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What is GSM?

The GSM Group’s study of a digital mobile system with international roaming, good network connectivity, and better sound quality began in the 1980s. Today, based on the idea of ​​the globalization of mobile radio networks, the GSM symbol stands for Global System for Mobile Communication.

The frequency band of the GSM system in 1990 was 900 MHZ with the abbreviation 900 GSM, and in 1991, with the upgrade of the system, the frequency band was changed to 1800 MHZ with the symbol 1800 DCS.

In the United States in 1995, the system was used with the 1900 MHz frequency band and the acronym 1900 PCS. The GSM system is an example of second-generation mobile technology that covers audio applications well. But it has limited support for the Internet and data transmission. GSM is based on technology similar to the TDMA system, except that it uses higher frequencies. The system also uses a small cell technology to increase capacity.

Introduction

In recent decades, the use of mobile phones has become very popular. In addition to telecommunication services, these phones also can move long distances. Until today, the mobile communication system has had various applications based on the existing capabilities. The first generation of analog mobile telecommunication systems was based on cellular technology. Examples of these systems were NMT, AMPS, and TACS.

The capacity of these systems was very low, and it was used only in the transmission of audio data. The interference problem was one of the most important problems of such systems, which allowed the installation of more stations from these systems.

There was no special security mechanism in these systems. Considering the problems of this generation and eliminating these disadvantages, another generation of mobile telecommunications called the second generation 2G entered the field. In this generation, analog networks gave way to digital networks. In digital networks, the traffic channel is digital, meaning that the speech in them is coded. The most important standard of the second generation is GSM, which has gained global acceptance and has become the most widely used cellular communication system in the world.

GSM architecture

The GSM system is made up of a combination of 3 main subsystems:
1. Network subsystem
2. Radio subsystem
3. Support and maintenance subsystem

In the GSM system for communication of network operators with different sources and equipment of cellular infrastructure, not only air interface but also several other main interfaces are defined to connect different parts of this system (you can see these connections in the figure above).

The following are three important interfaces in the GSM system:

• Interface A between MSC and BSC
• A-bis interface between BSC and BTS.
• The UM interface between BTS and MS.
• There is another interface called MAP, a protocol that is exchanged between MSC, VLR, HLR, EIR, and AUC elements.

Network subsystem:

The system includes equipment and functions related to end-to-end calls, customer management, and mobility and also acts as an interface between the GSM system and landline call centers (PSTNs).
The network subsystem is a switching subsystem that includes MSCs, VLRs, HLRs, AUCs, and EIRs.

The following is a brief definition of each of these elements:

MSC or Mobile Switching Services Center performs call setup functions, connects with fixed telephone centers. And functions such as subscriber billing are also responsible for this center.
The HLR or HOME location registrar of a centralized database contains the information of all subscribers registered in a PLMN. There may be more than one HLR in a PLMN, but each specified subscriber can only access one HLR.
The VLR, or visitor location recorder, of a database, contains information about mobile phones that are currently moving in the MSC domain. When an MS enters a new MSC domain, the VLR to which the MSC is connected requests the MS information from the HLR.

The HLR will also provide the MS information to the MSC in which the MS is located. If an MS wants to make a call, the VLR will provide all the information needed to make the call, and there is no need to ask the HLR at any time.

VLR In one sentence, it can be said that an HLR is distributed and includes detailed information about the location of a mobile phone.
The AUC, or authentication center, connects to the HLR and is responsible for preparing the HLR and the authentication parameters and encryption keys used for security purposes.
The EIR, or Equipment Identifier, is a database in which International Mobile Equipment Identification (IMEI) numbers are stored for each registered mobile device.

In this way, each subscriber is assigned an identification code. These codes in the EIR center are divided into three categories:

• Safelist – includes phones that can be used on the network without any restrictions.
• Greylist – includes phones that can be used on the network but must be tracked. For example, is the phone stolen?
• Blacklist – Includes phones that are not available on the network. Like stolen or defective phones.

