Second-generation cellular communication standards are widespread not only in Russia, but also in other countries. The most famous 2G standard is GSM (Global System for Mobile Communications). About 80% of cellular networks around the world are built according to this standard. GSM networks are used by 3 billion people in more than 212 countries around the world. Such widespread use allows the use of international between mobile operators, which makes it possible for a subscriber to use his phone in almost any corner of the world. Moreover, it is the possibility (including international) that is the main distinguishing feature of the GSM standard from.
The development of the GSM standard began back in 1982 by a standardization organization. In 1991, the world's first GSM network was put into operation in Finland. By the end of 1993, the number of subscribers using this standard exceeded one million. By this time, GSM networks were deployed in 73 countries around the world.
GSM networks allow providing a wide range of services:
Thanks to this, GSM has gained a strong position in the cellular market. Moreover, we can say with confidence that this standard will be the leading one for the next few years.
So, let's consider the main elements that make up the GSM system:
The GSM network is divided into 2 systems. Each of these systems includes a number of functional devices, which, in turn, are components of a mobile radio communication network.
These systems are:
Visiting location register ()
Authentication Center ()
Subscriber equipment identification register ()
Is a database containing information on the identification numbers of GSM mobile phones. This information is required to block stolen pipes. is not a required element of the network. There are only a few operators in the world that have implemented it in their network.
We all use mobile phones, but at the same time hardly anyone thinks - how do they work? In this article, we will try to figure out how, in fact, communication is realized with respect to your mobile operator.
When you make a call to your interlocutor, or someone calls you, your phone is connected via the radio channel to one of the antennas of the neighboring base station (BS, BS, Base Station).Each base station of cellular communication (in the common people - cell towers) includes from one to twelve transceiver antennas with directions in different directions in order to provide high-quality communication to subscribers within the radius of their operation. Such antennas are called by experts in their own jargon "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 goes through the cable directly to the control unit of the base station. The base station is a collection of sectors and a control block. In this case, a certain part of a settlement or territory is served by several base stations at once connected to a special block - 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 its part, controllers (there can also be several of them) are connected to the most important block - Mobile services Switching Center (MSC), which for simplicity of perception is usually called simply "Switch"... 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 only one switch. Large ones, such as our operators of the "big three" MTS, Beeline or MegaFon, who simultaneously serve millions of subscribers, use several MSC devices connected to each other at once.
Let's see why such a complex system is needed and why it is impossible to connect base station antennas to the switch directly? To do this, you need to talk about another term, called in technical language handover... It characterizes the handover of service in mobile networks on a relay basis. In other words, when you move along the street on foot or in a vehicle and talk on the phone, so that your conversation is not interrupted, you should promptly 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 sectors of the base stations were directly connected to the switch, he would have to carry out this handover procedure for all its subscribers himself, and the switch already has enough tasks. Therefore, to reduce the likelihood 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 coverage 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-end" devices - LAC and MSC. If handover occurs between different BSs, then LAC is taken over, and so on.
The switch is nothing more than the main "brain" of GSM networks, so its operation should be considered in more detail. A cellular network switch undertakes approximately the same tasks as a PBX in the networks of wire operators. It is he who understands where you are making a 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. There is a special number on the SIM card of your mobile operator, which is called IMSI - 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 this basis that operators distinguish subscribers from each other.
When the phone is turned on, 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. In this case, two additional devices that are connected directly to the switch come into play - HLR (Home Location Register) and VLR (Visitor Location Register)... Translated into Russian, this, respectively, Home subscribers register and Guest subscriber register... The HLR stores the IMSI of all subscribers on 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 a subscriber with this number exists in its database, and if the fact of its presence is confirmed, the system checks whether it 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 the diagram:
Thus, we have briefly described how GSM cellular networks work. In fact, this description is rather superficial, since if we delve into the technical details in more detail, then the material would have turned out 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.
If we talk about generations of mobile communications, then 2G is the most developed and widely represented in Russia. The main second generation standards in Russia are GSM 900/1800 and CDMA 450. Both GSM and CDMA are used for voice calls, text messages and mobile Internet access. Although the second generation cannot provide the same speeds as, say, 3G, or 4G, but this is the only type of cellular communication that is present in all regions of the Russian Federation, even in the most remote ones. The largest mobile providers in the Russian Federation are MegaFon, MTS, Beeline, VimpelCom and Tele2. On average, the coverage of the territory of the Russian Federation is 85%, but MTS, for example, provides coverage for 100% of Russia.
(Click on the image to see full size)
The GSM standard in Russia uses frequencies of 900 and 1800 MHz. Since all mobile phones are duplex devices, two frequencies are used for communication at once, one for receiving, the other for transmitting data. By the way, with the method of triangulation over cell towers, these two frequencies are used. CDMA uses two frequencies in the 450 and 850 MHz bands, with the same duplex allocation. The largest CDMA provider is SKYLINK. As we noted, these standards are used primarily for voice calls, text messages and mobile Internet access. Internet access is realized on GPRS and EDGE technologies.
The third generation of mobile communications or 3G, which is widely used all over the world, is also represented in Russia. The largest 3G networks in the country operate on WCDMA technology and, according to the decision of the SCRF, operate at frequencies of 2000-2100 MHz. 3G should be understood as 3G with all the add-ons: HSUPA, HSPDA HSPA +, which are often mistakenly referred to as. The data transfer rates in such networks are incomparably higher than in the GSM network, and vary in the range of 2-14 Mbit / s. This generation of mobile communications allows us to enjoy fast mobile internet and make video calls.
The largest operators of the 3G services market in Russia are MTS, MegaFon, VimpelCom, Beeline and SKYLINK. Together these companies provide 3G network operation in more than 120 largest cities of the Russian Federation. The coverage of 3G networks is not so great and is concentrated mainly in densely populated cities. 3G is often used to organize covert wireless video surveillance, as the transmission speed allows streaming video, and low power consumption increases the operating time of a hidden camera. This partly explains the popularity.
