WCDMA or GSM Standard: What's the Difference Between Them? What is GSM.


GSM technology

GSM refers to networks of the second generation (2 Generation), although for 2010 it is conditionally in the 2.75G phase due to numerous extensions (1G - analog cellular, 2G - digital cellular, 3G - broadband digital cellular, switched by multi-purpose computer networks, including the Internet). Cell phones are produced for 4 frequency bands: 850 MHz, 900 MHz, 1800 MHz, 1900 MHz. Depending on the number of bands, phones are divided into classes and frequency variations depending on the region of use:

Single-band - the phone can operate on one of the frequencies. Currently not available, but it is possible to manually select a specific frequency in some phone models, such as Motorola C115, or using the phone's engineering menu;

dual-band (DualBand) _ for Europe, Asia, Africa, Australia 900/1800 and 850/1900 for America and Canada;

triband (TriBand) _ for Europe, Asia, Africa, Australia 900/1800/1900 and 850/1800/1900 for America and Canada;

QuadBand _ support all 850/900/1800/1900 bands.

The GSM standard uses GMSK modulation with the value of the normalized bandwidth BT _ 0.3, where B is the filter bandwidth at the level of minus 3 dB, T is the duration of one bit of a digital message.

Today, GSM is the most common communication standard. According to the GSM Association (GSMA), this standard accounts for 82% of the global mobile communications market, 29% of the world's population uses global GSM technologies. The GSMA currently includes operators in over 210 countries and territories. GSM originally stood for "Groupe Spécial Mobile", after the name of the analysis group that created the standard. It is now known as "Global System for Mobile Communications" (Global System for Mobile Communications), although the word "Communication" is not included in the abbreviation. The development of GSM began in 1982 by a group of 26 European national telephone companies. The European Conference of Postal and Telecommunications Administrations (CEPT) sought to build a single 900 MHz cellular system for all European countries.

GSM's achievements have become "one of the most compelling demonstrations of what kind of cooperation in European industry can be achieved in the global market." In 1989, the European Telecommunications Standards Institute (ETSI) took responsibility for the further development of GSM. In 1990, the first recommendations were published. The specification was published in 1991. Commercial GSM networks began operating in European countries in mid-1991. GSM was developed later than conventional cellular communications and was better designed in many ways. The North American counterpart, PCS, has grown standards from its roots, including TDMA and CDMA digital technologies, but for CDMA the real increased serviceability has never been proven.

Their main requests are related to the quality of services, support, pricing and other factors. When you choose a network operator, you also have to choose between a GSM or WCDMA network.

You have probably come across these terms before when choosing a new mobile phone, connecting to providers for the first time, or changing providers. But do you know what they mean and what is the difference between them? To make the right choice, you should consider in more detail how GSM differs from WCDMA and which one is better.

What is GSM?

GSM acts as the Global System for Mobile Communications and is now considered the communications standard on a global scale, especially in Asia and Europe, with availability in over 210 countries worldwide. It operates on four different frequency bands: 900 MHz and 1800 MHz in Europe and Asia, and 850 MHz and 1900 MHz in North and South America. The GSM Association is an international organization founded in 1987 that is dedicated to developing and overseeing the expansion of the wireless use of this standard.

GSM uses a variant of TDMA (Time Division Multiple Access), which divides the frequency bands into multiple channels. In this technology, voice is converted into digital data that is transmitted over a channel and time slot. At the other end, the receiver only listens for the assigned time slot, and the call combines both signals. Obviously, this happens in a very short time, and the recipient does not notice the "gap" or time division.

What is WCDMA?

CDMA, or Code Division Multiple Access, became a standard developed and patented by Qualcomm, and subsequently used as the basis for the CDMA2000 and WCDMA standards for 3G. However, due to its proprietary nature, WCDMA technology has not received the global adoption that GSM has. It is currently used by less than 18% of networks worldwide, mostly in the US, but also in South Korea and Russia. What is the difference between GSM and WCDMA from a technical point of view?

In WCDMA networks, digital calls are overlaid on top of each other, assigning unique codes to differentiate them. Each call signal is encoded with a different key and then they are transmitted simultaneously. Each receiver has a unique key capable of splitting the combined signal into its individual calls.

Both standards are multi-access, which means that multiple calls can go through the same tower. But as you can see, the main difference between the two has to do with how the data is converted into radio waves that your phone broadcasts and receives.

The main reason telcos have had trouble rolling out the new format quickly is the difference in the frequency bands they use. Because of this, GSM-only phones could not communicate with WCDMA networks, and vice versa. To get around this, most device manufacturers had to apply multiple frequency bands for 2G and 3G networks. This ensured that mobile phones could be used on virtually any network and anywhere in the world.

WCDMA vs GSM: What's the difference?

Before the advent of 4G LTE technology, the obvious difference between GSM and WCDMA devices was related to the SIM card. GSM phones came with a SIM card slot, but CDMA devices did not.

In other words, WCDMA is phone based with a subscriber number associated with a specific 3G capable device. If you want to switch to another phone, you will need to contact the provider, deactivate the old device and activate the new one. On the other hand, in GSM devices, the number is associated with the SIM card, so when switching to another device, all you have to do is put the SIM card into the new phone.

