Principle of operation cellular communications

The basic principles of cellular telephony are quite simple. The Federal Communications Commission originally established geographic coverage areas for cellular radio systems based on modified 1980 Census data. The idea behind cellular communications is that each area is subdivided into hexagonal-shaped cells that fit together to form a honeycomb-like structure, as shown in the figure. 6.1, a. The hexagonal shape was chosen because it provides the most efficient transmission, approximately matching the circular radiation pattern while eliminating the gaps that always appear between adjacent circles.

A cell is defined by its physical size, population, and traffic patterns. The Federal Communications Commission does not regulate the number of cells in a system or their size, leaving operators to set these parameters in accordance with expected traffic patterns. Each geographic area is allocated a fixed number of cellular voice channels. Physical Dimensions Cells depend on subscriber density and call structure. For example, large cells (macrocells) typically have a radius of 1.6 to 24 km with a base station transmitter power of 1 W to 6 W. The smallest cells (microcells) typically have a radius of 460 m or less with a base station transmitter power of 0.1 W to 1 W. Figure 6.1b shows a cellular configuration with two cell sizes.

Figure 6.1. – Honeycomb structure of cells a); honeycomb structure with honeycombs of two sizes b) classification of honeycombs c)

Microcells are most often used in regions with high density population. Due to their short range, microcells are less susceptible to interference that degrades transmission quality, such as reflections and signal delays.

A macro cell can be superimposed on a group of micro cells, with the micro cells serving slow moving mobile devices and the macro cell serving fast moving mobile devices. The mobile device is able to determine the speed of its movement as fast or slow. This allows you to reduce the number of transitions from one cell to another and the correction of location data.

The algorithm for moving from one cell to another can be changed at short distances between the mobile device and the microcell base station.

Sometimes the radio signals in a cell are too weak to provide reliable communications indoors. This is especially true for well-shielded areas and areas with high levels of interference. In such cases, very small cells are used - picocells. Indoor picocells can use the same frequencies as regular cells of this region, especially in favorable environments, such as underground tunnels.

When planning systems using hexagonal-shaped cells, base station transmitters can be located in the center of the cell, on the edge of the cell, or at the top of the cell (Figure 6.2 a, b, c, respectively). Cells with a transmitter in the center typically use omnidirectional antennas, while cells with transmitters on an edge or vertex typically use sectorial directional antennas.

Omnidirectional antennas radiate and receive signals equally in all directions.

Figure 6.2 – Placement of transmitters in cells: in the center a); on edge b); at the top c)

In a cellular communication system, one powerful fixed base station located high above the city center can be replaced by numerous identical low-power stations that are installed in the coverage area at sites located closer to the ground.

Cells using the same group of radio channels can avoid interference if they are properly spaced. In this case, frequency reuse is observed. Frequency reuse is the allocation of the same group of frequencies (channels) to several cells, provided that these cells are separated by significant distances. Frequency reuse is facilitated by reducing the coverage area of ​​each cell. The base station of each cell is allocated a group of operating frequencies that differ from the frequencies of neighboring cells, and the base station antennas are selected in such a way as to cover the desired service area within its cell. Since the service area is limited to the boundaries of a single cell, different cells can use the same group of operating frequencies without interference, provided that two such cells are located at a sufficient distance from each other.

Geographic service area cellular system, containing several groups of cells is divided into clusters (Figure 6.3). Each cluster consists of seven cells, which are allocated the same number of full-duplex communication channels. Cells with the same letter designations use the same group of operating frequencies. As can be seen from the figure, the same frequency groups are used in all three clusters, which makes it possible to triple the number available channels mobile communications. Letters A, B, C, D, E, F And G represent seven frequency groups.

Figure 6.3 – Principle of frequency reuse in cellular communications

Consider a system with a fixed number of full-duplex channels available in some area. Each service area is divided into clusters and receives a group of channels that are distributed between N honeycombs of the cluster, grouping into non-repeating combinations. All cells have the same number of channels, but they can serve single-sized areas.

Thus, the total number of cellular channels available in the cluster can be represented by the expression:

F=GN (6.1)

Where F– the number of full-duplex cellular communication channels available in the cluster;

G– number of channels in a cell;

N– number of cells in the cluster.

If the cluster is "copied" within a given service area m times, then the total number of full duplex channels will be:

C = mGN = mF (6.2)

Where WITH– total number of channels in a given zone;

m– number of clusters in a given zone.

From expressions (6.1) and (6.2) it is clear that the total number of channels in a cellular telephone system is directly proportional to the number of “repetitions” of a cluster in a given service area. If the cluster size is reduced while the cell size remains the same, more clusters will be needed to cover a given service area and the total number of channels in the system will increase.

The number of subscribers who can simultaneously use the same group of frequencies (channels), while not being in neighboring cells of a small service area (for example, within a city), depends on the total number of cells in a given area. Typically the number of such subscribers is four, but in densely populated regions it can be much higher. This number is called frequency reuse factor or FRF – Frequency reuse factor. Mathematically it can be expressed by the relation:

(6.3)

Where N– the total number of full-duplex channels in the service area;

WITH– the total number of full-duplex channels in the cell.

