Network subsystem

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The mobile switching center is at the heart of the GSM network subsystem (NSS). In the U.S., the MSC is called the mobile telephone switching office (MTSO).

The MSC/MTSO is basically a traditional PSTN switch that handles all of the traditional wired functions plus some wireless functions. It handles the unique wireless functions like registration (How can I sign up for this service?), authentication (Who are you and what do you want?), location updating (Where are you?), handovers (What cell do you need next?), and call routing to roaming subscribers (You’re not at home, so how do I find you?). These functions and others are at the heart of a contemporary wireless network.

In addition to all of the user functions, the MSC handles all of the connectivity between the user and the PSTN, ISDN, packet networks, and the Internet. The MSC also handles all of the SS7 signaling between the functional elements in the GSM wireless architecture.

GSM Network Subsystem Components

NSS Components

At the core of roaming is the home location register (HLR), which contains all of the information about the subscriber, including the whereabouts of the subscriber, a critical part of the roaming service. Also contained in another MSC is the visitor location register (VLR), which tells the HLR where the roaming subscriber is located.

All calls are directed to the HLR because that is where the phone is located. Since the HLR has gotten word from the VLR that the user has roamed, the HLR can redirect the call. For example, consider a user with a Vermont-based phone (an 802 area code). If someone calls the user while he/she is in Los Angeles, the call will be routed to an HLR in Vermont. The Vermont HLR will know that the user is now associated with an MSC and BSC in the Los Angeles area and redirect the call to that location. The concept is simple, but the technology and infrastructure required to make it happen can be quite complex.

Other registers besides the HLR and VLR provide essential services to users and service providers. These include the equipment identity register (EIR) and the authentication (AuC) registers.

The EIR is the database of the valid mobile equipment as identified by its international mobile equipment identity. In the GSM database, the IMEI will be invalidated if it is stolen or not type-approved by the subscriber. This allows the service provider to track down the stolen equipment or at least to disallow the use of stolen equipment in the network.

The authentication center (AuC) holds the matching half of the secret key the SIM card uses for authentication and encryption. The AuC uses a CAVE algorithm to validate the SIM card in the handset before allowing the user to get to the appropriate services.