Network coverage

Network coverage is independent of whether it is GSM or WCDMA. What is the difference in this case? This characteristic rather depends on the infrastructure that the operator has. GSM networks are much more popular around the world, except in the US, where Verizon Wireless, the (W)CDMA network, boasts the largest number of subscribers in the country.

International roaming

When connecting domestically, it does not matter which network you use, as long as its coverage is sufficient. So, in Russia you can freely use WCDMA or GSM. What is the difference outside the country?

When it comes to international roaming, GSM has a lot of advantages: there are many more of these networks around the world, as well as many roaming rates between these providers. With a GSM phone, you also have the advantage that you can purchase a local SIM card wherever you are (provided you're using an unlocked device). In turn, you may not be able to get full access to the WCDMA data connection, depending on the device and network compatibility.

4G, WCDMA or GSM: what's the difference in the near future?

With the advent of 4G and the adoption of LTE and LTE-Advanced as the standard by most network operators around the world, the GSM vs. WCDMA debate is taking less time. Today, you may notice that the latest smartphones designed for WCDMA networks also come with SIM card slots to take advantage of the network's 4G LTE capabilities.

The difference between GSM or WCDMA devices means that they cannot be interchanged even now and will never be cross-compatible, but this will not matter in the near future. This is due to the fact that modern developers continue to move towards a full transition to 4G LTE. This technology has obvious advantages.

Thus, in international roaming, the main factor is the quality of the voice call and the satisfaction of user needs for 3G data. These parameters can be equally good in GSM or WCDMA networks. What is the difference? The 3G modems built into these devices can show high functionality. But when it comes to factors such as availability, coverage, and price, 4G offers the best deal.

Lecture Search

Literature

1. Babkov V.Yu., Tsikin I.A. Cellular systems of mobile radio communication.- SP.:

Publishing House of the Polytechnic University, 2011.-426 p.

2. Kuznetsov M.A., Ryzhkov A.E. Modern technologies and standards

mobile communications. - St. Petersburg: Link, 2006.

3. Volkov A.N., Ryzhkov A.E., Sivers M.A. UMTS. Cellular standard

third generation. ‒ St. Petersburg, Link, 2008.

4. Standards and 4G radio access networks: LTE, WiMAX / A.E. Ryzhkov,

M.A.Sivers and others - St. Petersburg, Link, 2012 - 226p.

5. Nikitina A.V., Ryzhkov A.E. Radio access networks of the fourth generation.

LTE standard: technologies and procedures. - St. Petersburg, SPbGUT Publishing House, 2012 - 88s.

Answers:

Principles of cellular communication. Cellular technologies. Characteristics of traffic in cellular networks.

cellular, mobile network- one of the types of mobile radio communication, which is based on cellular network. The key feature is that the total coverage area is divided into cells (cells) determined by the coverage areas of individual base stations (BS). The cells partially overlap and together form a network. On an ideal (flat and undeveloped) surface, the coverage area of ​​one BS is a circle, so the network composed of them looks like hexagonal cells (honeycombs).

The network consists of transceivers spaced apart in space operating in the same frequency range, and switching equipment that allows you to determine the current location of mobile subscribers and ensure communication continuity when a subscriber moves from the coverage area of ​​one transceiver to the coverage area of ​​another.

The main components of a cellular network are cell phones and base stations, which are usually located on rooftops and towers. When turned on, the cell phone listens to the air, finding a signal from the base station. The phone then sends its unique identification code to the station. The telephone and the station maintain constant radio contact, periodically exchanging packets. Communication between the phone and the station can go on an analog protocol (AMPS, NAMPS, NMT-450) or digital (DAMPS, CDMA, GSM, UMTS). If the phone goes out of range of the base station (or the quality of the radio signal of the service cell deteriorates), it establishes communication with another (Eng. handover).

Cellular networks can consist of base stations of different standards, which allows you to optimize the network and improve its coverage.

Cellular networks of different operators are connected to each other, as well as to the fixed telephone network. This allows subscribers of one operator to make calls to subscribers of another operator, from mobile phones to landlines and from landlines to mobiles.

Operators can conclude roaming agreements with each other. Thanks to such contracts, the subscriber, being outside the coverage area of ​​his network, can make and receive calls through the network of another operator.

What is GSM

As a rule, this is carried out at increased rates. The possibility of roaming appeared only in 2G standards and is one of the main differences from 1G networks.

Operators can share network infrastructure, reducing network deployment and operating costs.

The main characteristics of the GSM standard.

GSM(from the name of the group Groupe Special Mobile, later renamed Global System for Mobile Communications) ( Russian SPS-900) is a global standard for digital mobile cellular communication, with time division (TDMA) and frequency division (FDMA). Developed under the auspices of the European Telecommunication Standards Institute (ETSI) in the late 80s.

GSM refers to networks of the second generation (2 Generation) (1G - analog cellular communication, 2G - digital cellular communication, 3G - broadband digital cellular communication switched by multi-purpose computer networks, including the Internet).

Cell phones are produced for 4 frequency bands: 850 MHz, 900 MHz, 1800 MHz, 1900 MHz.

In the GSM standard, GMSK modulation is used with a normalized bandwidth of BT - 0.3, where B is the filter bandwidth at a level of minus 3 dB, T is the duration of one bit of a digital message.