With the projected increase in cellular traffic, the increased demand for service is met by reducing the size of the cell, dividing it into several cells, each with its own base station. Effective cell separation allows the system to handle more calls as long as the cells are not too small. If the cell diameter becomes less than 460 m, then the base stations of neighboring cells will influence each other. The relationship between frequency reuse and cluster size determines how scale cellular system in case of increasing subscriber density. The fewer cells in a cluster, the greater the likelihood of mutual influences between channels.

Because cells are hexagonal in shape, each cell always has six equally spaced adjacent cells, and the angles between the lines connecting the center of any cell to the centers of neighboring cells are multiples of 60°. Therefore, the number of possible cluster sizes and cell layouts is limited. To connect cells to each other without gaps (in a mosaic way), the geometric dimensions of the hexagon must be such that the number of cells in the cluster satisfies the condition:

(6.4)

Where N– number of cells in the cluster; i And j– non-negative integers.

Finding a route to the nearest cells with a shared channel (the so-called first-tier cells) occurs as follows:

Move to i cells (through the centers of neighboring cells):

Move to j cells forward (through the centers of neighboring cells).

For example, the number of cells in the cluster and the location of the first tier cells for the following values: j = 2. i = 3 will be determined from expression 6.4 (Figure 6.4) N = 3 2 + 3 2 + 2 2 = 19.

Figure 6.5 shows the six closest cells using the same channels as the cell A.

The process of handing over from one cell to another, i.e. when a mobile device moves from base station 1 to base station 2 (Figure 6.6) includes four main stages:

1) initiation - the mobile device or network detects the need for handover and initiates the necessary network procedures;

2) resource reservation - using appropriate network procedures, network resources necessary for service transfer (voice channel and control channel) are reserved;

3) execution – direct transfer of control from one base station to another;

4) termination - excess network resources are released, becoming available to other mobile devices.

Figure 6.6 – Handover

Almost everyone used a cell phone, but few people thought about how it all works? In this literary opus we will try to consider how communication occurs 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 channel with one of the antennas of the nearest base station.

Each of base stations contains from one to twelve transceiver antennas directed in different directions to provide communication to subscribers from all directions. In professional jargon, antennas are also called “sectors”. 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 set of sectors and a control block is usually called - BS, Base Station, base station. Several base stations, whose antennas serve a specific territory or area of ​​the city, are connected to a special unit - the so-called LAC, Local Area Controller, often simply called controller. Up to 15 base stations are usually connected to one controller.

In turn, the controllers, of which there may also be several, are connected to the very central “brain” unit - MSC, Mobile services Switching Center, Control Center Mobile services , popularly known as switch. The switch provides output (and input) to city telephone lines, to other cellular operators and so on.

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

Small GSM networks use only one switch; larger ones, serving more than a million subscribers, can use two, three or more M.S.C., united with each other.

Why such complexity? It would seem that you could simply connect the antennas to the switch - and that’s it, there would be no problems... But it’s not so simple. It's all about one simple English word - handover. This term refers to handover in cellular networks. That is, when you are walking down the street or driving a car (train, bicycle, roller skates, asphalt paver...) and at the same time talking on the phone, then in order for the connection not to be interrupted (and it is not interrupted), you need to switch in time 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 switchings would have to be managed by the switch, which already has something to do. A multi-level network design 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 handles the transfer of the phone, without affecting the “superior” devices - L.A.C. And M.S.C.. Accordingly, if the transition occurs between different B.S., then it is controlled L.A.C. 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 telephone networks. It is he who determines where you call, who calls you, and is responsible for the work additional services, and, in the end, generally determines whether it is possible to call or not.

Let's stop at the last point - what happens when you turn on your phone?

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

Here we briefly looked at how it works cellular network. In fact, everything there is much more complicated, but if we describe everything as it is in detail, then this presentation may well exceed “War and Peace” in volume.

Next, we will look at how (and most importantly, why!) the operator debits money from our account. As you've probably already heard, tariff plans there are three different types– the so-called “credit”, “advance” and “prepaid”, from English Pre-Paid, that is, prepaid. What's the difference? Let's look at how money can be written off during a conversation:

Let's say you called somewhere. It was recorded on the switchboard that subscriber such and such called there and talked for, say, forty-five seconds.

The first case is that you have a credit or advance payment system. In this case, the following happens: data about your and not only your calls is accumulated in the switch and then, in the order of the general queue, is 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, writes off subscription fees, writes off money for services, and so on.

Information transfer speed from M.S.C. V Billing depends on how much computing power you have billing, or, in other words, how quickly he manages to convert technical data about calls made into direct money. Accordingly, the more subscribers talk, or the more “slow” the billing, the slower the queue will move, and accordingly, the greater the delay between the conversation itself and the actual debiting of money for this conversation. This fact is associated with the dissatisfaction often expressed by some subscribers - “They say they are stealing money! I didn’t speak for two days - a certain amount was written off...” But it does not take into account at all that for conversations that took place, for example, three days ago, the money was not immediately written off... People try not to notice good things... And these days, for example, billing could simply not work - because of an accident, or because it was somehow modernized.

