GSM (global system for mobile communication)

The Global System for Mobile Communication (GSM) is a standard developed by the European Telecommunications Standards Institute (ETSI) for digital cellular networks. It was first introduced in 1991 and has since become the most widely used mobile communication standard in the world. GSM provides both voice and data communication services, including text messaging (SMS), multimedia messaging (MMS), and internet access. In this article, we will explore the various components of GSM and how they work together to provide reliable mobile communication.

GSM Network Architecture:

The GSM network consists of three main components: the Mobile Station (MS), the Base Station Subsystem (BSS), and the Network Subsystem (NSS). The MS is the user's mobile phone, while the BSS and NSS are responsible for managing the communication between the mobile phone and the rest of the network.

The Mobile Station:

The mobile station, also known as the user equipment (UE), is the physical device that the user uses to communicate with the network. It consists of two parts: the mobile equipment (ME) and the subscriber identity module (SIM).

The mobile equipment includes the radio transmitter and receiver, the display, and the keypad. It is responsible for transmitting and receiving signals between the mobile phone and the base station.

The SIM is a small card that contains the user's subscriber information, such as the phone number and network authentication key. It allows the user to connect to the network and make and receive calls.

Base Station Subsystem:

The Base Station Subsystem (BSS) is responsible for managing the communication between the mobile phone and the rest of the network. It consists of two main components: the Base Transceiver Station (BTS) and the Base Station Controller (BSC).

The Base Transceiver Station (BTS) is a radio transmitter and receiver that communicates directly with the mobile station. It is responsible for transmitting and receiving signals between the mobile phone and the rest of the network. The BTS also manages the handover process, which occurs when the user moves out of range of one base station and into the range of another.

The Base Station Controller (BSC) is responsible for controlling and managing the BTS. It manages the handover process between the BTS and other base stations, and it also manages the allocation of radio channels to the mobile station.

Network Subsystem:

The Network Subsystem (NSS) is responsible for managing the overall operation of the network. It consists of several components, including the Mobile Switching Center (MSC), the Home Location Register (HLR), the Visitor Location Register (VLR), and the Authentication Center (AUC).

The Mobile Switching Center (MSC) is the central component of the network. It manages the communication between the mobile station and the rest of the network. The MSC is responsible for routing calls to and from the mobile station, managing the handover process, and managing the allocation of resources to the mobile station.

The Home Location Register (HLR) is a database that contains information about each subscriber, such as the phone number, subscriber identity, and service profile. It is used to authenticate the user and to provide information about the user's location to the network.

The Visitor Location Register (VLR) is a temporary database that contains information about subscribers who are currently located in the area covered by a particular MSC. It is used to manage the handover process and to provide information about the user's location to the network.

The Authentication Center (AUC) is a database that contains authentication and encryption keys used to authenticate the user and to encrypt the communication between the mobile station and the rest of the network.

GSM Radio Interface:

The GSM radio interface is the part of the network that manages the communication between the mobile station and the base station. It consists of several components, including the frequency bands and channel spacing, the time division multiple access (TDMA) technology, and the coding and modulation techniques.

Frequency Bands and Channel Spacing:

GSM operates in several frequency bands, including the 900 MHz band and the 1800 MHz band. Each band is divided into a number of radio channels, which are used to carry voice and data communication. The channels are spaced 200 kHz apart in the 900 MHz band and 400 kHz apart in the 1800 MHz band.

Time Division Multiple Access (TDMA):

GSM uses a technology called time division multiple access (TDMA) to allow multiple users to share the same frequency channel. TDMA divides each channel into eight time slots, with each time slot lasting for 577 microseconds. Each user is assigned one or more time slots within a channel to transmit and receive information.

Coding and Modulation:

GSM uses several coding and modulation techniques to ensure reliable communication between the mobile station and the base station. These techniques include frequency modulation (FM) for voice communication, Gaussian minimum shift keying (GMSK) for data communication, and error correction coding to ensure that data is transmitted without errors.

GSM Security:

GSM provides several security features to protect the user's communication from interception and eavesdropping. These features include authentication, encryption, and subscriber identity confidentiality.

Authentication:

Authentication is the process of verifying the identity of the user and ensuring that the user is authorized to access the network. GSM uses a challenge-response mechanism to authenticate the user. The Authentication Center (AUC) generates a random challenge, which is sent to the mobile station. The mobile station calculates a response using a secret key stored in the SIM card and sends it back to the network. The network verifies the response and grants access to the user if it is correct.

Encryption:

Encryption is the process of encoding the user's communication to prevent eavesdropping. GSM uses the A5 encryption algorithm to encrypt the communication between the mobile station and the base station. The A5 algorithm uses a secret key stored in the SIM card to generate a stream of bits, which are used to encrypt the communication.

Subscriber Identity Confidentiality:

Subscriber identity confidentiality is the process of hiding the user's identity from the network. GSM uses a temporary mobile subscriber identity (TMSI) to identify the user instead of the permanent subscriber identity (IMSI). The TMSI is assigned by the VLR and is used for communication within the current network. The IMSI is only used during the initial authentication process and is not transmitted during normal communication.

Conclusion:

In conclusion, GSM is a standard for digital cellular networks that provides reliable voice and data communication services. The GSM network consists of three main components: the mobile station, the base station subsystem, and the network subsystem. The GSM radio interface uses TDMA technology to allow multiple users to share the same frequency channel, and it uses several coding and modulation techniques to ensure reliable communication. GSM also provides several security features, including authentication, encryption, and subscriber identity confidentiality, to protect the user's communication from interception and eavesdropping.