duration, destination, and other relevant details for accurate billing and charging of subscribers.
  10. Call Handover and Management:
    • During the call, the GMSC may be involved in call management functions, including call handovers between different network elements or MSCs to ensure a seamless and uninterrupted communication experience for the subscribers.
  11. Call Termination and Teardown:
    • When the call is completed, the GMSC facilitates the termination and teardown of the call. This involves releasing the established connections, updating billing records, and freeing up resources for subsequent calls.

In summary, the Gateway Mobile Switching Center (GMSC) functions as a gateway between GSM networks and external networks, managing the routing of calls, interfacing with the Home Location Register (HLR), establishing connections, and facilitating communication with other network elements to ensure efficient and accurate call handling in mobile telecommunications.