Fourth generation networks are also actively developing. The first companies to start building such a network are Yota and Freshtel, after which such giants as MTS and MegaFon joined the development of this generation of communications in the Russian Federation. Also in Russia, production facilities were recently organized, which develop and assemble equipment for base stations of the fourth generation, as well as produce all the peripheral equipment necessary for this. The first city where the 4G network was launched was Novosibirsk, and after the fourth generation of mobile communications appeared in Moscow. 4G is represented by two standards - LTE (791-862 MHz) and Wi-Max (2500-2600 MHz). Today the 4G network is fully deployed in such cities as: Moscow, St. Petersburg, Sochi, Samara, Novosibirsk, Ufa and Krasnodar.
The above were the most common standards for cellular communications, but it is worth noting that the Russian Federation has also created its own global positioning system, called. It was created to replace the American GPS satellite navigation system. GLONASS is very different from GPS. The American system operates on three channels and uses 3 different frequencies: 1575.42, 1227.60 and 1176.45 MHz, and is divided into civil and military sectors, and the frequency 1575.42 MHz is reserved for the operation of the rescue service. GLONASS, in turn, works with two channels, their frequencies: 1602-1615 and 1246-1256 MHz. GLONASS is most popular in circumpolar regions, as the orbits of GLONASS satellites are higher than GPS orbits and have better visibility. However, it is worth noting that GPS is more accurate in determining the coordinates.
In general, we can say that Russia has good coverage with various standards and generations of cellular communications, and the high rates cannot but please active users of mobile gadgets.
Chapter 1. DIGITAL CELLULAR MOBILE RADIO SYSTEM OF GSM STANDARD
1.1. General characteristics of the GSM standard
In accordance with the 1980 CEPT recommendation regarding the use of the mobile spectrum in the frequency range 862-960 MHz, the GSM standard for the digital pan-European (global) cellular terrestrial mobile system provides for the operation of transmitters in two frequency ranges: 890-915 MHz (for mobile station transmitters - MS), 935-960 MHz (for base station transmitters - BTS).
The GSM standard uses narrowband time division multiple access (NB TDMA). The TDMA frame structure contains 8 time positions on each of 124 carriers.
To protect against errors in radio channels when transmitting information messages, block and convolutional interleaving coding is used. Improving the efficiency of coding and interleaving at low speed of movement of mobile stations is achieved by slow switching of operating frequencies (SFH) during a communication session at a rate of 217 hops per second.
To combat the interference fading of received signals caused by multipath propagation of radio waves in urban conditions, the communication equipment uses equalizers that provide equalization of pulse signals with a standard deviation of the delay time of up to 16 μs.
The synchronization system is designed to compensate for the absolute signal delay time up to 233 μs, which corresponds to the maximum communication range or maximum cell (cell) radius of 35 km.
In the GSM standard, Gaussian Frequency Shift Keying (GMSK) is selected. Speech processing is carried out within the framework of the adopted system of discontinuous transmission of speech (DTX), which ensures that the transmitter is turned on only in the presence of a speech signal and the transmitter is turned off during pauses and at the end of a conversation. A speech codec with regular impulse excitation / long-term prediction and linear predictive coding with prediction (RPE / LTR-LTP-codec) is selected as a speech-transforming device. The total speed of conversion of speech, about a signal is 13 kbps.
The GSM standard achieves a high degree of security for message transmission; the messages are encrypted using the public key encryption algorithm (RSA).
In general, the communication system operating in the GSM standard is designed for its use in various fields. It provides users with a wide range of services and the ability to use a variety of equipment for the transmission of voice messages and data, call and alarm signals; connect to public switched telephone networks (PSTN), data networks (PDN) and integrated services digital networks (ISDN).
Main characteristics of the GSM standard
| Frequencies of transmission of a mobile station and reception of a base station, MHz | 890-915 |
| Frequencies of reception of a mobile station and transmission of a base station, MHz | 935-960 |
| Duplex spacing of receiving and transmitting frequencies, MHz | 45 |
| Rate of transmission of messages in the radio channel, kbit / s | 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 | 16-20 |
| Modulation type | GMSK |
| Modulation index | BT 0.3 |
| Pre-modulation Gaussian filter bandwidth, kHz | 81,2 |
| Frequency hops per second | 217 |
| Time diversity TDMA frame (transmit / receive) for a mobile station | 2 |
| Speech codec type | RPE / LTP |
| Maximum cell radius, km | up to 35 |
| Combined TDMA / FDMA channel arrangement |
1.2. Structural diagram and composition of equipment for communication networks
The functional structure and interfaces adopted in the GSM standard are illustrated by the block diagram of Fig, 1.1, in which MSC (Mobile Switching Center) is a mobile switching center; BSS (Base Station System) - base station equipment; OMC (Operations and Maintenance Center) - control and service center; MS (Mobile Stations) - mobile stations.
Functional interfacing of system elements is carried out by a number of interfaces. All network functional components in the GSM standard interact in accordance with the CCITT SS N 7 signaling system (CCITT SS. N 7).
The mobile switching center serves a group of cells and provides all kinds of connections that a mobile station needs in the process. MSC is similar to ISDN switching office and is an interface between fixed networks (PSTN, PDN, ISDN, etc.) and a mobile network. It provides call routing and call control features. In addition to performing the functions of a conventional ISDN switching station, the MSC is responsible for the functions of switching radio channels. These include "handover", during which continuity of communication is achieved when a mobile station moves from cell to cell, and switching of working channels in a cell when interference or malfunctions occur.
Each MSC provides services to mobile subscribers located within a certain geographic area (for example, Moscow and the region). The MSC manages the call setup and routing procedures. For the public switched telephone network (PSTN), the MSC provides SS N 7 signaling, call transfer, or other types of interfaces as required by a specific project.
MSC generates the data necessary for billing the communication services provided by the network, accumulates data on the conversations that took place and transfers them to the settlement center (billing center). The MSC also compiles the statistics needed to monitor and optimize the network.
The MSC also maintains security procedures used to control access to radio links.