GSM is by far the most common communication standard. According to the GSM Association (GSMA), this standard accounts for 82% of the global mobile communications market, 29% of the world's population uses global GSM technologies. The GSMA currently includes operators in over 210 countries and territories.

Construction principle
The principle of building cellular systems in brief is as follows: within the network coverage area, a certain number of relatively low-power stationary transceiver stations (base stations) are installed, each of which has a small coverage area (usually several kilometers). At the same time, the coverage areas of neighboring stations overlap each other somewhat to ensure the possibility of moving a subscriber from one area to another without losing communication. For this overlap to be possible, neighboring stations must use different operating frequencies. A minimum of three different frequencies is required to completely cover a certain area so that the triangular stations can have service area overlap. The fourth station can again use one of these three frequencies, since it borders only two zones. With this approach, the shape of the coverage area of ​​each base station is a hexagon, and the location of these zones exactly repeats the structure of bee honeycombs, which gave the name to communication systems with a similar construction principle.

Today GSM- the fastest growing cellular communication system. New, somewhat revolutionary technical innovations compatible with GSM, can and will be presented in the near future. All this serves as a solid foundation for the technology to GSM has become the single real standard for digital cellular systems around the world.

Now GSM is developing towards the third generation of cellular systems. The most significant difference between the developed technology GSM and the third generation of systems consists in the requirement for very high data transfer rates, up to 2 Mbps. This means that in order to provide a large network coverage area GSM-900, DCS-1800 and PCS-1900 can be used as components of third generation cellular communication systems. This also means that the high-speed data interface for 3G cellular systems must be designed to be compatible with GSM.

Main characteristics of the standard GSM.

· Frequencies of transmission of a mobile station and reception of a base station, MHz 890-915

· Frequencies of mobile station reception and base station transmission, MHz 935-960

Duplex spacing of reception and transmission frequencies, MHz 45

· Rate of message transmission in the radio channel, kbps 270, 833

· Speech codec conversion rate, kbps 13

Communication channel bandwidth, kHz 200

Maximum number of communication channels 124

The maximum number of channels organized in the base station is 16-20

Services provided[edit]

GSM provides support for the following services:

· Data transfer services (synchronous and asynchronous data exchange, including packet data transfer - GPRS). These services do not guarantee the compatibility of terminal devices and only provide the transfer of information to and from them.

· Transmission of speech information.

· Sending short messages (SMS).

· Sending fax messages.

Additional (optional) services:

· Identification of the calling number and restriction of such identification.

· Unconditional and conditional call forwarding to another number.

· Call waiting and holding.

· Conference communication (simultaneous voice communication between three or more mobile stations).

Ban on user-defined services (international calls, roaming calls, etc.)

and many other services.

Advantages and disadvantages[edit]

Advantages of the GSM standard:

· Smaller compared to analog standards (NMT-450, AMPS-800) size and weight of telephones with more time without recharging the battery. This is achieved mainly due to the equipment of the base station, which constantly analyzes the level of the signal received from the subscriber's device. In cases where it is higher than required, a command is automatically sent to the cell phone to reduce the radiated power.

· Good communication quality with a sufficient density of base stations.

· Large network capacity, the possibility of a large number of simultaneous connections.

· Low level of industrial interference in these frequency ranges.

· Improved (compared to analog systems) protection against eavesdropping and illegal use, which is achieved through the use of encryption algorithms with a shared key. [ clarify]

· Efficient coding (compression) of speech. EFR technology was developed by Nokia and subsequently became the industry standard encoding/decoding for GSM technology (see GSM-FR, GSM-HR and GSM-EFR)

· Wide distribution, especially in Europe, a large selection of equipment.

· Possibility of roaming.

This means that a subscriber of one of the GSM networks can use a cellular phone number not only at home, but also move around the world, moving from one network to another without parting with his subscriber number. The process of switching from network to network occurs automatically, and the user of a GSM phone does not need to notify the operator in advance (in the networks of some operators, there may be restrictions on the provision of roaming to their subscribers, more detailed information can be obtained by contacting your GSM operator directly)

Disadvantages of the GSM standard:

· Speech distortion during digital processing and transmission.

· Communication is possible at a distance of no more than 120 km from the nearest base station, even when using amplifiers and directional antennas. Therefore, to cover a certain area, more transmitters are needed than in the NMT-450 and AMPS.

GSM structure[edit]

Main article: GSM core network

GSM network structure

The GSM system consists of three main subsystems:

base station subsystem (BSS - Base Station Subsystem),

switching subsystem (NSS - Network Switching Subsystem),

· the technical service center (OMC - Operation and Maintenance Center).

Terminal devices - mobile stations (MS - Mobile Station), also known as mobile (cellular) phones, are allocated in a separate class of GSM equipment.

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GSM (from Groupe Special Mobile, which was later renamed Global System for Mobile Communications) is a digital communication standard developed in the late 80s of the last century.

Income

GSM should be attributed to networks of the second generation, that is, 2G - digital cellular communications.

The standard got its name in honor of the analysis group that created the standard (Groupe Special Mobile). Its development began in 1982. The goal is to build a single cellular system for all European countries in the 900 MHz band. Commercial GSM networks began operating in mid-1991.

