How is cellular communication
Do you know what happens after you have dialed a friend on your mobile phone? How does the cellular network find it in the mountains of Andalusia or on the coast of a distant Easter Island? Why sometimes suddenly the conversation is interrupted?
A large area of the inhabited part of our country is covered by Base Stations (BS). In the field they look like red and white towers, and in the city they are hidden on the roofs of non-residential houses. Each station catches a signal from mobile phones at a distance of up to 35 kilometers and communicates with a mobile phone via service or voice channels.
After you have dialed a friend’s number, your phone contacts the Base Station (BS) closest to you via the service channel and asks you to select the voice channel. The Base Station sends a request to the controller (BSC), and it forwards it to the switch (MSC). If your friend is a subscriber of the same cellular network, the switch will check with the Home Location Register (HLR), find out where the called subscriber is currently (at home, in Turkey or in Alaska), and transfer the call to the appropriate switch, from where Forward to the controller and then to the Base Station. The base station will contact the mobile phone and connect you with a friend. If your friend is a subscriber of another network or you call a landline phone, your switch will contact the corresponding switch of another network.
Complicated? Let's see more in detail. Base Station is a pair of iron cabinets locked in a well-conditioned room. Given that in Moscow it was on the street +40, I wanted to live a little in this room. Usually, Base Station is located either in the attic of a building or in a container on the roof.
Base Station Antenna is divided into several sectors, each of which "shines" in its own direction. The vertical antenna communicates with the telephones, the round one connects the Base Station with the controller.
Each sector can serve up to 72 calls simultaneously, depending on the settings and configuration. A Base Station can consist of 6 sectors, so one Base Station can serve up to 432 calls, however, usually a smaller number of transmitters and sectors are installed at the station. Cellular operators prefer to install more BS to improve the quality of communication.
Base Station can operate in three bands:
- 900 MHz - the signal at this frequency spreads further and penetrates better into buildings;
- 1800 MHz - the signal propagates over shorter distances, but allows you to install a larger number of transmitters on 1 sector;
- 2100 MHz - 3G Network
This is what a cabinet with 3G equipment looks like.
900 MHz transmitters are installed on Base Stations in fields and villages , and in a city where Base Stations are stuck like needles at a hedgehog, communication is mainly carried out at 1800 MHz, although transmitters of all three bands can be present at any Base Station at the same time.
The signal with a frequency of 900 MHz can beat up to 35 kilometers , although the "range" of some Base Stations standing along the tracks can reach up to 70 kilometers, by reducing the number of simultaneously serviced subscribers at the station by half. Accordingly, our phone with its small built-in antenna can also transmit a signal over a distance of 70 kilometers ...
All Base Stations are designed to provide optimal radio coverage at ground level. Therefore, despite the range of 35 kilometers, the radio signal is simply not sent to the altitude of the aircraft. However, some airlines have already begun to install low-power base stations on their aircraft that provide coverage inside the aircraft. Such a BS is connected to a landline cellular network using a satellite channel. The system is complemented by a control panel that allows the crew to turn the system on and off, as well as certain types of services, for example, turn off the voice on night flights.
The phone can measure the signal level from 32 Base Stations simultaneously . Information about the 6 best (in terms of signal) it sends over the service channel, and already the controller (BSC) decides which BS to transmit the current call (Handover) if you are in motion. Sometimes the phone may be mistaken and transfer you to the BS with the worst signal, in this case, the conversation may be interrupted. It may also turn out that at the Base Station that your phone has chosen, all voice lines are busy. In this case, the conversation is also interrupted.
I was also told about the so-called " problem of the upper floors ." If you live in a penthouse, then sometimes, when moving from one room to another, the conversation may be interrupted. This happens because in one room the phone can “see” one BS, and in the second - another, if it goes to the other side of the house, and, moreover, these 2 Base Stations are far away from each other and are not registered as " neighboring "at the cellular operator. In this case, the transfer of the call from one BS to another will not occur.
Communication in the metro is provided in the same way as on the street: Base Station - controller - switch, with the only difference that small Base Stations are used there, and in the tunnel coverage is provided not by an ordinary antenna, but by a special radiating cable
As I wrote above, one BS can make up to 432 calls simultaneously. Usually this power is enough for the eyes, but, for example, during some holidays, the BS may not be able to cope with the number of people willing to call. This usually happens on New Year's Eve, when everyone starts to congratulate each other.
SMS messages are transmitted through service channels . On March 8 and February 23, people prefer to congratulate each other via SMS, sending funny rhymes, and their phones often cannot agree with the BS on the allocation of a voice channel.
I was told an interesting case. From one district of Moscow, complaints from subscribers began to arrive that they could not get through to anywhere. Technical experts began to understand. Most of the voice channels were free, and all service were busy. It turned out that next to this BS was an institute in which exams were held and students were constantly exchanging text messages.
The phone divides long SMS into several short and sends each separately. Technical staff advise sending such greetings via MMS. It will be faster and cheaper.
From the Base Station the call goes to the controller. It looks as boring as the BS itself - it's just a set of cabinets.
Depending on the equipment, the controller can service up to 60 Base Stations . The connection between the BS and the controller (BSC) can be carried out via a radio-relay channel or via optics. The controller controls the operation of radio channels, incl. controls subscriber movement, signal transmission from one BS to another.
The switch looks much more interesting.
Each switch serves from 2 to 30 controllers. It already occupies a large room filled with various closets with equipment.
The switch controls the traffic. Remember the old movies, where people first dialed up to the "girl", and then she connected them to another subscriber by flipping the wiring? Modern switches also do the same.
To control the network, the company has several cars, which they affectionately call " hedgehogs ". They move around the city and measure the signal level of their own network, as well as the level of the network of colleagues from the Big Three.
The whole roof of such a car is equipped with antennas.
Inside there is equipment that carries hundreds of calls and removes information.
Round-the-clock monitoring of switches and controllers is carried out from the Flight Control Center of the Network Control Center (CSC) .
There are 3 main areas of control over the cellular network : accidents, statistics and feedback from subscribers .
Just as in airplanes, there are sensors on all the equipment of the cellular network that send a signal to the CCX and output information to the computers of dispatchers. If any equipment fails, a light will flash on the monitor.
The CCS also keeps track of statistics on all switches and controllers. He analyzes it, comparing with previous periods (hour, day, week, etc.). If the statistics of one of the nodes began to differ sharply from the previous indicators, then the “light bulb” will start flashing again on the monitor.
Feedback is accepted by the subscriber service operators. If they cannot solve the problem, then the call is transferred to a technician. If it turns out to be powerless, then an “incident” is created in the company, which is decided by the engineers who operate the relevant equipment.
The switches are monitored around the clock by 2 engineers.
The graph shows the activity of Moscow switches. It is clearly seen that at night almost no one calls.
Controllers for controllers (sorry for tautology) are carried out from the second floor of the Network Control Center.