People have long since learned to communicate at a distance. In ancient times, a messenger was sent with news, later letters were written. Now, to say a few words to a distant friend, you can just call him. The main thing is to have cellular telephone. But how do they connect to each other if they don't even have wires? In this story, I will tell you how the phone works.

What it is?

A mobile phone is more like a walkie-talkie than a regular corded phone. Radio waves are used to transmit the signal.

The difference is that the walkie-talkies are connected to one antenna, and can only be connected by catching a signal from it. Cell phones are not tied to a specific station. While moving, they connect to the antenna from which the strongest signal is received, so we can use communication almost all over the world without changing the SIM card. Antennas, or base stations, have been built all over the world, hiding in billboards, clocks, poles, and even trees. Each of them is responsible for its own zone, which has the shape of a hexagon. In the diagrams, these territories bordering each other resemble a honeycomb. Hence the name - cellular communication.

Who was first?

Who do you think was the first to talk on a cell phone? Of course, it was an employee of Motorola, which released them. In 1973, while on the streets of New York, he called and boasted of a call from an unusual phone at that time to his main competitor. This phone became the prototype of the first mobile phone that hit the stores 10 years later.

For the phone to work, you need to insert a SIM card into it. It contains information about the subscriber, that is, about the person who uses it. The mobile phone starts checking all the frequencies available to it, there are about 160 of them. The six best signals are recorded on the SIM card, these are the signals of your network.

After you have dialed your buddy's number, your phone transmits information about you to the antenna with the strongest signal. Your operator (for example, MTS or Beeline) recognizes you, finds a free channel on which your conversation can take place, and connects you. All this takes only a few seconds.

The conversation itself is a rather complicated technical process. Our voice is broken into segments lasting 20 milliseconds and converted to digital format, then encoded by a special system. Encrypted signals are processed again to remove extraneous noise.

Now cellular telephone serves not only for conversations. One small device fits such simple mechanisms as a simple clock, alarm clock, calculator, calendar, flashlight, as well as complex cameras, Internet access, a player and much more.

Almost everyone used a cell phone, but few people thought - how does it all work? In this literary opus, we will try to consider how communication takes place from the point of view of your telecom operator.

When you dial a number and start calling, well, or someone calls you, your device communicates via radio with one of the antennas of the nearest base station.

Each of base stations contains from one to twelve transmitting and receiving antennas directed in different sides to provide communication to subscribers from all directions. Antennas are also called "sectors" in professional jargon. You yourself have probably seen them many times - large gray rectangular blocks.

From the antenna, the signal is transmitted via cable directly to the control unit of the base station. The combination of sectors and the control block is usually called - BS, Base Station. Several base stations, whose antennas serve any particular territory or area of ​​the city, are connected to a special unit - the so-called LAC, Local Area Controller, "local area controller", often referred to simply controller. Up to 15 base stations are usually connected to one controller.

In turn, the controllers, which can also be several, are connected to the most central "brain" unit - MSC, Mobile services Switching Center, Control Center mobile services , commonly known as switch. The switch provides output (and input) to city telephone lines, to other operators cellular communication and so on.

That is, in the end, the whole scheme looks something like this:

Small GSM networks use only one switch, larger networks serving more than a million subscribers can use two, three or more MSc, combined with each other.

Why such complexity? It would seem that you can simply connect the antennas to the switch - and that's it, there would be no problems ... But not everything is so simple. The point here is one simple English word - handover. This term refers to handover in cellular networks. That is, when you walk down the street or drive a car (train, bicycle, roller skates, asphalt paver ...) and at the same time talk on the phone, then in order for the connection not to be interrupted (and it is not interrupted), you need to switch Your phone from one sector to another, from one BS to another, from one Local Area to another, and so on. Accordingly, if the sectors were directly connected to the switch, then all these switching would have to be controlled by the switch, which already has something to do. A multi-level network scheme makes it possible to evenly distribute the load, which reduces the likelihood of equipment failure and, as a result, loss of communication.

Example - if you and your phone move from the coverage area of ​​one sector to the coverage area of ​​another, then the BS control unit is engaged in transferring the phone, without affecting the "superior" devices - LAC and MSc. Accordingly, if the transition occurs between different BS, then it is controlled LAC and so on.