The MSC not only participates in call control, but also manages the location registration and handover procedures in addition to the base station subsystem (BSS) handover. Mobile station location registration is necessary to ensure call delivery to roaming mobile subscribers from PSTN subscribers or other mobile subscribers. The call handover procedure allows connections to be maintained and a conversation to be maintained when a mobile station moves from one service area to another. Calls in cells controlled by one base station controller (BSC) are handled by this BSC. When calls are transferred between two networks controlled by different BSCs, the primary control is at the MSC. The GSM standard also provides call transfer procedures between networks (controllers) belonging to different MSCs. The switching center continuously monitors mobile stations using position registers (HLR) and movement registers (VLR). The HLR stores that part of the location information of a mobile station that allows the switching center to deliver the call to the station. The HLR contains the International Mobile Subscriber Identity (IMSI). It is used to identify the mobile station at the Authentication Center (AUC) (Fig. 1.2, 1.3).
Composition of temporary data stored in HLR and VLR
In practice, HLR is a reference database of subscribers permanently registered in the network. It contains identification numbers and addresses, as well as subscriber authenticity parameters, the range of communication services, and special routing information. Subscriber roaming (wandering) data are recorded, including temporary mobile subscriber identification number (TMSI) and associated VLR.
All MSC and VLR networks have remote access to the data contained in the HLR, and if there are several HLRs in the network, the database contains only one subscriber record, therefore each HLR represents a certain part of the overall network subscriber database. The subscriber database is accessed by IMSI or MSISDN (mobile subscriber number in the ISDN network). The database can be accessed by MSCs or VLRs belonging to other networks as part of providing inter-network roaming to subscribers.
The second main device providing control over the movement of a mobile station from zone to zone is the VLR movement register. It enables the operation of the mobile station outside the HLR controlled area. When, in the process of moving, a mobile station moves from the coverage of one base station controller BSC, which unites a group of base stations, to the coverage of another BSC, it is registered with a new BSC, and information about the communication area number is entered in the VLR, which will ensure the delivery of calls
mobile station. For the safety of the data located in the HLR and VLR, in the event of failures, the memory devices of these registers are protected.
The VLR contains the same data as the HLR, however, this data is contained in the VLR only as long as the subscriber is in the area controlled by the VLR.
In a GSM mobile network, cells are grouped into geographic areas (LAs), which are assigned their own identification number (LAC). Each VLR contains subscriber data in multiple LAs. When a mobile subscriber moves from one LA to another, its location data is automatically updated in the VLR. If the old and new LA are managed by different VLRs, then the data on the old VLR is erased after it is copied to the new VLR. The subscriber's current VLR address contained in the HLR is also updated.
The VLR also provides a roaming mobile station number (MSRN) assignment. When the mobile station receives an incoming call, the VLR selects its MSRN and transmits it to the MSC, which routes the call to base stations near the mobile subscriber.
The VLR also allocates handover numbers when handing connections from one MSC to another. In addition, the VLR manages the distribution of new TMSIs and submits them to the HLR. It also manages the authentication procedures during call processing. At the decision of the operator, the TMSI can be periodically changed to complicate the subscriber identification procedure. The VLR database can be accessed through IMSI, TMSI, or MSRN. In general, the VLR is a local mobile subscriber database for the area where the subscriber is located, which eliminates persistent HLR inquiries and reduces call handling time.
To exclude unauthorized use of the resources of the communication system, authentication mechanisms are introduced - subscriber authentication. The Authentication Center consists of several blocks and generates keys and authentication algorithms. With its help, the authority of the subscriber is checked and his access to the communication network is carried out. The AUC decides on the parameters of the authentication process and determines the encryption keys of the subscriber stations based on the database concentrated in the Equipment Identification Register (EIR).
Each mobile subscriber for the time of using the communication system receives a standard subscriber authentication module (SIM), which contains: an international identification number (IMSI), its own individual authentication key (Ki), an authentication algorithm (A3).
With the help of the information recorded in the SIM, as a result of the mutual data exchange between the mobile station and the network, a complete authentication cycle is carried out and the subscriber's access to the network is allowed.
The procedure for checking the subscriber's authenticity by the network is implemented as follows. The network transmits a random number (RAND) to the mobile station. On it, using Ki and the A3 authentication algorithm, the response value (SRES) is determined, i.e.
SRES = Ki * [RAND]
The mobile station sends the calculated SRES value to the network, which compares the received SRES value with the SRES value calculated by the network. If both values are the same, the mobile station starts sending messages. Otherwise, the communication is interrupted and the indicator of the mobile station indicates that the identification has not taken place. To ensure privacy, the SRES is computed within the SIM. Unclassified information (eg Ki) is not processed in the SIM module.
EIR - Equipment Identification Register, contains a centralized database for confirming the authenticity of the International Mobile Station Equipment Identification Number (1ME1). This database applies exclusively to the equipment of the mobile station. The EIR data base consists of lists of 1ME1 numbers, organized as follows:
WHITE LIST - Contains 1ME1 numbers that are known to be assigned to authorized mobile stations.
BLACK LIST - contains 1ME1 numbers of mobile stations that are stolen or denied service for another reason.
GRAY LIST - contains 1ME1 numbers of mobile stations that have problems identified by software data, which is not a reason for being included in the "black list".
The EIR database is accessed remotely by the MSCs of the network as well as the MSCs of other mobile networks.
As with the HLR, a network can have more than one EIR, with each EIR managing specific 1ME1 groups. The MSC includes a translator, which, upon receiving the 1ME1 number, returns the EIR address that controls the corresponding part of the equipment database.
The IWF is an internetworking functional joint, which is one of the constituent parts of the MSC. It provides subscribers with access to protocol and data rate conversions so that they can be transferred between its GSM terminal equipment (DIE) and conventional fixed network terminal equipment. The gateway also "takes out" the modem from its equipment bank for interfacing with the corresponding fixed network modem. The IWF also provides direct-connect-type interfaces for customer-supplied equipment, such as PAD packet data over the X.25 protocol.