As of this writing, GSM is the most widely used communication standard in the world. It accounts for more than 80% of the entire global mobile communications market.

Services provided by GSM

Main services:

  • Transmission of speech information.
  • Sending fax messages.

Additional services include:

  • Definition of incoming numbers.
  • Voice communication with several subscribers (conferencing).
  • Prohibition of certain services.

GSM standards

Mobile phones are released with support for 4 frequencies: 850 MHz, 900 MHz, 1800 MHz and 1900 MHz.

Phones are divided into classes depending on the number of bands that devices support. For example, a phone that operates on one frequency is called a single band, and one that operates on three frequencies is called a triband. In some models, you can select a specific frequency.

Advantages and disadvantages of GSM

Let's start with the disadvantages:

  • Possible speech distortion during digital processing and data transmission.
  • Ability to communicate at a distance of no more than 120 km from the nearest station.

Now for the benefits:

  • Smaller size and weight of telephones that use analog standards.
  • Good protection against eavesdropping and illegal use.
  • Very widespread throughout the world.

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GSM communication operators in Russia

As of November 2007 in Russia there were about 168 million mobile subscribers. At the same time, 85% of them are clients of the Big Three GSM operators - Mobile Telesystems (MTS), MegaFon and VimpelCom.

Despite the fact that annual growth rates are constantly declining, the level of penetration of cellular services in Russia as a whole is 107%, while in the Moscow licensed zone (MLZ) this figure was 164%.

Leadership in the growth of the subscriber base on a nationwide scale is held by Megafon, and in the MLZ it is inferior to MTS in this indicator. Among federal and regional operators, Tele2, NTK, Baikalwestcom and Yeniseitelecom demonstrate the highest growth rates of subscribers on an annualized basis.

Regional GSM-operators that are not included in the "big cellular three" are looking for a way to compete with the giants of the market. Most of the independent GSM-operators in Russia appeared in the last few years on the basis of operators of the outdated AMPS standard. All of them in 2001-2002. received licenses from the Ministry of Communications, giving them the right to work in the GSM-1800 standard.
Now these companies are launching GSM-networks one after another, but their subscribers, having found themselves in other regions, are forced to pay $1-$1.5 per minute for roaming communications. Now these companies intend to agree on uniform roaming tariffs for each other, which will allow network subscribers to feel as good as MTS, VimpelCom and MegaFon clients when moving around the country, for whom uniform and relatively low intranet roaming tariffs are one of the key advantages of the Big Three operators.

Open Joint Stock Company "Mobile TeleSystems" (MTS)- the largest mobile operator in Russia and the CIS countries, serving more than 74 million subscribers. The MTS license portfolio includes most of the regions of Russia, Ukraine, Belarus, Uzbekistan and Turkmenistan, and the population living in the MTS network coverage area is more than 230 million people.
Mobile TeleSystems was founded in October 1993. November 19, 1993 MTS received the first license for the provision of GSM cellular services. On May 15, 1994, the first calls were made in the MTS network, and already on July 7, 1994, MTS began to connect the first subscribers.
In June 2002, MTS launched a network in the Republic of Belarus. In March 2003, MTS acquired a controlling stake in UMC, the leading mobile operator in Ukraine.

JSC "Megafon" is an all-Russian mobile operator of the GSM 900/1800 standard. Formed in May 2002. The licensed territory of OJSC MegaFon covers 100% of the territory of Russia - all 89 constituent entities of the Russian Federation, where 145 million people live. MegaFon is the first all-Russian mobile operator of the GSM 900/1800 standard.

OJSC VimpelCom is a mobile operator in Russia, providing its services under the Beeline trademark. Cellular communications licenses of the VimpelCom group of companies cover the territory where 94% of the Russian population lives, including Moscow, the Moscow Region and St. Petersburg. The Beeline network operates on the territory of 76 constituent entities of the Russian Federation.
VimpelCom was founded on September 15, 1992. In June 1997, the first Russian network of the GSM-1800 standard - BEELINE 1800 was successfully launched. On October 21, 1998, the company successfully launched the first stage of the GSM-900/1800 dual-band network in Moscow.
On March 24, 1999, JSC VimpelCom became a member of the GSM Operators Association, which unites companies operating in the GSM-900 and GSM-1800 standards in Russia and a number of CIS countries.

CJSC "Middle Volga Interregional Association of Radio Telecommunication Systems" (SMARTS) was founded in May 1991 in Samara. The founders of the company are 95% individuals. Now the SMARTS GSM network covers 16 regions of Russia. To date, SMARTS has entered into roaming agreements with almost all Russian networks in 74 regions. The company has global roaming in 78 countries.

OJSC "Uralsvyazinform" is the largest operator of mobile communications and Internet services in the Ural region. The company operates on the territory of seven constituent entities of the Russian Federation with a total area of ​​1.9 million square meters. km with a population of more than 15 million people

NSS Nizhny Novgorod Cellular Communications- at the end of June 1995, the company began working with subscribers.

GSM-STORE - communication salon, online store

In 1999, the company established communication with the world through international roaming.