The operation of the switch should be considered in a little more detail. A switch in a cellular network performs almost the same functions as a PBX in wired networks. telephone networks. It is he who determines where you call, who calls you, is responsible for the work additional services, and, in the end - in general, determines whether it is possible to call or not.

On the last paragraph stop - what happens when you turn on your phone?

Here, turn on your phone. Your SIM card has a special number, the so-called IMSI - International Subscriber Identification Number, International Subscriber Identification Number. This number is unique for every SIM card in the world, and it is by this number that operators distinguish one subscriber from another. When the phone is turned on, it sends this code, the base station transmits it to LAC, LAC– to the switch, in turn. This is where two additional modules associated with the switch come into play - HLR, Home Location Register and VLR Visitor Location Register. Respectively, Register of Home Subscribers and Register of Guest Subscribers. AT HLR stored IMSI all subscribers who are connected to this operator. AT VLR in turn contains data on all subscribers who, in this moment use the network given operator. IMSI transferred to HLR(of course, in a highly encrypted form; we will not go into details of encryption in detail, we will only say that another block is responsible for this process - AuC, Authentication Center), HLR, in turn, checks whether it has such a subscriber, and, if so, whether it is blocked, for example, for non-payment. If everything is in order, then this subscriber is registered in VLR and from that moment can make calls. Large operators may have not one, but several in parallel HLR and VLR. And now let's try to display all of the above in the figure:

Here we briefly reviewed how it works cellular network. In fact, everything is much more complicated there, but if you describe everything as it is thoroughly, then this presentation may well exceed War and Peace in terms of volume.

Next, we will consider how (and most importantly, why!) The operator writes off money from our account. As you have probably heard, tariff plans there are three different types- the so-called "credit", "advance" and "prepaid", from English Pre-paid i.e. prepaid. What is the difference? Consider how money can be written off during a conversation:

Let's say you made a phone call. It was fixed on the switch - such and such a subscriber called there, talked, say, forty-five seconds.

The first case - you have a credit or advance payment system. In this case, the following happens: data about your and not only your calls are accumulated in the switch and then, in the order of the general queue, are transferred to a special block called billing, from English to bill - to pay bills. Billing is responsible for all issues related to subscribers' money - calculates the cost of calls, debits the monthly fee, debits money for services, and so on.

Transfer rate of information from MSc in Billing depends on what computing power billing, or, in other words, with what speed he manages to translate technical data about calls made into direct money. Accordingly, the more subscribers talk, or the more "brake" billing, the slower the queue will move, respectively, the greater the delay between the conversation itself and the actual debiting of money for this conversation. This fact is connected with the dissatisfaction often expressed by some subscribers - “Supposedly, they steal money! I didn’t talk for two days - they wrote off a certain amount ... ”. But at the same time, it does not take into account at all what conversations that took place, for example, three days ago, the money was not written off right away ... People try not to notice good things ... And these days, for example, billing could simply not work - due to an accident, or due to the fact that it was somehow modernized.

In the opposite direction - from billing to MSc- there is another queue in which billing informs the switch about the status of subscribers' accounts. Again, a fairly common case - the account debt can reach several tens of dollars, and you can still call on the phone - this is just because the “reverse” queue has not yet come up and the switchboard does not yet know that you are a malicious defaulter and You should have been banned for a long time.

Advance tariffs differ from credit tariffs only in the way of settlement with the subscriber - in the first case, a person deposits some amount into the account, and money for calls is gradually deducted from this amount. This method is convenient because it allows you to plan and limit your communication costs to some extent. The second option is a credit one, in which the total cost of all calls for any period (“ billing cycle”), usually for a month, is issued as an invoice, which the subscriber must pay. The credit system is convenient because it insures you against those cases when you urgently need to call, and the money in the account suddenly runs out and the phone is blocked.

Prepays are arranged in a completely different way:

In prepaid billing as such is commonly referred to as " Prepaid by platform».