EC - echo canceller, used by the MSC on the PSTN side for all telephone channels (regardless of their length) due to physical delays in the propagation paths, including the radio channel, of GSM networks. A typical echo canceller can provide 68 milliseconds of cancellation between the EC output and the fixed-line telephone. The total delay in the GSM channel in the forward and backward directions caused by signal processing, speech coding / decoding, channel coding, etc. is about 180 ms. This delay would not be noticeable to the mobile subscriber if the hybrid transformer with path conversion from two-wire to four-wire mode was not included in the telephone channel, which must be installed in the MSC, since the standard connection to the PSTN is two-wire. When two subscribers of a fixed network are connected, there are no echoes. Without turning on the EU, the delay from the propagation of signals in the GSM path will irritate subscribers, interrupt speech and distract attention.
OMC - the center of operation and maintenance, is the central element of the GSM network, which provides control and management of other network components and quality control of its work. OMC connects with other components of the GSM network via X.25 packet transmission channels. The OMC provides alarm handling functions to alert maintenance personnel and logs emergency information to other network components. Depending on the nature of the malfunction, the OMC makes it possible to ensure its elimination automatically or with the active intervention of personnel. OMC can provide a check of the condition of the network equipment and the progress of the call to the mobile station. OMC allows you to manage the load in the network. The efficient management function includes the collection of statistical data on the load from the components of the GSM network, recording them in disk files and displaying them for visual analysis. OMC provides management of software changes and databases on the configuration of network elements. The software can be loaded into memory from the OMS to other network elements or from them to the OMS.
NMC is a network management center that allows for rational hierarchical management of the GSM network. It provides network-wide operations and maintenance, supported by CHI centers that are responsible for managing regional networks. The NMC provides traffic management for the entire network and provides network supervisory control for complex emergencies such as node failure or overload. In addition, it monitors the status of the automatic control devices used in the network equipment and displays the status of the network for the NMC operators. This allows operators to control regional problems and, if necessary, provide assistance to the local self-government bodies responsible for a specific region. In this way, NMC staff are aware of the state of the entire network and can instruct MHO staff to change their strategy for solving a regional problem.
The NMC focuses on signaling routes and connections between nodes to avoid congestion conditions on the network. Also monitored
connection routes between GSM network and PSTN to avoid propagation of congestion conditions between networks. In doing so, NMC personnel coordinate network management issues with other NMC personnel. The NMC also provides traffic management capability for base station subsystem (BSS) network equipment. NMC operators in extreme situations may employ management procedures such as "priority access", where only high priority subscribers (emergency services) can access the system.
The NMC can take over responsibility in any region when the local MLA is unattended, with the MLA acting as a transit point between the NMC and the network equipment. The NMC provides operators with functions similar to those of the OMC.
The NMC is also an important network planning tool, as the NMC monitors the network and how it operates at the network level, and therefore provides network planners with data that will determine its optimal development.
BSS - base station equipment, consists of a base station controller (BSC) and transceiver base stations (BTS). The base station controller can control multiple transmit / receive units. The BSS manages the allocation of radio channels, monitors connections, adjusts their sequencing, provides frequency hopping mode, signal modulation and demodulation, message coding and decoding, speech coding, adaptation of the transmission rate for voice, data and call, and determines the sequence of transmission of paging messages.
The BSS together with the MSC, HLR, VLR performs some functions, for example: the release of the channel is mainly under the control of the MSC, but the MSC can request the base station to provide the release of the channel if the call does not go through due to radio interference. The BSS and MSC jointly prioritize information transmission for some categories of mobile stations.
TCE is a transcoder that converts the output signals of the MSC voice and data transmission channel (64 kbps PCM) to the form corresponding to the GSM recommendations for the radio interface (Rec. GSM 04.08). In accordance with these requirements, the transmission rate of digital speech is 13 kbps. This channel for transmitting digital voice signals is called "full rate". In the future, the standard provides for the use of a half-speed voice channel (transmission rate 6.5 kbps).
Reducing the transmission rate is achieved by using a special speech-converting device using linear predictive coding (LPC), long-term prediction (LTP), residual impulse excitation (RPE - sometimes called RELP).
The transcoder is usually located together with the MSC, then the transmission of digital messages in the direction to the base station controller - BSC is carried out with the addition of additional bits to the stream with a transmission rate of 13 kbit / s (stuffing) up to a data transmission rate of 16 kbit / s. It is then compressed at a factor of 4 onto a standard 64 kbps channel. This is how the 30-channel PCM line defined by the GSM Recommendations is formed, which provides the transmission of 120 voice channels. A sixteenth channel (64 kbps), a "time slot", is allocated separately for signaling information and often contains SS N7 or LAPD traffic. The other channel (64 kbit / s) can also carry data packets conforming to the CCITT X.25 protocol.
Thus, the resulting transmission rate for the specified interface is 30x64 kbps + 64 kbps + 64 kbps = 2048 kbps.
MS - a mobile station, consists of equipment that serves to organize access for subscribers of GSM networks to existing fixed telecommunication networks. Within the framework of the GSM standard, five classes of mobile stations are adopted from the 1st class model with an output power of 20 W installed on the vehicle to the portable model 5th class with a maximum power of 0.8 W (Table 1.1). When transmitting messages, an adaptive control of the transmitter power is provided to ensure the required quality of communication.
The mobile subscriber and the station are independent from each other. As already noted, each subscriber has his own international identification number (IMSI) recorded on his smart card. This approach allows radiotelephones to be installed, for example, in taxis and rental cars. Each mobile station is also assigned its own international identification number (1ME1). This number is used to prevent a stolen or unauthorized station from accessing the GSM networks.
Table 1.1
| Power class | Maximum transmitter power level | Permissible deviations |
| 1 | 20 watts | 1.5 dB |
| 2 | 8 watts | 1.5 dB |
| 3 | 5 watts | 1.5 dB |
| 4 | 2 watts | 1.5 dB |
| 5 | 0.8 watts | 1.5 dB |
1.3. Network and radio interfaces
When designing digital cellular systems for mobile communication of the GSM standard, three types of interfaces are considered: for connection with external networks; between different equipment of GSM networks; between the GSM network and external equipment. All existing internal interfaces of GSM networks are shown in the block diagram in Fig. 1.1. They fully comply with the requirements of ETSI / GSM Recommendations 03.02.
External network interfaces
PSTN connection
The connection to the public telephone network is carried out by MSC over a 2 Mbit / s communication line in accordance with the SS N 7 signaling system. Electrical characteristics of the 2 Mbit / s interface comply with CCITT G.732 Recommendations.