In the opposite direction - from billing to M.S.C.- there is another queue in which billing informs the switchboard about the status of subscribers' accounts. Again, a fairly common case - the debt on the account can reach several tens of dollars, but you can still make phone calls - this is precisely because the “reverse” queue has not yet arrived and the switchboard does not yet know that you are a malicious defaulter and You should have been blocked a long time ago.

Advance tariffs differ from credit tariffs only in the method 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 credit, in which the total cost of all calls for any period (“ billing cycle"), usually per month, is issued in the form of an invoice that the subscriber must pay. The credit system is convenient because it insures you against those cases when you urgently need to make a call, but the money in your account suddenly runs out and your phone is blocked.

Prepaids are designed completely differently:

In the prepaide billing as such is usually called " Pripad platform».

Immediately at the moment the telephone connection starts, a direct connection is established between switch And prepaid platform. No queues, data is transmitted in both directions directly during the conversation, in real time. In connection with this, prepaids have the following characteristic features: the absence subscription fee(since there is no such thing as billing period), a limited range of additional services (they are technically difficult to charge in “real time”), the inability to “go into the red” - the conversation will simply be interrupted as soon as the money in the account runs out. Clear dignity preipedes is the ability to accurately control the amount of money in the account, and, as a result, your expenses.

IN preipedes sometimes a funny phenomenon is observed - if prepaid platform for some reason refuses to work, for example, due to overload, then, accordingly, for subscribers prepaid tariffs at this time all calls become absolutely free. Which, in fact, makes them – the subscribers – happy.

But how is our money calculated when we talk while in roaming? And how does the phone generally 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 СС – Country Code(3 digits) and NC – Network Code(5 digits) – clearly characterize the operator to which you are connected this subscriber. According to these five numbers VLR finds the guest operator HLR home operator and looks in it - but, in fact, can this subscriber use roaming with this operator? If yes, then IMSI is registered with VLR guest operator, and in HLR home - link to the same guest VLR to know where to look for the subscriber.

The situation with writing off money in billing is also not very simple. Due to the fact that calls are processed by the guest switch, but the “home” switch counts the money billing, large delays in debiting funds are quite possible - up to a month. Although there are systems, for example, “ Camel2”, which even in roaming work on the prepaid principle, that is, they write off money in real time.

Here another question arises - what is the money written off 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 to home
+
Cost of an outgoing call from a guest operator

Outgoing call to 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 only on two things - on which operator the subscriber is connected to at home and which operator the subscriber uses when away. This reveals one very important thing - the cost of a minute in roaming absolutely does not depend on the tariff plan chosen by the subscriber.

I would like to add one more remark - 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 recipient pays the call is like someone coming from home. This is one of the disadvantages of the GSM standard - that communication in this case goes through the house. Although technically it is quite possible to arrange a connection “directly”, which operator will do this if you can leave everything as it is and make money?

One more question, in Lately often of interest to owners of more than one mobile phone– how much will a forwarded call from one phone to another cost? And it’s quite possible to answer this question:

Let’s say call forwarding is set from phone B to phone C. A call is made from phone A to phone B - accordingly, the call is forwarded to phone C. In this case, they pay:

Phone A – as for outgoing to phone B
(actually, this is logical - after all, that’s what he’s calling)
Phone B – pays the 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 from the same region, then this component is zero).
Phone C – pays as for incoming calls from phone A

In conclusion, I would like to mention one more subtle point - how much will forwarding in roaming cost? And here's where the fun begins:

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

this implies practical advice– when traveling, it is advisable to disable all types of forwarding (you can leave only unconditional - in this case, a “roaming loop” does not work), especially forwarding to voicemail- otherwise, later you can wonder for a long time - “Where did that money go, huh?”

List of terms used in the text:

AuC– Authentification Center, Authentication Center, is responsible for encoding information when transmitted over the network and received from the network
Billing– Billing, accounting system Money from the operator
B.S.– Base Station, base station, several transceiver antennas belonging to one control device.
Camel2– one of the Prepaid systems, which implements instant debiting of funds in roaming
CC– Country Code, country code in the GSM standard (for Russia – 250)
GSM– Global System for Mobile Communications, the most widespread cellular communication standard in the world
Handover – transfer of handset control 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 for GSM standard services, is unique for each subscriber
L.A.C.– Local Area Controller, Local Zone Controller, devices, work manager a certain number of base stations whose antennas serve a certain area.
Local Area– Local zone, an area served by BSs that are part of the same LAC
M.S.C.- Mobile services Switching Center, Mobile Services Control Center, 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 – the electronic unit, inserted into the phone on which the subscriber’s IMSI is recorded.
VLR– Visitor Location Register, a register of active subscribers – contains information about all subscribers who are currently using the services of this operator.

Telephone communication is the transmission of voice information over long distances. With the help of telephony, people have the opportunity to communicate in real time.

If at the time of the emergence of technology there was only one method of data transmission - analog, then at the moment a variety of communication systems are successfully used. 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 method?

Good old wired (analog) telephony

The term “telephone” communication most often refers to analog communication, a method of data transmission that has become commonplace over almost a century and a half. When using this, information is transmitted continuously, without intermediate encoding.

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

Mobile (cellular) communications

Subscribers of cellular operators mistakenly believe that they have “cut the wire” connecting them to telephone exchanges. In appearance, everything is so - a person can move anywhere (within 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 many differences from the operation of analogue networks. The signal actually “floats in the air”, only from the caller’s phone it goes to the transceiver, which, in turn, communicates with similar equipment closest to the called subscriber... through fiber optic networks.

