Definition: The GMSC acts as an interface between the GSM network and other networks, such as the Public Switched Telephone Network (PSTN) or another mobile network. Its primary function is to route incoming calls from other networks to the appropriate Mobile Switching Center (MSC) within the GSM network.
Functions:
Call Routing: When a call is initiated from a fixed-line telephone or another mobile network destined for a mobile subscriber, the call is directed to the GMSC. The GMSC determines the appropriate MSC where the mobile subscriber is located based on the subscriber's location and other relevant information.
Number Translation: The GMSC may perform number translation functions, converting numbers from the format used in other networks to the format used within the GSM network.
Components:
Database: The GMSC relies on various databases to determine the correct MSC for routing calls. Key databases include:
Home Location Register (HLR): Stores subscriber data, including location information. The GMSC consults the HLR to determine the current MSC associated with the mobile subscriber's location.
Gateway Location Register (GLR): In some architectures, the GMSC might consult the GLR, which provides information about the current MSC serving a particular geographical area or cell.
Signaling Interface: The GMSC communicates with other components of the GSM network through various signaling interfaces. Common interfaces include:
BSSAP (Base Station System Application Part): This interface is used to communicate with the Base Station Subsystem (BSS) for mobile station management.
MAP (Mobile Application Part): MAP is a signaling protocol used between various components in the GSM network, including the GMSC, MSC, and HLR, for various operations like subscriber location updates, call setup, etc.
Call Flow:
When a call is initiated from an external network, the call setup request is received by the GMSC.
The GMSC queries the relevant databases (HLR or GLR) to determine the current MSC serving the called mobile subscriber.
Based on the information retrieved from the database, the GMSC routes the call to the appropriate MSC.
The MSC then handles further call setup procedures, such as paging the mobile subscriber, establishing a voice channel, and completing the call.
Integration with Other Networks: The GMSC plays a crucial role in ensuring seamless communication between GSM networks and other networks like PSTN or ISDN. It performs necessary protocol conversions and ensures that calls are appropriately routed to the intended mobile subscriber.