Sibirtelecom OJSC is the largest telecommunications operator in the Siberian Federal District. The company operates on an area of ​​about 5 thousand square kilometers with a population of about 21 million people.

TELE2, known as Comviq until 1993, was founded in Sweden in 1981. In Russia, TELE2 is the owner of 12 Russian mobile communication companies. Russia's first mobile communication network TELE2 was launched in Irkutsk on April 1, 2003.

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General characteristics of the GSM standard-Lesson 9 —>> Coding Session 13

Organization of physical and logical communication channels in the GSM standard Frequency plan of the GSM standard. Structure of logical communication channels. The structure of logical control channels. Organization of physical channels. Radio signal modulation

Lesson #11

GSM frequency plan
Physical channel in the GSM standard is a combination of time and frequency division of signals and is defined as a sequence of RF channels (with the possibility of frequency hopping) and time windows of a TDMA frame.
GSM standard designed to create cellular mobile communication systems (MCSS) in the following frequency bands: 890 ... 915 MHz - for transmission by mobile stations (uplink); 935 ... 960 MHz - for transmission by base stations (downlink).
Initially GSM networks will occupy the 10 MHz band in the frequency bands 905…915 MHz (for transmission by mobile stations) and 950…960 MHz (for transmission by base stations) and will operate in parallel with the existing European national networks of analog METS standards NMT 900, TACS, ETACS, C- 900.

MCSS frequency plans, including the GSM standard, are shown in fig. 48.

GSM is a pan-European standard for digital
SSPS AMPS - standard for analog SSPS North
Americas D-AMPS (ADS) - the standard for digital METS North
Americas TACS(ETACS) is the UK standard for analog
JCSS
NMT 900 - Scandinavian standard for analog MCTS HCMTS, NAMTS - Japanese analog CMTS standards
SMPS frequency plans
Each of the lanes dedicated to GSM, is divided into frequency channels. The channel spacing is 200 kHz, which allows organizing 124 frequency channels in GSM, which are distributed in accordance with the placement of cells. The frequencies allocated for the transmission of the mobile station to the base station and in the opposite direction are grouped in pairs, organizing a duplex channel with a spacing of 45 MHz. These pairs of frequencies are also preserved during frequency hops. Each cell is characterized by a fixed assignment of a certain number of pairs of frequencies from 1 to 15 (no more).
If we designate Fl (n) - the number of the carrier frequency in the band 890 ... 915 MHz, Fu (n) - the number of the carrier frequency in the band 955.-.960 MHz, then the channel frequencies are determined by the following formulas:
Fl(n) = 890.2 + 0.2(n - 1), MHz Fu(n) == Fl(n) + 45, MHz 1< n < 124.
Each frequency carrier contains 8 physical channels located in 8 time windows within a TDMA frame and in a sequence of frames. Each physical channel uses the same time window in every TDMA time frame.
Prior to the formation of a physical channel, messages and data presented in digital form are grouped and combined into two types of logical channels: communication channels - for the transmission of coded speech or data (TSN), control channels - for the transmission of control and synchronization signals (CCH).
More than one type of logical channel can be placed on the same physical channel, but only with their appropriate combination.

AT GSM standard There are two main types of logical communication channels:
TCH/F (Full rate traffic channel)
22.8 kbps full rate messaging channel (other designation W)
TCH/H (Half rate traffic channel)
11.4 kbps half rate messaging channel (other designation Lm).
One physical channel may be a full rate messaging channel or two half rate channels.

In the first case, the communication channel occupies one time window; in the second, two communication channels occupy the same time window, but interleaved in adjacent frames (i.e., each channel is through a frame).

Structure of logical communication channels
The GSM standard distinguishes two main types of logical communication channels:
TCH / F (Full Rate Traffic Channel) - a message transmission channel with a full speed of 22.8 kbps (another designation is Bm);

TCH / H (Half Rate Traffic Channel) - a message transmission channel with a half speed of 11.4 kbit / s (another designation is m).
One physical channel may be a full rate messaging channel or two half rate channels. In the first case, the communication channel occupies one time window; in the second, two communication channels occupy the same time window, but with interleaving in adjacent frames (i.e., each channel through a frame).

The following types of communication channels are intended for the transmission of coded speech and data:

TCH/FS (Full Rate Traffic Channel for Speech)
—a channel for transmitting voice at full rate;

TCH/HS (Half Rate Traffic Channel for Speech) :
— channel for transmitting voice at half speed;

TCH/F 9.6 (Full Rate Traffic Channel for 9.6 kbit/s User Data)
— data link with a full rate of 9.6 kbps;

TCH/F 4.8 (Full Rate Traffic Channel for 4.8 kbit/s User Data)
— full rate data link 4.8 kbps;

TCH/F 2.4 (Full Rate Traffic Channel for 2.4 kbit/s User Data)
— data link with a full rate of 2.4 kbps;

TSN/N 4.8 (Half Rate Traffic Channel for 9.6 kbit/s User Data)
— 4.8 kbps half rate data link;

CH/H 2.4 (Half Rate Traffic Channel for 9.6 kbit/s User Data)
— 2.4 kbit/s half rate data link.

The digital voice rate on the TCH/FS channel is 13 kbps (increased to 22.8 kbps on the TCH/F channel due to coding).