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

The technology provides greater mobility compared to analog data transmission, but carries the same risks of unwanted interference and the possibility of wiretapping.

Cell Signal Path

Let's take a closer look at exactly how the signal reaches the called subscriber.

1. The user dials a number.
2. His phone establishes radio contact with a nearby base station. They are located on high-rise buildings, industrial buildings and towers. Each station consists of transceiver antennas (from 1 to 12) and a control unit. Base stations that serve one territory are connected to the controller.
3. From the base station control unit, 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: intercity, city, international, and other mobile operators. Depending on the size of the network, it may involve either one or several switches connected to each other using wires.
4. From “your” switch, the signal is transmitted via high-speed cables to the switch of another operator, and the latter easily determines in the coverage area of ​​which controller the subscriber to whom the call is addressed is located.
5. The switch calls the desired controller, which sends the signal to the base station, which “interrogates” the mobile phone.
6. The called party receives an incoming call.

This multi-layer network structure allows the load to be evenly distributed between all its nodes. This reduces the likelihood of equipment failure and ensures uninterrupted communication.

It's clear how cellular communications work. What are its pros and cons? The technology provides greater mobility compared to analog data transmission, but carries the same risks of unwanted interference and the possibility of wiretapping.

Satellite connection

Let's see how satellite communications, the highest level of development of radio relay communications today, works. A repeater placed in orbit is capable of covering a huge area of ​​the planet's surface on its own. A network of base stations, as is the case with 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 subscriber who is a legal entity can attach an entire mini-PBX to one repeater antenna (this is the now familiar “dish”), but one must take into account the volume of incoming and outgoing messages, as well as the size of the files that need to be sent.

Disadvantages of technology:

• 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 repeater, the time it takes for functionality to be fully restored will take a very long time.
• the cost of borderless communication services often exceeds more conventional 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 terrestrial communication lines. The advantages of this 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 territories;
• communication with hard-to-reach and remote areas;
• reservation of channels that can be used in the event of a breakdown of terrestrial communications;
• flexibility of network technical characteristics, allowing it to be adapted to almost any requirements.

Disadvantages of technology:

• serious weather dependence. A magnetic storm or other cataclysm can leave a subscriber without communication for a long time;
• if something physically fails on the satellite repeater, the period until the system’s functionality is fully restored will take a long time;
• the cost of borderless communication services often exceeds more conventional bills.

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

Millions of people around the world use mobile phones because mobile phones have made it much easier to communicate with people around the world.

Mobile phones these days come with a range of features, and more are becoming available every day. Depending on your mobile phone model, you can do the following:

Save important information
Take notes or make a to-do list
Record important meetings and turn on alarms for reminders
use a calculator for calculations
send or receive mail
search for information (news, statements, jokes and much more) on the Internet
play games
watch TV
send messages
Use other devices such as MP3 players, PDAs and GPS navigation systems.

But haven't you ever wondered how a mobile phone works? And what makes it different from a simple landline phone? What do all these terms PCS, GSM, CDMA and TDMA mean? This article will talk about new features of mobile phones.

Let's start with the fact that a mobile phone is essentially a radio - a more advanced type, but a radio nonetheless. The telephone itself was created by Alexander Graham Bell in 1876, and wireless communication a little later by Nikolai Tesla in the 1880s (the Italian Guglielmo Marconi first began talking about wireless communication in 1894). It was destined for these two great technologies to come together.

In ancient times, when there were no mobile phones, people installed radio phones in their cars to communicate. This radiotelephone system operated using one main antenna installed on a tower outside the city and supported about 25 channels. To connect to the main antenna, the phone had to have a powerful transmitter - with a radius of about 70 km.

But not many could use such radio phones due to the limited number of channels.

The genius of the mobile system lies in dividing the city into several elements (“cells”). This promotes frequency reuse throughout the city, so millions of people can use mobile phones at the same time. “Honeycomb” was not chosen by chance, since it is the honeycomb (hexagon-shaped) that can most optimally cover the area.

In order to better understand the operation of a mobile phone, it is necessary to compare CB radio (i.e. regular radio) and cordless telephone.

Full-duplex portable device versus half-duplex - radiotelephones, like simple radios, are half-duplex devices. This means that two people use the same frequency, so they can only speak in turns. A mobile phone is a full-duplex device, which means that a person uses two frequencies: one frequency is for hearing the person on the other side, the other is for speaking. Therefore, you can talk on mobile phones at the same time.

Channels - a radiotelephone uses only one channel, a radio has about 40 channels. A simple mobile phone may have 1,664 channels or more.

In half-duplex devices, both radio transmitters use the same frequency, so only one person can talk. In full duplex devices, the 2 transmitters use different frequencies so people can talk at the same time. Mobile phones are full duplex devices.

In a typical US mobile phone system, a mobile phone user uses about 800 frequencies to talk around town. A mobile phone divides a city into several hundreds. Each cell has a specific size and covers an area of ​​26 km2. Honeycombs are like hexagons enclosed in a lattice.

