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How does a GSM cellular network work.

When making a call or receiving a call, the subscriber’s phone establishes a radio connection from one of the antennas of a nearby base station (BS – Base Station). The GSM cellular communication system includes a set of base stations, each of which may include 1-12 transmit-receive antennas. To ensure high-quality communication in the radius of their action, the antennas have a versatile orientation. Antennas are rectangular structures that can be seen on special masts or on the cover of tall buildings. Such antennas generate signals and transmit them via a special cable to the BS control unit. A base station is a combination of antennas and a control unit. There are territories that can be served by several base stations at once, connected to the local area controller (LAC). One controller can combine up to 15 base stations in a certain territory. Local area controllers are switched with the Mobile Services Switching Center (MSC), or simply a “switch”, which, in turn, has input and output connections to any existing types of cellular and wired communications. GSM regional cellular networks can use only one mobile services control center. At the same time, large mobile operators (for example, MTS, Beeline or Megafon), with several million subscribers, use several interconnected MSC centers.

To understand the hierarchy of such a complex system, it is necessary to use the meaning of the technical term handover, which refers to the handover function of a subscriber in cellular networks according to the relay principle. This means that if the client moves along the street and simultaneously talks on the phone, then to maintain the continuity of the conversation, it is necessary to timely switch the subscriber’s phone from one sector (cell) of the base station to another, as well as from the control of one BS or LAC to another, etc. .d. Therefore, if direct connection of the base station sectors to the switch were used, then, despite the abundance of other tasks, the latter would have to independently carry out the handover procedure for all existing subscribers. To ensure uniform loading of equipment and reduce the likelihood of its failures from overloads, the organization of GSM mobile networks is based on a multi-level principle. In other words, when a subscriber moves from the coverage area of ​​one sector of the base station to the coverage area of ​​another, the control is performed by the control unit of the BS, while the “superior” hierarchical LAC or MSC devices are not involved. Similarly, when handover between different base stations, LAC is already working, etc.

The switch performs the same functions as the PBX in wired networks, and is the main control device of GSM networks. The mobile services center determines the addressees of the call, regulates the operation of additional services and directly decides whether the subscriber has the right to make a call at a given time. So, you pressed the “magic button” and your phone turned on. On the SIM card, which is located in the subscriber’s phone, there is a special IMSI (International Subscriber Identification Number), which means “International Subscriber Identification Number”. IMSI is a unique number for all existing mobile networks around the world, by which mobile operators uniquely identify subscribers. At the moment the phone’s power button is pressed, it sends the IMSI code to the base station, which, in turn, sends it first to the LAC, and that one even further down the hierarchy to the switch. At the same time, two more additional devices participate in the process – HLR (Home Location Register) and VLR (Visitor Location Register), which are connected directly to the switch. HLR stands for “Home Subscriber Register” and stores the IMSI codes of all subscribers of its own network, and VLR (“Guest Subscriber Register”) contains information about all subscribers who use the network of this mobile operator at a particular time.

When transmitting the IMSI code to the HLR, the encryption system provided by the AuC (Authentication Center) is used. Initially, the HLR checks for the presence of a subscriber with this number in its database, and if so, whether the subscriber has the right to use the network services at the moment, or, for example, currently has a financial lock. If the verification has ended positively for the subscriber, his number is redirected to VLR, after which the client can make calls or use other cellular services.

Thus, we superficially examined the basic principle of the operation of GSM cellular networks, as a more in-depth description of technical details is many times more voluminous and less clear to most readers.

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