Channels of connection can transmit a wide range of informational messages, but they are not used to transmit control signals.

In addition, different protocols can be used for data transmission over communication channels, for example, ITU-T X.25.

Structure of logical control channels

Control channels (CCH) provide transmission of control and synchronization signals. There are four types of control channels:

BCCH (Broadcast Control Channels) - channels for transmitting control signals;
CCSN (Common Control Channels) - common control channels;
SDCCH (Standalone Dedicated Control Channels) - individual control channels;
ACCH (Associated Control Channels) - combined control channels.

Signaling channels controls are used only in the direction from the base station to all mobile stations. They carry information that mobile stations need to operate in the system. There are three types of channels for transmitting control signals BCCH:

FCCH (Frequency Correction Channel) - frequency adjustment channel, which is used for carrier synchronization at the mobile station. This channel transmits an unmodulated carrier with a fixed frequency offset relative to the nominal value of the frequency of the communication channel;
SCH (Synchronization Channel) - synchronization channel, through which information is transmitted to the mobile station about the frame (time) synchronization;
BCCH (Broadcast Control Channel) - transmission control channel, provides the transmission of basic transmission control commands (the number of common control channels of those that are combined with other channels, including physical ones, etc.).

Three types of common SSCH control channels are used:
PCH (Paging Channel) - call channel, is used only in the direction from the base station to the mobile to call it;
RACH (Random Access Channel) - parallel access channel, is only used in the direction from the mobile station to the base station to request the assignment of an individual control channel;
AGCH (Access Grant Channel) - authorized access channel, is used only for transmission from the base station to the mobile to allocate a special control channel that provides direct access to the communication channel.

Dedicated individual control channels are used in two directions for communication between the base and mobile stations.

There are two types of such channels:

SDCCH / 4 (Stand-alone Dedicated Control Channel) - individual control channel, consists of four subchannels;
SDCCH / 8 (Stand-alone Dedicated Control Channel) - individual control channel, consists of eight subchannels.

These channels are for setting the type of service required by the user. They provide a request to the mobile station about the required type of service, control of the correct response of the base station and allocation of a free communication channel, if possible.

Shared control channels are also used bi-directionally between the base station and the mobile station.

In the forward direction, they transmit a control command from the base station, and in the reverse direction, information about the status of the mobile station. There are two types of ASSN:

FACCH (Fast Associated Control Channel) - a fast associated control channel, is used to transmit commands when a mobile station moves from cell to cell, i.e. when "handover" (handover, handoff) of the mobile station;
SACCH (Slow Associated Control Channel) - slow associated control channel, in the forward direction transmits commands to set the output power level of the transmitter of the mobile station.

What is GSM in a phone?

In the reverse direction, the mobile station sends data regarding the level of the set output power, the level of the radio signal measured by the receiver and its quality.

A shared control channel always contains one of two channels: a communication channel or an individual control channel.

Shared control channels are always combined with communication channels or with individual control channels. There are six types of integrated control channels:

FACCH/F merged with TCH/F;
FACCH/H combined with TCH/H;
SACCH/TF merged with TCH/F;
SACCH/TH combined with TSN/H;
SACCH/C4 merged with SDCCH/4;
SACCH/C8 merged with SDCCH/8.

Organization of physical channels
As a rule, logical channels are combined into groups, which are transmitted by TDMA multiframes.

For example, on the main (zero) frequency in the cell (BCCH carrier) in the zero timeslot, the FCCH + SCH + BCCH + CCCH group is transmitted (in the downlink direction). This group is also called a combined BCCH/CCCH channel. This channel is for all mobile stations served by the cell. Thus, the phone always "knows" where to get system information about the cell in order to gain access to the network.

As can be seen from the figure, the multiframe is divided into 5 groups of 10 frames each, the last frame remains empty. The mobile station determines the frequency of the BCCH carrier by searching for a Frequency correction Burst that is transmitted on the FCCH. It then receives and decodes the current frame number and BSIC identifier over the SCH, which is necessary for correct synchronization with the BTS. The FCCH and SCH channels occupy the first 2 frames in each group. The remaining 8 frames form 2 blocks of four frames. The first block of the first group is for the BCCH channel. Through it, the MS determines the ability to access a given cell and decodes the system information of the cell. The remaining 9 blocks (call signaling blocks) are used to transmit the PCH and AGCH channels that are part of the common control channel CCCH.

Radio signal modulation
Modulation (lat. modulatio - dimension)- the process of changing one or more parameters of a high-frequency carrier oscillation according to the law of a low-frequency information signal.

The transmitted information is embedded in the control signal, and the role of the information carrier is performed by a high-frequency oscillation called the carrier.
Modulation can be carried out by changing the amplitude, phase or frequency of the high-frequency carrier.
This technique provides several important advantages:
Allows you to generate a radio signal that will have properties corresponding to the properties of the carrier frequency. You can read about the properties of waves of different frequency ranges, for example, here.
Allows the use of small antennas, because the size of the antenna must be proportional to the wavelength.
Allows you to avoid interference with other radio signals.