Because mobile phones and stations use low-power transmitters, non-adjacent cells may use the same frequencies. The two cells may use the same frequencies. The cellular network consists of powerful high-speed computers, base stations (multi-frequency VHF transceivers) distributed throughout the entire working area of ​​the cellular network, mobile phones and other high-tech equipment. We'll talk about base stations further, but now let's look at the “cells” that make up a cellular system.

One cell in an analog cellular system uses 1/7 of the available two-way communication channels. This means that each cell (out of 7 cells in the grid) uses 1/7 of the available channels, which have their own set of frequencies and therefore do not overlap each other:

A mobile phone user usually receives 832 radio frequencies for talking around the city.
Each mobile phone uses 2 frequencies per call - the so-called. two-way channel - therefore, for each mobile phone user there are 395 communication channels (the remaining 42 frequencies are used by the main channel - we will talk about it later).

Thus, each cell has up to 56 available communication channels. This means that 56 people will be able to talk on mobile phones at the same time. The first mobile technology, 1G, is considered an analogue of the cellular network. Since digital information transmission (2G) began to be used, the number of channels has increased significantly.

Mobile phones have built-in low-power transmitters, so they operate at 2 signal levels: 0.6 watts and 3 watts (for comparison, here is a simple radio that operates at 4 watts). Base stations also use low-power transmitters, but they have their own advantages:

The transmission of the base station and mobile phone signal within each cell does not allow you to move far from the cell. In this way, both cells can reuse the same 56 frequencies. The same frequencies can be used throughout the city.
The charge consumption of a mobile phone, which usually runs on battery power, is not significantly high. Low-power transmitters mean small batteries, which makes mobile phones more compact.

A cellular network needs a number of base stations, regardless of the size of the city. A small city should have several hundred towers. All mobile phone users in any city are managed by one main office, which is called the Mobile Phone Switching Center. This center controls all telephone calls and base stations in a given area.

Mobile phone codes

Electronic Sequence Number (ESN) is a unique 32-bit number programmed into the mobile phone by the manufacturer.
Mobile Identification Number (MIN) is a 10-digit code derived from a mobile phone number.
The System Identification Code (SID) is a unique 5-digit code assigned to each FCC company. The last two codes, MIN and SID, are programmed into the cell phone when you purchase the card and turn on the phone.

Each mobile phone has its own code. Codes are needed to recognize phones, mobile phone owners and mobile operators. For example, you have a mobile phone, you turn it on and try to make a call. Here's what happens during this time:

When you first turn on the phone, it looks for an identification code on the main control channel. A channel is a special frequency that mobile phones and the base station use to transmit signals. If the phone cannot find the control channel, then it is out of reach and the message “no network” is displayed on the screen.
When the phone receives an identification code, it checks it against its own code. If there is a match, the mobile phone is allowed to connect to the network.
Along with the code, the phone requests access to the network and the Mobile Switching Center records the phone's position in the database, so the Switching Center knows which phone you are using when it wants to send you a service message.
The switching center receives calls and can calculate your number. At any time, he can look up your phone number in his database.
The switching center contacts your mobile phone to tell you which frequency to use and after the mobile phone communicates with the antenna, the phone gains access to the network.

The cell phone and base station maintain constant radio contact. A cell phone periodically switches from one base station to another, which emits a stronger signal. If a cell phone moves out of the field of a base station, it establishes a connection with another, nearby base station, even during a conversation. The two base stations "communicate" through the Switching Center, which transmits a signal to your mobile phone to change frequency.

There are cases when, when moving, the signal moves from one cell to another, belonging to another mobile operator. In this case, the signal does not disappear, but is transferred to another mobile operator.

Most modern cell phones can operate in several standards, which allows you to use roaming services in different cellular networks. The switching center whose cells you are now using contacts your switching center and asks for code confirmation. Your system transfers all the data about your phone to another system and the Switching Center connects you to the cells of the new mobile operator. And the most amazing thing is that all this is done within a few seconds.

The most annoying thing about all this is that you can pay a pretty penny for roaming calls. On most phones, when you first cross the border, the roaming service is displayed. Otherwise, you better check your mobile coverage map so that you don’t have to pay “inflated” tariffs later. Therefore, check the cost of this service immediately.

Please note that the phone must work in more than one band if you want to use the roaming service, because different countries use different bands.

In 1983, the first analog mobile telephone standard, AMPS (Advanced Mobile Telephone Service), was developed. This analog mobile communication standard operates in the frequency range from 825 to 890 MHz. In order to maintain competition and keep prices in the market, the US federal government required that there be at least two companies engaged in the same business in the market. One such company in the United States was the Local Telephone Company (LEC).

Each company had its own 832 frequencies: 790 for calls and 42 for data. To create one channel, two frequencies were used at once. The frequency range for the analog channel was typically 30 kHz. The transmission and reception range of the voice channel is separated by 45 MHz, so that one channel does not overlap the other.

A version of the AMPS standard called NAMPS (Narrowband Advanced Communications System) uses new digital technologies to allow the system to triple its capabilities. But even though it uses new digital technologies, this version remains just analogue. Analog standards AMPS and NAMPS operate only at 800 MHz and cannot yet offer a wide variety of functions, such as Internet connectivity and mail.