The center checks which of the above three lists is the identification code assigned to the mobile phone. And thus determines the status of the permission to use the phone on the network.

Some notable points:

• IMEI identification code is assigned based on the mobile phone serial number.
• In case of loss of mobile phone, its serial number can be reported to one of the mobile subscriber affairs centers to be followed up.

Another component of the Echo Canceller network subsystem is that it reduces annoying issues (such as sound reflection). That arise over the mobile network when connecting to a PSTN circuit.
The IWF or intranet function is also the interface between MSC and other PSTN and ISDN networks.

Radio subsystem

Includes equipment and radio link connection management functions, such as handover management. This subsystem includes BSC, BTS, and MS. MS is subcontracted to the radio system and is always the last route of a call, protecting the establishment of a call and the network subsystem to manage mobility. It has network end capabilities as well as user end. Each cell in the GSM system has a BTS with multiple receivers and transmitters. Each geographical area is divided into cells, and each cell is covered and serviced by a BTS station.

A BSC controls a group of BTSs. There are several configurations for BSC-BTS. Some of these configurations are designed for high traffic conditions, and some for areas with medium traffic. A BSC also controls functions such as handover and power control. BSC and BTS are also known as BSS. From MSC’s point of view, BSS seems to be an interface that communicates with MSs in a specific area. BSS is constantly associated with radio channel management, transmission functions, radio link control, quality estimation, and system provision for handovers. BSS can cover N cells, of which N can be one or more.

Maintenance and Support Center Subsystem:

(OMC) includes maintenance and support functions for GSM equipment and supports the network operator interface.
The OMC connects to all equipment within the switching and BSC system. The OMC performs a country’s GSM monitoring functions (such as billing), and another of its most important functions is its HLR maintenance function.

Depending on the size of the network, each country can have more than one OMC. The NMC Network Management Center also does global and centralized network management, and OMC is also responsible for regional network management. In the mobile network, the first part directly connected to the mobile phone is the common term of the mobile antenna and in the specialized interpretation of BTS (base transceiver station). Finally, by the transmission lines of this device to another device called BSC, which is the task of management Connects several BTS. The second step after the mobile antenna (BTS) in the network is a device called BSC.

(Base Station Controller) which stands for BSC. As its name implies, controlling several BTS is the responsibility of one BSC. Its work is very important because it is desirable to set several important network parameters for the quality of calls and calls.

You were defined in this device. For example, you are talking on your mobile phone, sitting in a moving car, and talking on several streets, but you are still talking. In this case, you have passed several mobile antennas. Each mobile antenna has taken you to another antenna, cleared your traffic channel, and delivered it to another antenna. This on-the-go call management is known as HAND OVER, and the BSC task is related to that BTS. Also, the power of radiation (mobile antenna board) is defined in this device. Through the BSC, the output of one antenna is attenuated so that its frequency does not interfere with other antennas.

The capacity of BSCs is defined based on the TRX of the antennas connected to it. Information is transmitted between MS and BTS using frequency carriers to communicate in the mobile system. The communication path from MS to BTS is called Uplink, and the communication path from BTS to MS is called Downlink.

Telephone conversations have separate channels for transmitting information in the above two paths, i.e., one channel is used for transmitting information in the Uplink path, and the other is used for transmitting information in the Downlink path. Such a connection is called Duplex. To avoid interference, the distance between the Uplink and Downlink frequencies is always the same, which is called a double distance.

Conclusion

This standard has major differences from the first generation, which is analog and has many advantages over other technologies of this generation. Including its digitality and the quality of excellent audio quality with mobile phones, upgradeability, international roaming, etc.

In other words, it is considered a second-generation mobile system and is a commonly accepted standard for digital communications. However, the capacity and bit rate of GSM data transmission is very weak. And with the growth of mobile phone systems, increasing Internet users. And rising expectations and The needs of users, such as the demand for high-quality Internet access through wireless systems, have led to the design of new generation systems that can meet these needs.