GSM architecture

The GSM architecture is divided into three main subsystems:

Additionally, there is the Operating Support Subsystem (OSS) to manage and maintain the network.

The Mobile Station is the user’s device, made up of two parts:

A. Mobile Equipment (ME):
- Portable or vehicle-mounted handheld device (e.g., a mobile phone).
- Uniquely identified by an IMEI (International Mobile Equipment Identity) number.
- Used for voice and data transmission.
- Monitors power and signal quality of surrounding cells to assist in smooth handover between cells.
- Can send 160-character long SMS messages.
B. Subscriber Identity Module (SIM):
- A smart card containing the IMSI (International Mobile Subscriber Identity) number.
- Enables users to send and receive calls and use other subscriber services.
- Protected by a password or PIN.
- Contains encoded network identification details.
- Portable: can be moved from one mobile device to another.

The BSS is responsible for communication between the Mobile Station and the Network Switching Subsystem.

Base Transceiver Station (BTS):
- Facilitates wireless communication between the mobile device (UE) and the network.
- Handles radio signals to and from the mobile station.
Base Station Controller (BSC):
- Controls one or more BTSs.
- Manages radio resources including setting up radio channels, frequency hopping, and handovers.
- Acts as a link between the BTS and the Mobile Switching Center (MSC).

The NSS handles the core network functions such as call routing and subscriber management.

Mobile Switching Center (MSC):
- Primary node for routing voice calls, SMS, and other services (conference calls, fax, circuit-switched data).
- Interfaces with other networks such as PSTN, ISDN, and data networks.
Home Location Register (HLR):
- Central database that contains subscriber information, such as user profiles and service permissions.
Visitor Location Register (VLR):
- Temporary database storing information about subscribers currently in the MSC’s service area.
Authentication Center (AuC):
- Validates SIM cards attempting to connect to the network to ensure security.
Equipment Identity Register (EIR):
- Maintains a list of valid IMEI numbers.
- Helps identify stolen or unauthorized mobile equipment.

Operations and Maintenance Center (OMC):
- Connected to all switching and base station equipment.
- Responsible for monitoring and maintaining the network to ensure smooth operation.

Um Interface: Air interface between Mobile Station (MS) and BTS. It is called Um because it corresponds to the mobile analog of the ISDN U interface.
Abis Interface: Connects the BTS to the BSC within the Basic Station Subsystem (BSS).
A Interface: Provides communication between the BSS and the MSC in the Network Switching Subsystem (NSS).

The MSC connects the GSM network to external public networks such as:

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