The data stream transmitted in WiMax networks corresponds to a frequency in the region of 11 kHz. If we try to transmit this low frequency signal over the air, we will need an antenna of the following dimensions:

An antenna 24 kilometers long does not seem comfortable enough to use.
If we transmit this signal superimposed on a carrier frequency of 2.5 GHz (the frequency used in Yota WiMax), then we need an antenna 12 cm long.

We all use mobile phones, but rarely does anyone think - how do they work? In this article, we will try to figure out how communication is actually implemented with respect to your mobile operator.

When you make a call to your interlocutor, or someone calls you, your phone is connected via radio to one of the antennas nearby base station (BS, BS, Base Station).Each cellular base station (in the common people - cell towers) includes from one to twelve transceivers antennas having directions in different directions in order to provide high-quality communication to subscribers within their range. Specialists in their jargon call such antennas "sectors", which are gray rectangular structures that you can see almost every day on the roofs of buildings or special masts.


The signal from such an antenna is sent via cable directly to the control unit of the base station. The base station is a combination of sectors and a control unit. At the same time, a certain part of the settlement or territory is served by several base stations connected to a special unit at once - local zone controller(abbreviated LAC, Local Area Controller or just "controller"). As a rule, one controller unites up to 15 base stations of a certain area.

For their part, the controllers (there may also be several) are connected to the main unit - Mobile services control center (MSC, Mobile services Switching Center), which for ease of perception is called simply "commutator". The switch, in turn, provides input and output to any communication lines - both cellular and wired.

If you display what is written in the form of a diagram, you get the following:
Small-scale GSM networks (usually regional) can use just one switch. Large ones, such as our “big three” operators MTS, Beeline or MegaFon, serving millions of subscribers at the same time, use several MSC devices interconnected at once.

Let's see why such a complex system is needed and why it is impossible to connect the base station antennas to the switch directly? To do this, you need to talk about another term called in technical language handover (handover). It characterizes handover in mobile networks according to the handover principle. In other words, when you move down the street on foot or in a vehicle and talk on the phone at the same time, so that your conversation is not interrupted, you should timely switch your device from one BS sector to another, from the coverage area of ​​​​one base station or controller local zone to another, etc. Therefore, if the base station sectors were connected directly to the switch, it would have to carry out this handover procedure for all its subscribers itself, and the switch already has enough tasks. Therefore, in order to reduce the probability of equipment failures associated with its overloads, the scheme for constructing GSM cellular networks is implemented according to a multi-level principle.

As a result, if you and your phone move from the service area of ​​one BS sector to the coverage area of ​​another, then this movement is carried out by the control unit of this base station, without touching more “high-priced” devices - LAC and MSC. If the handover occurs between different BSs, then LAC is already taken for it, etc.

The switch is nothing more than the main "brain" of GSM networks, so its operation should be considered in more detail. The cellular network switch takes on approximately the same tasks as the PBX in the networks of wired operators. It is he who understands where you are making the call or who is calling you, regulates the work of additional services and, in fact, decides whether you can currently make your call or not.

Now let's see what happens when you turn on your phone or smartphone?

So, you pressed the "magic button" and your phone turned on. On the SIM card of your mobile operator there is a special number called IMSI - International Subscriber Identification Number (International Subscriber Identification Number). It is a unique number for each SIM-card not only for your operator MTS, Beeline, MegaFon, etc., but a unique number for all mobile networks in the world! It is on it that operators distinguish subscribers among themselves.

When you turn on the phone, your device sends this IMSI code to the base station, which transmits it further to the LAC, which, in turn, sends it to the switch. At the same time, two additional devices connected directly to the switch come into our game - HLR (Home Location Register) and VLR (Visitor Location Register). Translated into Russian, this is, respectively, Register of home subscribers and Register of guest subscribers. HLR stores the IMSI of all subscribers in its network. The VLR contains information about those subscribers who currently use the network of this operator.

The IMSI number is transmitted to the HLR using an encryption system (another device is responsible for this process AuC - Authentication Center). At the same time, HLR checks whether there is a subscriber with this number in its database, and if the fact of its presence is confirmed, the system looks at whether he can currently use communication services or, say, has a financial block. If everything is normal, then this subscriber goes to VLR and after that he gets the opportunity to call and use other communication services.

For clarity, we will display this procedure using a diagram:

Thus, we briefly described the principle of operation of GSM cellular networks. In fact, this description is rather superficial, because if we delve into the technical details in more detail, then the material would turn out to be many times more voluminous and much less understandable for most readers.

In the second part, we will continue our acquaintance with the operation of GSM networks and consider how and for what the operator debits funds from our account with you.

(2 Generation) (1G - analog cellular, 2G - digital cellular, 3G - broadband digital cellular switched by multi-purpose computer networks, including the Internet).

Depending on the number of bands, phones are divided into classes and frequency variations depending on the region of use.

  • Single-band - the phone can operate in one frequency band. Currently not available, but it is possible to manually select a specific frequency range in some phone models, such as Motorola C115, or using the phone's engineering menu.
  • Dual band (Dual Band) - for Europe, Asia, Africa, Australia 900/1800 and 850/1900 for America and Canada.
  • Tri-band (Tri Band) - for Europe, Asia, Africa, Australia 900/1800/1900 and 850/1800/1900 for America and Canada.
  • Quad Band - support all 850/900/1800/1900 bands.