Digital mobile phones belong to the second generation (2G) of mobile technology. They use the same radio technology as analog phones, but in a slightly different way. Analogue systems do not fully utilize the signal between the phone and the mobile network - analogue signals cannot be jammed or manipulated as easily as digital signals can. This is one reason why many cable companies are switching to digital - so they can use more channels in a given band. It's amazing how effective a digital system can be.

Many digital mobile systems use frequency modulation (FSK) to transmit and receive data through the analog AMPS portal. Frequency modulation uses 2 frequencies, one for logic one, the other for logic zero, choosing between the two, when transmitting digital information between the tower and the mobile phone. In order to convert analog information into digital and vice versa, modulation and a coding scheme are required. This suggests that digital mobile phones must be able to process data quickly.

In terms of complexity per cubic inch, mobile phones are among the most complex modern devices. Digital mobile phones can perform millions of calculations per second in order to encode or decode a voice stream.

Any regular phone consists of several parts:

The chip (board) that is the brain for the phone
Antenna
Liquid crystal display (LCD)
Keyboard
Microphone
Speaker
Battery

The microcircuit is the center of the entire system. Next, we will look at what types of chips there are and how each of them works. The analog-to-digital and back-to-digital conversion chip encodes the outgoing audio signal from an analog system to a digital one and the incoming signal from a digital system to an analog one.

A microprocessor is a central processing device responsible for performing the bulk of information processing work. It controls the keyboard and display, and many other processes.

The ROM chips and memory card chip allow you to store mobile phone operating system data and other user data, such as phone book data. Radio frequency controls power and charging and handles hundreds of FM waves. The high-frequency amplifier controls the signals that are received or reflected by the antenna. Screen size has increased significantly since mobile phones have become more functional. Many phones have notebooks, calculators, and games. And now many more phones are connected to a PDA or Web browser.

Some phones store certain information, such as SID and MIN codes, in built-in flash memory, while others use external cards such as SmartMedia cards.

Many phones have speakers and microphones so tiny that it's hard to imagine how they make sound at all. As you can see, the speakers are the same size as a small coin, and the microphone is no larger than a watch battery. By the way, such watch batteries are used in the internal chip of a mobile phone to operate the watch.

The most amazing thing is that 30 years ago many of these parts occupied an entire floor of the building, but now all this fits in the palm of a person.

There are three most common ways 2G mobile phones use radio frequencies to transmit information:

FDMA (Frequency Division Multiple Access) TDMA (Time Division Multiple Access) CDMA (Code Division Multiple Access)

Although the names of these methods seem so confusing, you can easily guess how they work simply by breaking the name down into individual words.

The first word, frequency, time, code, indicates the access method. The second word, division, means that it separates calls based on access method.

FDMA places each phone call on a separate frequency. TDMA allocates each call a specific time on its assigned frequency. CDMA assigns a unique code to each call and then transmits it to a free frequency.

The last word of each method, multiple, means that each hundredth can be used by several people.

FDMA

FDMA (Frequency Division Multiple Access) is a method of using radio frequencies where only one subscriber is in the same frequency band, different subscribers use different frequencies within a cell. Is an application of frequency division multiplexing (FDM) in radio communications. To better understand how FDMA works, we need to look at how radios work. Each radio station sends its signal to free frequency bands. The FDMA method is used primarily for transmitting analog signals. And although this method can undoubtedly transmit digital information, it is not used because it is considered less effective.

TDMA

TDMA (Time Division Multiple Access) is a method of using radio frequencies when there are several subscribers in the same frequency slot, different subscribers use different time slots (intervals) for transmission. It is an application of Time Division Multiplexing (TDM) to radio communications. When using TDMA, a narrow frequency band (30 kHz wide and 6.7 milliseconds long) is divided into three time slots.

A narrow frequency band is usually understood as “channels”. Voice data converted into digital information is compressed, causing it to take up less space. Therefore, TDMA operates three times faster than an analog system using the same number of channels. TDMA systems operate on the 800 MHz (IS-54) or 1900 MHz (IS-136) frequency range.

GSM

TDMA is currently the dominant technology for mobile cellular networks and is used in the GSM (Global System for Mobile Communications) standard (Russian SPS-900) - a global digital standard for mobile cellular communications, with channel sharing based on the TDMA principle and a high degree of security thanks to public key encryption. However, GSM uses TDMA and IS-136 access differently. Let's imagine that GSM and IS-136 are different operating systems that run on the same processor, for example, both Windows and Linux operating systems run on an Intel Pentium III. GSM systems use an encoding method to secure phone calls from mobile phones. The GSM network in Europe and Asia operates at a frequency of 900 MHz and 1800 MHz, and in the USA at a frequency of 850 MHz and 1900 MHz and is used in mobile communications.

Blocking your GSM phone

GSM is the international standard in Europe, Australia, most of Asia and Africa. Mobile phone users can buy one phone that will work anywhere the standard is supported. In order to connect to a specific mobile operator in different countries, GSM users simply change the SIM card. SIM cards store all the information and identification numbers that are needed to connect to a mobile operator.