Right at the start telephone connection establishes a direct link between switch and solder platform. No queues, data is transmitted in both directions directly during the conversation, in real time. In connection with this, the following characteristic features are inherent in solders - this is the absence subscription fee(because there is no such thing as billing period), a limited set of additional services (it is technically difficult to charge them in the “real time” mode), the impossibility of “going into the red” - the conversation will simply be interrupted as soon as the money in the account runs out. Clear dignity prepaid is the ability to accurately control the amount of money in the account, and, as a result, their expenses.

AT soldered sometimes there is some funny phenomenon - if prepaid platform for some reason refuses to work, for example, due to overload, then, accordingly, for subscribers prepaid tariffs during this time all calls become absolutely free. What, in fact, their - subscribers - can not but rejoice.

But how is our money calculated when we talk, being in roaming? And how does the phone work in roaming? Well, let's try to answer these questions:

Number IMSI consists of 15 digits, and the first 5 digits, the so-called SS - Country Code(3 digits) and NC-Network Code(5 digits) - clearly characterize the operator to which this subscriber is connected. These five numbers VLR finds a guest operator HLR home operator and looks in it - but, in fact, is it possible for this subscriber to use roaming with this operator? If yes, then IMSI prescribed by VLR guest operator, and HLR home - a link to the same guest VLR to know where to look for the caller.

With the write-off of money in billing, the situation is also not very simple. Due to the fact that the calls are processed by the guest switch, but the money is counted by its own, “home” billing, quite possible big delays in the write-off of funds - up to a month. Although there are systems, for example, " camel2”, which also work in roaming on the principle of prepaid, that is, they write off money in real time.

Here another question arises - what is the money debited for? roaming? If “at home” everything is clear - there are clearly defined tariff plans, then with roaming the situation is different - a lot of money is written off and it is not clear why. Well, let's try to figure it out:

All phone calls in roaming are divided into 3 main categories:

Incoming calls - in this case, the cost of the call consists of:

Costs international call from home to guest region
+
Cost of an incoming call from a guest operator
+
Some surcharge depending on the specific guest operator

Outgoing call home:

Cost of an international call from the guest region home
+
Cost of an outgoing call from a guest operator

Outgoing call by guest region:

Cost of an outgoing call from a guest operator
+
Some surcharge depending on the specific operator

As you can see, the cost of calls in roaming depends on only two things - on which operator the subscriber is connected to at home and which operator the subscriber uses at home. This reveals one very important thing– the cost of a minute in roaming is absolutely independent of the tariff plan chosen by the subscriber.

I would like to add one more note - if two phones of one operator are roaming together with another operator (well, for example, two friends went on vacation), then it will be very expensive for them to talk to each other - the caller pays as for outgoing home, and the receiving call - as for incoming from home. This is one of the disadvantages of the GSM standard - the fact that communication in this case goes through the house. Although it is technically quite possible to arrange a connection “directly”, but which of the operators will go for it if you can leave everything as it is and make money?

Another question that has often been of interest to owners of more than one mobile phone– how much will a redirected call from one phone to another cost? And the answer to this question is quite realistic:

Let's say call forwarding to phone C is set from phone B. Phone A calls to phone B - accordingly, the call is redirected to phone C. In this case, they pay:

Phone A - as for outgoing to phone B
(actually, this is logical - after all, he calls him)
Phone B - pays the call forwarding price
(usually a few cents per minute)
+
the cost of an international call from the region where B is registered to the region where C is registered
(if the phones are in the same region, then this component is equal to zero).
Phone C - pays as for incoming calls from phone A

At the end of the topics, I would like to mention one more subtle point - how much will call forwarding cost in roaming? And here the most interesting begins:

For example, the phone has call forwarding on a busy condition to home number. Then at incoming call the so-called " roaming loop"- the call will go to home phone via guest switch, respectively, the cost of such a forwarded call for roamer will be equal to the sum of the costs of incoming and outgoing home calls, plus the cost of the call forwarding itself. And what is funny at the same time - the roamer may not even know that such a call took place, and subsequently be surprised when he sees the bill for communication.

From this follows practical advice - when traveling, it is advisable to turn off all types of call forwarding (you can leave only unconditional - in this case, the "roaming loop" does not work), especially call forwarding to voicemail- otherwise, you can later be surprised for a long time - "Where did this money go, huh?"