Commercial GSM networks began operating in European countries in the middle of the year. GSM was developed later than analog cellular and was better designed in many ways. The North American counterpart, PCS, has grown from its roots standards including TDMA and CDMA digital technologies, but for CDMA the potential improvement in QoS has never been proven.

GSM Phase 1

1982 (Groupe Spécial Mobile) - 1990 Global System for Mobile Communications. First commercial network in January Digital standard, supports data rates up to 9.6 kbps. Completely obsolete, production of equipment for it has been discontinued.

In 1991, GSM services "PHASE 1" were introduced.

Base station subsystem

Antennas of three base stations on a mast

BSS consists of the actual base stations (BTS - Base Transceiver Station) and base station controllers (BSC - Base Station Controller). The area covered by the GSM network is divided into hexagonal cells. The diameter of each hexagonal cell can be different - from 400 m to 50 km. The maximum theoretical cell radius is 120 km, which is due to the limited ability of the synchronization system to compensate for the signal delay time. Each cell is covered by one BTS, while the cells partially overlap each other, thereby retaining the possibility of MS handover when moving from one cell to another without breaking the connection ( The operation of a handover of a mobile phone (MS) from one base station (BTS) to another at the moment the mobile phone passes the range of the current base station during a call, or GPRS session, is called the technical term "Handover"). Naturally, in fact, the signal from each station propagates, covering the area in the form of a circle, but when crossing, regular hexagons are obtained. Each base has six neighbors due to the fact that the tasks of planning the placement of stations included such as minimizing signal overlap zones from each station. A greater number of neighboring stations than 6 does not bring any special benefits. Considering the boundaries of the signal coverage from each station already in the overlap zone, we just get - hexagons.

The base station (BTS) provides signal reception/transmission between the MS and the base station controller. BTS is autonomous and is built on a modular basis. Directional base station antennas can be located on towers, rooftops, etc.

The base station controller (BSC) controls the connections between the BTS and the switching subsystem. Its powers also include management of the order of connections, data transfer rate, distribution of radio channels, collection of statistics, control of various radio measurements, assignment and management of the Handover procedure.

Switching Subsystem

NSS consists of the following components.

Switching Center (MSC - Mobile Switching Center)

The MSC controls a certain geographic area with BTS and BSC located on it. It establishes a connection to and from the subscriber within the GSM network, provides an interface between GSM and PSTN, other radio networks, and data transmission networks. Also performs the functions of call routing, call control, handover when moving MS from one cell to another. After the call is completed, the MSC processes data on it and transfers it to the settlement center to generate an invoice for the services provided, collects statistical data. The MSC also constantly monitors the position of the MS using data from the HLR and VLR, which is necessary to quickly find and establish a connection with the MS in case of a call.

Home Location Register (HLR - Home Location Registry)

Contains a database of subscribers assigned to it. It contains information about the services provided to this subscriber, information about the status of each subscriber, which is necessary in case of a call, as well as the International Mobile Subscriber Identity (IMSI - International Mobile Subscriber Identity), which is used to authenticate the subscriber (using AUC). Each subscriber is assigned to one HLR. HLR data is available to all MSCs and VLRs in a given GSM network, and in the case of internetwork roaming, to MSCs of other networks.

Visitor Location Registry (VLR)

VLR monitors the movement of MS from one area to another and contains a database of moving subscribers currently in this area, including subscribers of other GSM systems - the so-called roamers. Subscriber data is deleted from the VLR if the subscriber has moved to another area. Such a scheme reduces the number of requests to the HLR of a given subscriber and, consequently, the call service time.

Equipment Identification Register (EIR)

Contains the database required for MS authentication by IMEI (International Mobile Equipment Identity). Forms three lists: white (allowed for use), gray (some problems with MS identification) and black (MS prohibited for use). Russian operators (and most of the operators of the CIS countries) use only white lists, which does not allow to solve the problem of mobile phone theft once and for all.

Authentication Center (AUC)

Here, the subscriber is authenticated, or rather, SIM (Subscriber Identity Module). Access to the network is allowed only after the SIM has passed the authentication procedure, during which a random RAND number is sent from the AUC to the MS, after which the AUC and MS are simultaneously encrypted with the Ki key for this SIM using a special algorithm. The MS and AUC then return "signed responses" - SRES (Signed Response), which are the result of this encryption, to the MSC. At the MSC, the responses are compared, and if they match, the authentication is considered successful.

OMC subsystem (Operations and Maintenance Centre)

It is connected to the rest of the network components and provides quality control and management of the entire network. Handles alarms that require human intervention. Provides a check of the network status, the possibility of passing the call. Performs software updates on all network elements and a number of other functions.

see also

  • List of GPS tracker models
  • GSM terminal

Notes

Links

  • Association GSMA (The GSM Association) (English)
  • 3GPP - Current GSM Standardization Level, Free Standards
  • 3GPP Specification Numbering Scheme
  • (English)
  • WHO booklet Building a Dialogue on the Risks of Electromagnetic Fields (pdf 2.68Mb)
  • “WHO Proposals for a Project to Study the Effects of Electromagnetic Fields; Impact of Radio Fields of Mobile Telecommunications on Health; Recommendations to State Authorities»


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