Unfortunately, the 850MHz/1900MHz GSM frequencies used in the US are not the same as the international system. So if you live in the US but really need a mobile phone abroad, you can buy a three or four band GSM phone and use it in your home country and abroad, or just buy a 900MHz/1800MHz GSM mobile phone for traveling abroad .

CDMA

CDMA (Code Division Multiple Access). Traffic channels with this method of dividing the medium are created by assigning each user a separate numeric code, which is distributed across the entire bandwidth. There is no time division, all subscribers constantly use the entire channel width. The frequency band of one channel is very wide, subscribers' broadcasts overlap each other, but since their codes are different, they can be differentiated. CDMA is the basis for IS-95 and operates on the 800 MHz and 1900 MHz frequency bands.

Dual band and dual standard mobile phone

When you go traveling, you undoubtedly want to find a phone that will work on several bands, in several standards, or will combine both. Let's take a closer look at each of these possibilities:

A multiband phone can switch from one frequency to another. For example, a dual band TDMA phone can use TDMA services on an 800 MHz or 1900 MHz system. A dual band GSM phone can use GSM service in three bands - 850 MHz, 900 MHz, 1800 MHz or 1900 MHz.
Multi-standard phone. “Standard” in mobile phones means the type of signal transmission. Therefore, a phone with AMPS and TDMA standards can switch from one standard to another if necessary. For example, the AMPS standard allows you to use an analog network in areas that do not support a digital network.
A multi-band/multi-standard phone allows you to change the frequency band and transmission standard.

Phones that support this feature automatically change bands or standards. For example, if a phone supports two bands, then it connects to the 800 MHz network if it cannot connect to the 1900 MHz band. When a phone has multiple standards, it first uses the digital standard, and if this is not available, it switches to the analogue one.

Mobile phones come in two- and three-band modes. However, the word "three-lane" can be deceiving. It may mean that the phone supports CDMA and TDMA standards, and the analog standard. And at the same time, it can mean that the phone supports one digital standard in two bands and an analog standard. For those traveling abroad, it is better to purchase a phone that operates on the 900 MHz GSM band for Europe and Asia and 1900 MHz for the US, and also supports the analog standard. In essence, this is a dual-band phone in which one of these modes (GSM) supports 2 bands.

Cellular and Personal Communications Service

Personal Communications Service (PCS) is essentially a mobile phone service that emphasizes personal communications and mobility. The main feature of PCS is that the user's telephone number becomes his personal communication number (PCN), which is “tied” to the user himself, and not to his phone or radio modem. A global traveler using PCS can freely receive phone calls and emails on their PCN.

Cellular communications were originally created for use in cars, while personal communications meant greater possibilities. Compared to traditional cellular communications, PCS has several advantages. Firstly, it is completely digital, which provides higher data transfer rates and facilitates the use of data compression technologies. Secondly, the frequency range used for PCS (1850-2200 MHz) allows reducing the cost of communication infrastructure. (Since the overall dimensions of PCS base station antennas are smaller than the overall dimensions of cellular network base station antennas, their production and installation are cheaper).

In theory, the mobile system in the US operates on two frequency bands - 824 and 894 MHz; PCS operates at 1850 and 1990 MHz. And since this service is based on the TDMA standard, the PCS has 8 time slots and the channel spacing is 200 KHz, as opposed to the usual three time slots and 30 KHz between channels.

3G is the latest technology in mobile communications. 3G means that the phone belongs to the third generation - the first generation is analog mobile phones, the second is digital. 3G technology is used in multimedia mobile phones, which are commonly called smartphones. Such phones have multiple bands and high-speed data transfer.

3G uses several mobile standards. The three most common are:

CDMA2000 is a further development of the 2nd generation CDMA One standard.
WCDMA (Wideband Code Division Multiple Access - broadband CDMA) is the radio interface technology chosen by most cellular operators to provide broadband radio access to support 3G services.
TD-SCDMA (English Time Division - Synchronous Code Division Multiple Access) is a Chinese standard for third generation mobile networks.

The 3G network can transfer data at speeds of up to 3 Mbps (so it only takes about 15 seconds to download an MP3 song lasting 3 minutes). For comparison, let's look at second-generation mobile phones - the fastest 2G phone can reach data transfer speeds of up to 144 Kb/s (it takes about 8 hours to download a 3-minute song). High-speed 3G data transfer is simply ideal for downloading information from the Internet, sending and receiving large multimedia files. 3G phones are a kind of mini-laptops that can handle large applications, such as streaming video from the Internet, sending and receiving faxes, and downloading e-mail messages with applications.

Of course, this requires base stations that transmit radio signals from phone to phone.

Cell phone base stations are cast metal or lattice structures that rise hundreds of feet into the air. This picture shows a modern tower that “serves” 3 different mobile operators. If you look at the base of the base stations, you can see that each mobile operator has installed its own equipment, which nowadays takes up very little space (at the base of older towers small rooms were built for such equipment).

Base station. photo from http://www.prattfamily.demon.co.uk

A radio transmitter and receiver are placed inside such a block, thanks to which the tower communicates with mobile phones. The radios are connected to the antenna on the tower by several thick cables. If you look closely, you will notice that the tower itself, all the cables and equipment of the companies at the base of the base stations are well grounded. For example, a plate with green wires attached to it is a copper ground plane.