ISDN connection
To connect to the ISDN networks being created, four 2 Mbit / s communication lines are provided, supported by the SS N 7 signaling system and meeting the CCITT Blue Book Recommendations Q.701-Q.710, Q.711-Q.714, Q.716, Q.781 , 0.782, 0.791, 0.795, 0.761-0.764, 0.766.
Connection to existing NMT-450 network
The mobile switching center connects to the NMT-450 network through four standard 2 Mbps lines and SS N7 signaling systems. At the same time, the requirements of the CCITT Recommendations on the Telephone User Part (TUP) and the Message Transfer Part (MTP) of the Yellow Book should be met. The electrical characteristics of the 2 Mbit / s line are in accordance with the CCITT G.732 Recommendations.
Connections to international GSM networks
Currently, the GSM network in Moscow is connected to the pan-European GSM networks. These connections are based on Signaling System Protocols (SCCP) and Mobile Internet Switching Protocols (GMSC).
Internal GSM - interfaces
The interface between MSC and BSS (A-interface) provides message transfer for BSS control, call transfer, traffic control. A-interface combines communication channels and signaling lines. The latter use the CCITT SS N7 protocol. The complete specification of the A-interface complies with the 08 series of the ETSI / GSM Recommendations.
The interface between the MSC and the HLR is shared with the VLR (B-interface). When the MSC needs to locate a mobile station, it refers to the VLR. If the mobile station initiates the positioning procedure with the MSC, it informs its VLR, which records all the changing information in its registers. This procedure occurs whenever the MS moves from one locating area to another. In case the subscriber requests special additional services or changes some of his data, the MSC also informs the VLR, which registers the changes and, if necessary, informs the HLR about them.
The interface between the MSC and the HLR (C-interface) is used to provide interoperability between the MSC and the HLR. The MSC may send an indication (message) to the HLR at the end of the session so that the subscriber can pay for the call. When the fixed telephony network is unable to execute the mobile subscriber call setup procedure, the MSC may request the HLR to locate the subscriber in order to place the call to the MS.
The interface between HLR and VLR (D-interface) is used to expand the exchange of data on the position of the mobile station, control the communication process. The main services provided to the mobile subscriber are the ability to send or receive messages regardless of location. For this, the HLR must supplement its data. The VLR informs the HLR about the position of the MS, controlling it and reassigning numbers to it in the wandering process, and sends all the necessary data to provide service to the mobile station.
The interface between MSCs (E-interface) provides interaction between different MSCs during the implementation of the HANDOVER procedure - "transferring" a subscriber from zone to zone when he moves during a communication session without interruption.
The interface between BSC and BTS (A-bis interface) is used for communication between BSC and BTS and is defined by the ETSI / GSM Recommendations for connection establishment and equipment control processes, transmission is carried out in digital streams at a rate of 2.048 Mbit / s. It is possible to use a 64 kbps physical interface.
The interface between the BSC and the OMC (O-interface) is intended for communication between the BSC and the OMC; it is used in the CCITT X.25 packet-switched networks.
The internal BSC interface of the base station controller provides communication between various BSC equipment and transcoding equipment (TCE); Uses the 2.048 Mbit / s PCM transmission standard and allows you to organize from four channels at a speed of 16 kbit / s one channel at a speed of 64 kbit / s.
The interface between MS and BTS (Um radio interface) is defined in series 04 and 05 of the ETSI / GSM Recommendations.
The network interface between the OMC and the network, the so-called control interface between the OMC and the network elements, is defined by ETSI / GSM Recommendations 12.01 and is analogous to the Q.3 interface, which is defined in the ISO OSI layered open network model.
The network connection with the OMC can be provided by the CCITT SS N7 signaling system or the X.25 network protocol. The X.25 network can connect to internetworks or PSDNs in open or closed modes.
GSM is a network and service management protocol that must also satisfy the Q.3 interface requirements as defined in ETSI / GSM Recommendations 12.01.
Interfaces between GSM network and external equipment
The interface between the MSC and the Service Center (SC) is required to implement the short message service. It is defined in ETSI / GSM Recommendations 03.40.
Interface to other OMC. Each network control and maintenance center must be connected to other OMCs operating networks in other regions or other networks. These connections are provided by X-interfaces in accordance with the CCITT M.ZO Recommendations. The OMC interface is used to interact with higher-level networks.
1.4. Service structure and data transmission in GSM standard
The GSM standard contains two classes of services: basic services and teleservices. The main services provide: data transmission (asynchronously) in duplex mode at rates of 300, 600, 1200, 2400, 4800 and 9600 bit / s via public telephone networks; data transmission (synchronously) in duplex mode at speeds of 1200, 2400, 4800 and 9600 bps via public telephone networks, public switched data networks (CSPDN) and ISDN; access using an adapter to packet asynchronous data transmission with standard rates of 300-9600 bps via public switched packet data networks (PSPDN), for example, Datex-P; synchronous full-duplex access to the packet data network with standard rates of 2400-9600 bps.
When transmitting data at 9.6 kbps, the full rate link is always used. In the case of transmission at speeds below 9.6 kbps, half-speed communication channels can be used.
The listed functions of data transmission channels are provided for terminal equipment that uses CCITT interfaces with V.24 or X.21 series specifications. These specifications define the issues of data transmission over conventional telephone channels. Teleservices provide the following services:
1) telephone communication (combined with an alarm service: apartment security, distress signals, etc.);
2) transmission of short messages;
3) access to the services "Videotex", "Teletex";
4) "Facsimile" service (group 3).
Additionally, a wide range of special services has been standardized (call transfer, notification of tariff charges, inclusion in a closed user group).
As the majority of subscribers are expected to use GSM services for business purposes, particular attention is paid to security aspects and the quality of services provided.
The block diagram of communication services in GSM PLMN is shown in Fig. 1.4 (GSM PLMN - GSM Public Land Mobile Network - communication network with ground mobile objects; TE (Terminal Equipment) - terminal equipment, MT (Mobile Terminal) - mobile terminal, IWF (Interworking Function) - gateway functional joint). Data transmission also includes a new type of service used in GSM - the transmission of short messages (transmission of service alphanumeric messages for certain groups of users).