List of terms used in the text:

AuC– Autentification Center, the Authentication Center, is responsible for encoding information when transmitted over the network and received from the network
Billing– Billing, cash accounting system for the operator
BS– Base Station, a base station, several transmitting and receiving antennas belonging to one control device.
camel2– one of the Prepaid systems, which implements instant withdrawal of funds in roaming
CC– Country Code, country code in the GSM standard (for Russia - 250)
GSM– Global System for Mobile Communications, the world's most widespread cellular communication standard
Handover - handset control transfer from one antenna/base station/LAC to another
HLR– Home Location Register, a register of home subscribers, contains detailed information about all subscribers connected to this operator.
IMEI– International Mobile Equipment Identification, international serial number equipment in the GSM standard, unique for each device
IMSI– International Mobile Subscriber Identification, the international serial number of a subscriber to GSM services, is unique for each subscriber
LAC– Local Area Controller, Local Area Controller, devices, work manager a certain number of base stations whose antennas serve a certain area.
local area– Local area, territory served by BSs that are part of the same LAC
MSc- Mobile services Switching Center, Mobile Services Control Center, the switch is the central link of the GSM network.
NC– Network Code, Network Code, the code of a specific operator in a given country in the GSM standard (for MTS - 01, BeeLine - 99).
Prepaid- Prepaid, prepayment - a billing system based on instant debiting of funds.
Roaming– Roaming, using the network of another, "guest" operator.
SIM- Subscriber Identification Module, Subscriber Identification Module, SIM card - an electronic unit inserted into the phone on which the subscriber's IMSI is recorded.
VLR– Visitor Location Register, register of active subscribers – contains information about all subscribers who currently use the services of this operator.

Telephone communication is the transmission of speech information over long distances. Telephony allows people to communicate in real time.

If at the time of the emergence of technology there was only one method of data transmission - analog, then in this moment successfully apply the most different systems communications. Telephone, satellite and mobile connection, as well as IP-telephony provide reliable contact between subscribers, even if they are in different parts of the world. How does it work telephone communications when using each of the methods?

Good old wired (analogue) telephony

The term "telephone" communication is most often understood as analog communication, a method of data transmission that has become familiar for almost a century and a half. When using this, information is transmitted continuously, without intermediate encoding.

The connection of two subscribers is regulated by dialing, and then communication is carried out by transmitting a signal from person to person over wires in the most literal sense of the word. Subscribers are no longer connected by telephone operators, but by robots, which greatly simplified and reduced the cost of the process, but the principle of operation of analog communication networks remained the same.

Mobile (cellular) communication

Subscribers of cellular operators mistakenly believe that they "cut the wire" connecting them to telephone exchanges. In appearance, everything is so - a person can move anywhere (within the signal coverage), without interrupting the conversation and without losing contact with the interlocutor, and<подключить телефонную связь стало легче и проще.

However, if we understand how mobile communications work, we will find not so many differences from the operation of analog networks. The signal actually "hovers in the air", only from the caller's phone it gets to the transceiver, which, in turn, communicates with the similar equipment closest to the called subscriber ... via fiber optic networks.

The data radio stage covers only the signal path from the phone to the nearest base station, which is connected to other communication networks in a completely traditional way. How cellular communication works is clear. What are its pros and cons?

The technology provides greater mobility than analog data transmission, but carries all the same risks of unwanted interference and the possibility of listening to lines.

Cell signal path

Let us consider in more detail exactly how the signal reaches the called subscriber.

1. The user dials a number.
2. His phone establishes a radio link with the nearest base station. They are located on high-rise buildings, industrial buildings and towers. Each station consists of transmitting and receiving antennas (from 1 to 12) and a control unit. Base stations that serve one area are connected to the controller.
3. From the control unit of the base station, the signal is transmitted via cable to the controller, and from there, also via cable, to the switch. This device provides signal input and output to various communication lines: long-distance, urban, international, and other mobile operators. Depending on the size of the network, it may involve either one or several switches connected to each other by wires.
4. From “their” switchboard, the signal is transmitted via high-speed cables to the switchboard of another operator, and the latter easily determines in which controller the subscriber to whom the call is addressed is located.
5. The switch calls the desired controller, which sends a signal to the base station, which "interrogates" the mobile phone.
6. The called party receives an incoming call.

