The Global System for Mobile Communications is known as GSM. Mobile communication is facilitated by the open-source, digital GSM cellular technology. So It utilizes four distinct frequency bands, including 850 MHz, 900 MHz, 1800 MHz, and 1900 MHz. It employs FDMA and TDMA in combination. So Every idea related to GSM architecture and its operation is covered in this article.

GSM is having 4 different sizes of cells are used in GSM

1. Macro : Base Station antenna is installed in this cell size.
2. Micro : Antenna height in this size cell is lower than typical roof height.
3. Pico : Small cells’ diameter of few meters.
4. Umbrella : It covers the shadowed (Fill the gaps between cells) regions.

Features of GSM

1. Clear voice clarity
2. Supports international roaming
3. Spectral / frequency efficiency
4. Ability to support multiple handheld devices.
5. Ease of accessing network
6. Low powered handheld devices.
7. Low service cost.
8. International ISDN compatibility.
9. New features and services.

GSM is nothing more than a more complex system that is further divided into three subsystems.

1. BSS : Base Station Subsystem is referred to as BSS. So Between a mobile phone and the network switching subsystem, BSS manages traffic and signaling. BTS and BSC are the two parts of BSS. 
2. NSS : Network and Switching Subsystem is referred to as NSS. So The central network of GSM is the NSS. That managed calls and mobility for mobile phones connected to the network. NSS is made up of various parts, including VLR, HLR, and EIR.
3. OSS : Operating Subsystem is what OSS stands for. So The network operator monitors and controls the system using the OSS, a functional entity. OMC is a component of OSS. So The goal of OSS is to provide customers with efficient support for all GSM maintenance services.

So Assume that the tower is connected to BTS through TRX, followed by BSC and MSC, and that there are three mobile stations that are connected to it. Let’s examine the operations of the various parts.

1. MS

Mobile System is referred to as MS. MS consists of the user tools and software required for connecting to a mobile network. Mobile Station (MS) is made up of Subscriber Identity Module (SIM) and Mobile Equipment (ME). So The tower to which these mobile stations are currently connected has a TRX connection to the BTS. A transceiver, or TRX, consists of a transmitter and a receiver. Transceiver has two sending and receiving capabilities.

2. BTS

Base Transceiver Stations, also known as BTSs, allow for wireless communication between user equipment and a network. The BTS is in every tower.

3. BSC

Base Station Controller is referred to as BSC. BSC has a number of BTS. So You can think of the BSC as a local exchange in your area with numerous towers and BTS on numerous towers.

4. MSC

MSC, or the mobile switching center, stands for. So And MSC is connected to features of communication switching like call setup, call release, and routing. All operations are carried out at the MSC level, including call tracing and call forwarding. Additional MSC components include VLR, HLR, AUC, EIR, and PSTN.

• VLR : Visitor Location Register is known as VLR. All mobile subscribers who are currently present in the MSC service area are listed precisely where they are in a database called VLR. Your entry is recorded in the VLR database if you move from one state to another state.
• HLR : Home Location Register is an abbreviation. So A database called HLR holds important information about GSM network users who are authorized to use the service. If you buy a SIM card from an HLR vendor. HLR is like a house that contains all of your information, such as your ID proof, the plan you are on, the caller tune you use, etc.
• AUC : Authentication Center is a common abbreviation. So The mobile subscriber who wants to connect to the network is authenticated by AUC.
• EIR : Equipment Identity Register is referred to as EIR. EIR is a database that maintains a list of everyone in the network who is allowed or banned. So You cannot enter the network and cannot place calls if you have been banned from it.
• PSTN : Public Switched Telephone Network is referred to as PSTN. PSTN and MSC are linked. PSTN was initially a fixed-line, analog telephone network. Its core network is now almost entirely digital and includes fixed telephones as well as mobile and other networks. The older landline phones that are still in use at our house are simply PSTN.

OMC : Operation Maintenance Center is referred to as OMC. Each MS, BSC, and MSC functioning properly within a GSM system is monitored and maintained by OMC.

So Three subsystem BSS, NSS and OSS are connected with each other via some interfaces. Total three interfaces are there:

1. Air Interface : UM interface is another name for air interface. Because it is a mobile analog to the U interface of ISDN, the interface between MS and BTS is known as the UM interface.
2. Abis Interface : It is an internal BSS interface that connects to BTS and BSC.
3. A interface : It provides communication between BSS and MSC.

Services of GSM

1. Bearer services/ data services:

So Data transmission is specified differently by GSM, with the original GSM allowing for rates of up to 9600 bits/s. Data transmission can be transparent or opaque with bearer services.

• Transparent bearer services:

Transparent bearer services only send data over the physical layer. If there are no transmission errors, data transmission has a constant delay at throughput.

•  Non-transparent bearer services:

Protocols at layers 2 and 3 2 3 are used by non-transparent bearer services to implement flow control and error correction. (Network layer and data link layer).

       2.Tele services:

We use teleservices both now and in the future.

• Video calls.
• Video text and face emoji.
• short text message(SMS).

     3.Supplementary services: 

            supplementary services it means advanced services.

• Conference calls.
• Call waiting.
• Call forwarding.

GSM security

• GSM provides several security measures that make use of private data kept in the AUC and the individual SIM.
• The SIM contains private information that is guarded against unauthorized access and is secured with a pin.

Advantages:

Security: In order to protect user data and privacy, GSM offers enhanced security features like authentication, encryption, and confidentiality.

Compatibility: Due to the widespread use of GSM around the globe, it is compatible with a wide range of networks and gadgets.

Roaming: Users can use their mobile phones abroad in nations that adhere to the GSM standard thanks to roaming.

Efficient use of bandwidth: So GSM makes effective use of the available bandwidth thanks to the time-division multiplexing (TDM) technique, which enables multiple users to share the same frequency channel at various times.

Wide range of features: So Call waiting, voicemail, conference calling, call forwarding, and other features are all supported by GSM.

Disadvantages:

Network congestion: During peak times, GSM networks may experience congestion, which may result in dropped calls or poor call quality.

Limited coverage: So In some remote locations, GSM networks may not have adequate coverage, which can make it challenging for users to make calls or access the internet.

Data transfer speed: So In some remote locations, GSM networks may not have adequate coverage, which can make it challenging for users to make calls or access the internet.

Security vulnerabilities: Even though GSM has improved security measures, it is still susceptible to eavesdropping and spoofing attacks.

Users who require high-speed internet access or other data-intensive applications may find it difficult to use GSM networks because of their limited capacity for handling large volumes of data.

In light of this, the architecture and functionalities of all GSM components are as follows.