The transmission of short messages uses the bandwidth of the signaling channels. Messages can be transmitted and received by the mobile station. Common control channels can be used for the transmission of short messages. The volume of messages is limited to 160 characters, which can be received during the current call or in an idle cycle. V
control of radio channels, protection against errors in the radio channel, coding and decoding of speech, monitoring and distribution of user data and calls, adaptation of the transmission rate between the radio channel and data, ensuring the parallel operation of loads (terminals), ensuring continuous operation while driving.
Three types of terminal equipment of a mobile station are used: MTO (Mobile Termination 0) - a multifunctional mobile station, which includes a data terminal with the ability to transmit and receive data and voice: MT1 (Mobile Termination 1) - a mobile station with the ability to communicate via a terminal with ISDN ; МТ2 (Mobile Termination 2) is a mobile station with the ability to connect a terminal for communication using the CCITT V or X series protocol.
Terminal equipment may consist of one or more types of equipment such as a dialer, data transmission equipment (DTE), telex, etc.
There are the following types of terminals: TE1 (Terminal Equipment 1) - terminal equipment that provides communication with ISDN; TE2 (Terminal Equipment 2) - terminal equipment that provides communication with any equipment through the CCITT V or X series protocols (communication with ISDN does not provide). Terminal TE2 can be connected as a load to MT1 (mobile station with ISDN communication capability) via a TA adapter.
The system of characteristics of the GSM standard, the adopted functional diagram of communication networks and a set of interfaces ensure high parameters of message transmission, compatibility with existing and future information networks, and provide subscribers with a wide range of digital communication services.
1.6. TDMA frame structure and signal generation in the GSM standard
As a result of the analysis of various options for constructing digital cellular systems for mobile communication (SSMS) in the GSM standard, time division multiple access (TDMA) is adopted. The general structure of time frames is shown in Fig. 1.6. The period length of the sequence in this structure, which is called a hyperframe, is equal to Tr = 3 h 28 min 53 s 760 ms (12,533.76 s). A hyperframe is divided into 2048 superframes, each of which has a duration Te = 12533.76 / 2048 = 6.12 s.
A superframe is composed of multiframes. To organize various communication and control channels in the GSM standard, two types of multiframes are used:
1) 26-position TDMA multiframe frames;
2) 51-position TDMA multiframe frames.
A superframe may contain 51 first type multiframes or 26 second type multiframes. Durations of multiframes, respectively:
1) Tm = 6120/51 = 120 ms;
2) Tm = 6120/26 = 235.385 ms (3060/13 ms). Duration of each TDMA frame
Tc = 120/26 = 235.385 / 51 = 4.615 ms (60/13 ms).
In the sequence period, each TDMA frame has its own sequence number (NF) from O to NFmax, where NFmax = (26x51x2048) -1 = 2715647.
Thus, a hyperframe consists of 2,715,647 TDMA frames. The need for such a large hyperframe period is due to the requirements of the applied cryptographic protection process, in which the frame number NF is used as an input parameter. TDMA frame is divided into eight time positions with a period
To = 60/13: 8 = 576.9 μs (15/26 ms)
Each time position is designated TN with a number from 0 to 7. The physical meaning of time positions, which are also called windows, is the time during which the carrier is modulated with a digital information stream corresponding to a voice message or data.
A digital information stream is a sequence of packets placed in these time slots (windows). Packets are formed slightly shorter than the intervals, their duration is 0.546 ms, which is necessary to receive a message in the presence of time dispersion in the propagation channel.
The information message is transmitted over the radio channel at a speed of 270.833 kbit / s.
This means that the time slot of a TDMA frame contains 156.25 bits.
The duration of one information bit is 576.9 μs / 156.25 = 3.69 μs.
Each time slot corresponding to a bit duration is designated BN with a number from 0 to 155; the last 1/4-bit interval is numbered 156.
To transmit information over communication and control channels, adjust carrier frequencies, provide time synchronization and access the communication channel in the TDMA frame structure, five types of time intervals (windows) are used:
NB is used to transmit information over communication and control channels, with the exception of the RACH access channel. It consists of 114 bits of encrypted message and includes an 8.25 bit guard interval (GP) of 30.46 μs duration. Information block 114 bits is divided into two independent blocks of 57 bits, separated by a 26-bit training sequence, which is used to set the equalizer in the receiver in accordance with the characteristics of the communication channel at a given time.
NB includes two Steeling Flag bits that indicate whether the group being transmitted contains voice or signaling information. In the latter case, the Traffic Channel is "stolen" to provide signaling.
Between the two groups of encrypted bits in the NB, there is a 26-bit training sequence known to the receiver. This sequence provides:
Estimation of the frequency of occurrence of errors in binary digits based on the results of comparing the received and reference sequences. In the process of comparison, the RXQUAL parameter is calculated, which is adopted for assessing the quality of communication. Of course, we are talking only about the assessment of the connection, and not about precise measurements, since only part of the transmitted information is checked. The RXQUAL parameter is used when entering a communication, when performing a "handover" procedure and when assessing the radio coverage area;
Estimation of the impulse response of the radio channel in the transmission interval NB for subsequent correction of the signal reception path through the use of an adaptive equalizer in the reception path;
Determination of signal propagation delays between base and mobile stations to estimate communication range. This information is necessary so that data packets from different mobile stations do not overlap when received at the base station. Therefore, more distant mobile stations must transmit their packets before stations in the immediate vicinity of the base station. The FB is designed to synchronize with the frequency of the mobile station. All 142 bits in this time interval are zero, which corresponds to an unmodulated carrier with an offset of 1625/24 kHz above the nominal carrier frequency. This is necessary to check if it works.
its transmitter and receiver with a small frequency separation of channels (200 kHz), which is about 0.022% of the nominal value of the 900 MHz bandwidth. The FB contains an 8.25 bit guard interval just like a normal timeslot. Repeated frequency control slots (FBs) form a frequency setting channel (FCCH).