A mobile phone, like any other electronic device, may experience problems:

Most often, these include corrosion of parts caused by moisture entering the device. If moisture gets into your phone, you need to make sure that the phone is completely dry before turning it on.
Excessive temperatures (for example, in a car) can damage the battery or electronic circuit board of the phone. If the temperature is too low, the screen may turn off.
Analog mobile phones often face the problem of "cloning". A phone is considered "cloned" when someone intercepts its identification number and can call other numbers for free.

Here's how "cloning" works: Before you call anyone, your phone transmits its ESN and MIN codes to the network. These codes are unique and it is thanks to them that the company knows to whom to send the invoice for calls. When your phone transmits MIN/ESN codes, someone can hear (using a special device) and intercept them. If these codes are used in another mobile phone, then you can make calls from it completely free of charge, since the owner of these codes will pay the bill.

In the theoretical part, we will not delve into the history of the creation of cellular communications, its founders, the chronology of standards, etc. For those who are interested, there is plenty of material both in printed publications and on the Internet.

Let's look at what a mobile (cell) phone is.

The figure shows the principle of operation in a very simplified way:

Fig.1 How a cell phone works

A cell phone is a transceiver operating at one of the frequencies in the range 850 MHz, 900 MHz, 1800 MHz, 1900 MHz. Moreover, reception and transmission are separated by frequency.

The GSM system consists of 3 main components such as:

Base station subsystem (BSS – Base Station Subsystem);

Switching/switching subsystem (NSS – NetworkSwitchingSubsystem);

Operation and Maintenance Center (OMC);

In a nutshell it works like this:

A cellular (mobile) phone interacts with a network of base stations (BS). BS towers are usually installed either on their ground masts, or on the roofs of houses or other structures, or on rented existing towers of all kinds of radio/TV repeaters, etc., as well as on high-rise chimneys of boiler houses and other industrial structures.

After turning on the phone and the rest of the time, it monitors (listens, scans) the airwaves for the presence of a GSM signal from its base station. The phone identifies its network signal using a special identifier. If there is one (the phone is in the network coverage area), then the phone selects the best frequency in terms of signal strength and at this frequency sends a request to the BS to register in the network.

The registration process is essentially an authentication (authorization) process. Its essence lies in the fact that each SIM card inserted into the phone has its own unique identifiers IMSI (International Mobile Subscriber Identity) and Ki (Key for Identification). These same IMSI and Ki are entered into the database of the authentication center (AuC) when manufactured SIM cards are received by the telecom operator. When registering a phone on the network, the identifiers are transmitted to the BS, namely the AuC. Next, the AuC (identification center) transmits a random number to the phone, which is the key to perform calculations using a special algorithm. This calculation occurs simultaneously in the mobile phone and the AuC, after which both results are compared. If they match, then the SIM card is recognized as genuine and the phone is registered on the network.

For a phone, the identifier on the network is its unique IMEI (International Mobile Equipment Identity) number. This number usually consists of 15 digits in decimal notation. For example 35366300/758647/0. The first eight digits describe the phone model and its origin. The rest are the phone's serial number and check number.

This number is stored in the phone's non-volatile memory. In outdated models, this number can be changed using special software and an appropriate programmer (sometimes a data cable), and in modern phones it is duplicated. One copy of the number is stored in a memory area that can be programmed, and a duplicate is stored in an OTP (One Time Programming) memory area, which is programmed once by the manufacturer and cannot be reprogrammed.

So, even if you change the number in the first memory area, when the phone is turned on, it compares the data in both memory areas, and if different IMEI numbers are detected, the phone is blocked. Why change all this, you ask? In fact, the legislation of most countries prohibits this. The phone's IMEI number is tracked online. Accordingly, if a phone is stolen, it can be tracked and confiscated. And if you manage to change this number to any other (work) number, then the chances of finding the phone are reduced to zero. These issues are dealt with by the intelligence services with appropriate assistance from the network operator, etc. Therefore, I will not go deeper into this topic. We are interested in the purely technical aspect of changing the IMEI number.

The fact is that under certain circumstances this number may be damaged as a result of a software failure or incorrect update, and then the phone is absolutely unsuitable for use. This is where all means come to the rescue to restore IMEI and the functionality of the device. This point will be discussed in more detail in the software phone repair section.

Now, briefly about voice transmission from subscriber to subscriber in the GSM standard. In fact, this is a technically very complex process, which is completely different from the usual voice transmission over analog networks such as, for example, a home wired/radio telephone. Digital DECT radiotelephones are somewhat similar, but the implementation is still different.

The fact is that the subscriber's voice undergoes many transformations before it is broadcast. The analog signal is divided into segments of 20 ms duration, after which it is converted to digital, after which it is encoded using encryption algorithms with the so-called. public key - EFR system (Enhanced Full Rate - an advanced speech coding system developed by the Finnish company Nokia).

All codec signals are processed by a very useful algorithm based on the DTX (Discontinuous Transmission) principle - intermittent speech transmission. Its usefulness lies in the fact that it controls the telephone transmitter, turning it on only when speech begins and turning it off during pauses between conversations. All this is achieved using the VAD (Voice Activated Detector) included in the codec – a speech activity detector.

For the receiving subscriber, all transformations occur in the reverse order.