Such a multi-layer structure of the network allows you to evenly distribute the load between all its nodes. This reduces the possibility of equipment failure and ensures uninterrupted communication.

How cellular communication works is clear. What are its pros and cons? The technology provides greater mobility than analog data transmission, but carries all the same risks of unwanted interference and the possibility of listening to lines.

Satellite connection

Let's see how satellite communications, the highest level of development of radio relay communications today, work. A repeater placed in orbit is capable of covering a vast area of ​​the planet's surface alone. A network of base stations, as in the case of cellular communications, is no longer needed.

An individual subscriber gets the opportunity to travel with virtually no restrictions, staying connected even in the taiga or the jungle. A legal entity subscriber can bind an entire mini-PBX to one repeater antenna (this is the already familiar “dish”), however, one should take into account the volume of incoming and outgoing, as well as the size of the files that need to be sent.

Technology cons:

• serious weather dependence. A magnetic storm or other cataclysm can leave a subscriber without communication for a long time.
• if something physically breaks down on a satellite transponder, the period that will pass before the functionality is fully restored will stretch for a very long time.
• the cost of communication services without borders often exceeds the more usual bills. When choosing a communication method, it is important to consider how much you need such a functional connection.

Satellite communications: pros and cons

The main feature of the "satellite" is that it provides subscribers with independence from land lines. The advantages of such an approach are obvious. These include:

• equipment mobility. It can be deployed in a very short time;
• the ability to quickly create extensive networks covering large areas;
• communication with hard-to-reach and remote territories;
• redundancy of channels that can be used in the event of a breakdown of terrestrial communications;
• the flexibility of the technical characteristics of the network, allowing it to be adapted to almost any requirements.

Technology cons:

• serious weather dependence. A magnetic storm or other cataclysm can leave a subscriber without communication for a long time;
• if something is physically out of order on the satellite transponder, the period that will pass until the system functionality is fully restored will stretch for a long time;
• the cost of communication services without borders often exceeds the more usual bills.

When choosing a communication method, it is important to consider how much you need such a functional connection.

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A huge number of BS - base stations have been installed on the territory of Russia. Probably, many of you have seen red and white structures towering in the fields or structures installed on the roofs of non-residential buildings. Each such base station is capable of picking up a signal from a cell phone at a distance of up to 35 km, communicating with it via service or voice channels.

After you dialed the number of the desired subscriber on your phone, the following happens: the mobile phone finds the nearest BS, contacts it via the service channel and requests a voice channel. After that, the BS sends a request to the controller (BSC), which then goes to the communicator. If the called party is served by the same operator as you, then the communicator will check the Home Location Register (HLR) database to find out exactly where the person you are calling is located and redirect the call to the correct switchboard, which will then transfer the call to the controller and then to the Base Station. And finally, the Base Station will contact the mobile phone of the right person and connect you with him. And if the one you want to talk to is a subscriber of another cellular operator, or you call a landline number, then the switch will “find” the corresponding switch of another network and turn to it. Sounds pretty confusing, right? Let's try to analyze this issue in more detail.

But back to the equipment. As we have already said, from the BS the call is transferred to the controller (BSC). Outwardly, it is not much different from the Base Station:

The number of base stations that are able to serve the controller can reach six dozen. The controller and the BS communicate via optical or radio relay channels. The controller controls the operation of the radio channels.

Below you can see what a switch is:

The number of controllers serviced by the switch varies from two to thirty. The switches are placed in large rooms filled with metal cabinets with equipment.

The task of the switch is to manage traffic. If earlier, in order to talk to each other, subscribers had to first contact the telephone operator, who then manually rearranged the necessary wires, now the switch does an excellent job with her role.

Inside the cars there are devices designed for data collection and processing:

Controllers and switches are under vigilant control 24 hours a day. Tracking is carried out in the so-called CKC (Air Control Center of the Network Control Center).