SB is used for time synchronization between base and mobile stations. It consists of a 64-bit sync sequence, carries information about the TOM frame number and the base station identification code. This interval is transmitted together with the frequency setting interval. The repeating synchronization intervals form a so-called synchronization channel (SCH).
DB provides link establishment and testing. In its structure, DB is the same as NB (Fig. 1.6) and contains a 26-bit beacon sequence. There are no control bits in the DB and no information is transmitted. DB only informs that the transmitter is functioning.
AB provides permission for the mobile station to access the new base station. AB is transmitted by the mobile station when requesting a signaling channel. This is the first packet transmitted by the mobile station, therefore the signal transit time has not yet been measured. Therefore, the package has a specific structure. The final 8-bit pattern is transmitted first, followed by the sync sequence for the base station (41 bits), which allows the base station to correctly receive the next 36 encrypted bits. The interval contains a large guard interval (68.25 bits, 252 μs duration), which provides (regardless of the signal transit time) sufficient time separation from packets of other mobile stations,
This guard interval corresponds to twice the maximum possible signal delay within one cell and thus sets the maximum allowable cell size. A feature of the GSM standard is the ability to provide communication for mobile subscribers in cells with a radius of about 35 km. The propagation time of the radio signal in the forward and reverse directions is 233.3 μs.
In the GSM structure, the time characteristics of the envelope of the signal emitted by the packets on the time slot of the TDMA frame, and the spectral characteristic of the signal are strictly defined. The envelope time mask for signals emitted in the AV interval of a full TDMA frame is shown in Fig. 1.7, and the envelope mask for the NB, FB, DB and SB signals of a full TDMA frame is shown in Fig. 1.8. The different envelope shapes of the emitted signals correspond to different lengths of the AV interval (88 bits) with respect to the other indicated intervals of the full TDMA frame (148 bits). The norms for the spectral characteristic of the emitted signal are shown in Fig. 1.9.
One of the features of signal formation in the GSM standard is the use of slow frequency hopping during a communication session. The main purpose of such hops (SFH - Slow Frequency Hopping) is to provide frequency diversity in radio channels operating in conditions of multipath propagation of radio waves. SFH is used in all mobile networks, which improves the coding and interleaving efficiency when subscriber stations are moving slowly. The principle of slow frequency hopping is that the message transmitted in the time slot of the TDMA frame (577 μs) allocated to the subscriber is transmitted (received) at a new fixed frequency in each subsequent frame. According to the frame structure, the time for frequency adjustment is about 1 ms.
During frequency hopping, a 45 MHz duplex separation is maintained between the transmit and receive channels. All active subscribers located in the same cell are assigned orthogonal shaping sequences, which eliminates mutual interference when receiving messages by subscribers in the cell. Frequency hopping sequence parameters (time-frequency matrix and start frequency) are assigned to each mobile station during channel establishment. The orthogonality of the frequency switching sequences in the cell is provided by the initial frequency shift of the same (according to the formation algorithm) sequence. Different shaping sequences are used in adjacent cells.
The combined TDMA / FDMA scheme of channel organization in the GSM standard and the principle of using slow frequency hops when transmitting messages in time frames are shown in Fig. 1.10,1.11.
For comparison, it can be noted that according to the results of experimental studies carried out on existing GSM networks, the spatial diversity of the receiving antennas at the base station gives a gain of 3-4 dB.
The adopted structure of TDMA frames and the principles of signal formation in the GSM standard, in combination with the drop coding methods, made it possible to reduce the signal-to-noise ratio required for reception to 9 dB, while in the standards of analog cellular communication networks it is 17-18 dB.
Literature for Chapter 1
1.1 M. Mouly, M.B. Pautet. The GSM System for Mobile Communications. 1992. p.p. 702.
1.2 Yu.A. Gromakov. Cellular mobile radio communication systems. Electronic communication technologies. Volume 48. Eco-Trends. Moscow. 1994.
1.3 A. Mehrotra. Cellular Radio: Analog and Digital Systems. Artech House, Boston-London. 1994. p.p. 460.
1.4 Yu.A. Gromakov. TDMA frame structure and signaling in the GSM standard. "Electrosvyaz". N 10. 1993. p. 9-12.
First time acronym Gsm was used in 1982 and stood for Groupe Speciale Mobile - the French name of the CEPT working group (Conference des administrations Europennes des Postes et Telecommunications - European Post and Telecommunications Administration).
The CEPT working group was tasked with developing specifications for a new digital standard for mobile communications in the 900 MHz range. Over time (1989) these works were transferred from CEPT to the new organization ETSI.
The birthday of GSM is considered to be 01.07.1991 - the first telephone call in this system was made in Helsinki (Finland).
The acronym GSM has changed to mean Global System for Mobile Communications.
GSM Kazakhstan is a GSM 900 mobile operator providing services under the Activ and Kcell brands. Founded on September 30, 1998. Shareholders of GSM Kazakhstan are the national telecom operator Kazakhtelecom JSC and the Finnish-Swedish-Turkish company FinTur.
The first among the operators of Kazakhstan to carry out the commercial launch of the "Mobile Video" service, services based on GPRS (MMS, WAP, Mobile Internet).
Radio communication systems networks are referred to in the technical literature as mobile, mobile and cellular networks. All names are used interchangeably, however, some discrepancies are outlined on this issue.
Wireless technologies are actively exploring the market of notebooks and PCs, whose users need high transfer speeds with limited mobility, both in speed of movement and in continuity of communication.
Based on this, everything that can be transferred and through which you can enter the communication network anywhere can be called mobile.
Traditional cellular communications can be called a mobile network.
The term cellular (cellular) means the division of a network into cells - cells (geographic areas). Each cell is assigned a frequency band that can be used in other cells.
Each cell has a base station, which contains radio transmitting and receiving equipment and provides radio communication with mobile phones located in this cell.
Figure 18. Cells in a mobile (mobile) communication system
Cell coverage depends on a number of factors:
base station transmitter power;
mobile phone power;
base station antenna heights;
terrain topology.
Cell sizes vary and therefore each cell can only serve a limited number of cellular telephones, which are called mobile terminals, mobile equipment ME, mobile stations MS.
The number of mobile terminals is 600 - 800. Cells are getting smaller in areas with higher population density. Cell coverage ranges from 100 m to tens of kilometers.
The choice of the hexagonal honeycomb is explained as follows.
A square cell (corresponding to city blocks) with a side will have four sides adjacent to it at a distance from its center to the centers of these four cells.
The centers of each of the four cells adjacent to the cell will be located at a distance from the center of the cell in question.
This configuration creates problems when switching to a new antenna for a subscriber moving away from the center of the cell.
For effective switching, it is desirable that the centers of all cells are at the same distance from each other. This is achieved with a hexagonal configuration.
With a hexagonal cell configuration, the distance between the centers of the cells will be equal. The base station BS antennas will be at the same distance from each other, regardless of the direction of movement of the mobile subscriber.
Considering the architecture and functionality of the GSM network, we will keep in mind that it is GSM that is the foundation of a number of more advanced technologies of the 2.5G generation, GPRS.
The GSM network consists of the following basic building blocks:
1. Transceiver BS;
2. BS controller;
3. Transcoding and Rate Adaptation Unit (TRAU).
4. Switching center MSC.
5. Home Register HLR (Home Location Register) - a network database that stores reference data on subscribers permanently registered in the area controlled by HLR (addresses, information about services).
6. Guest register VLR (Visitor Location Register) - a network database that stores information about the movements of subscribers. The accumulated information is stored as long as the subscriber is in the area controlled by the MSC.
7. Register of identification of equipment EIR (Equipment Identity Register).
8. Authentication Center AuC.
Figure 18. GSM 2G system architecture
For the purpose of study, it is convenient to consider the GSM-900 technology, since this technology, after minor modifications, is used in GSM-1800 and GSM-1900. GSM-1900 is also used in the USA under the name PSC-1900 (Personal Communication Services). GSM-1800 differs from GSM-900 in lower power of BS base stations, MS mobile terminals and smaller cell size.
Let's consider the principle of operation of GSM technology (Figure 18).
The mobile terminal MS (mobile station) communicates via the radio interface with the base transceiver station BTS (Base Transceiver Station).
MS consists of two parts: the tube itself, i.e. mobile equipment (terminal) ME (Mobile Equipment) and SIM-cards (Subscriber Identity Module).
A SIM card is a microcontroller placed in a small piece of plastic that stores a program for working with the GSM network and information about the subscriber and the operator.
The BTS is connected to the Base Station Controller (BSC), which provides a number of functions related to:
with radio resource management RR (Radio Resource);
with support for mobility MM (Mobile Management) in the coverage area of BTS stations;
a number of operational management functions for the entire radio network.
BTSs and BSCs form the Base Station Subsystem (BSS). The BSS provides radio access for the mobile terminal ME.
The rest of the network elements are responsible for the management functions and the databases required to establish a connection in the GSM network, such as encryption, authentication and roaming.
The Base Station Controller BSC is the network element that is the core of the GSM cellular radio network subsystem (BSS).
A SIM card (Subscriber Identity Module) is a subscriber identification module, a plastic card inserted into a ME mobile terminal and providing authorized access to a mobile (cellular) network.
The microchip of the SIM-card has dimensions of 85.5 × 54 × 0.76 mm, it is universal for different mobile devices. Protected by a special password or personal identification number, contains a unique international subscriber identifier IMSI (International Mobile Subscriber Identity).
Several BSs are connected to the Base Station Controller (BSC), which contains the control logic for each of these stations.
All BSCs are connected to the Mobile Switching Center (MSC), which manages the establishment of connections to and from mobile subscribers.
The MSC introduces the functionality of a standard switch and, in addition, a number of special functions for mobile communications.
These functions include, in particular, handover and roaming functions.
The function of handover (handover or handoff) is to delegate the call service to a new control cell during the connection of a mobile subscriber when moving from one cell to another.
In fact, handover means switching a subscriber from one radio channel and / or time slot to another, without notifying the subscriber about this change.
If the signal strength falls below a predetermined level (the user moves to another cell or approaches the border of the current cell), then it is checked whether the neighboring cell is receiving a signal with a higher level.
Upon confirmation of this, the service of the mobile subscriber is switched to this cell.
In modern technologies, the MAHO (Mobile Assisted Handover) method is used for this, in which the mobile terminal itself periodically measures the signal strength and quality of signals received both from the serving BS and from neighboring ones, and transmits a corresponding message to the network.
The nature of this message determines whether a handover should be performed or not.
In mobile technology, a subscriber moves from cell to cell within a network, as well as from one network to another. Movement (location) must be tracked with a certain precision in order to address calls (message) to it.
This problem is solved as follows.
1. The subscriber initially turns on his mobile terminal.
The device automatically sends a registration message to the local MSC. The message includes a unique subscriber identifier.
The message includes a unique subscriber identifier.
Based on this, the MSC can determine the HLR to which the subscriber belongs and send a registration message to the HLR to inform it which MSC is currently serving the subscriber.
2. HLR - transmits a de-registration message to the MSC that previously served this subscriber (if there is one) and sends an acknowledgment to the new MSC.
Each handset stores 15 digits of IMEI (International Mobile Equipment Identity) - a unique international identifier of a mobile terminal or 16 digits IMEISV (International Mobile Equipment Identity and Software Version Number) - a unique international identifier of a mobile terminal and software version number.
To find out the IMEI of your mobile phone, enter the combination "* # 06 #". It is useful to write down this number in case the mobile phone is stolen.
The EIR register contains three lists - black, gray and white.
Both the full IMEI number and the IMEISV number can be blacklisted. If the full IMEI number appears in the black list, then calls from this mobile terminal are prohibited.
If these values appear in the gray list, then calls can be allowed. But they can be prohibited at the discretion of the Operator.
When these values appear in the whitelist, calls are allowed.
The white list contains all series of equipment identification numbers for different countries.
The black list contains the identification numbers of mobile devices that are prohibited from using this network.
The greylisting contains information about defective or unlicensed (uncertified) hardware.
Authentication - checking that the access subject belongs to the identifier presented by him.
Authentication should not be confused with identification and authorization.
