Trunking networks. Systems of trunking radiotelephone communication. Recognition of iDEN in the world

Trunking communication is the most efficient type of two-way mobile communication, the most effective for coordinating mobile groups of subscribers. Trunked communication systems are less interesting for individual users (communication between them remains the prerogative of cellular radiotelephone systems); they are more promising and effective for corporate organizations, for group users - for instant communication between groups of users united on an organizational basis or simply by interests. Often, traffic (information transmission) is closed mainly within trunking systems, and subscribers' access to public telephone networks, although possible, is expected only in exceptional cases. But in principle, the operation of trunking systems is possible both in local (single-zone, corporate) and network (multi-zone, serving individual users) versions.

The trunking communication system (trunk - trunk, trunk) includes a base station (sometimes several) with repeaters and subscriber radio stations (trunk radiotelephones) with telescopic antennas.

The base station is connected to a telephone line and connected to a repeater with a long range - up to 50–100 km. Trunked radiotelephones are extremely reliable, compact and available in several variants:

l wearable - range 20–35 km, weight 300–500 g;

l transportable - range 35–70 km, weight about 1 kg;

l stationary - range 50–120 km, weight usually more than 1 kg.

The average possibilities of trunking communication in terms of territory coverage are shown in fig. 26.1.

Rice. 26.1. Trunking capabilities by area coverage

Generally speaking, trunking systems are characterized by equipment made using high technology, supported by good service for both the subscriber and the network operator, equipment that provides full duplex or half duplex radiotelephone communication with mobile objects, operation in analog and digital modes.

With trunking, a small number of radio channels are dynamically allocated to a large number of users. One channel accounts for up to 50 or more subscribers; since subscribers do not use the phone very intensively, and the base station operates in hub mode (that is, it distributes all radio channels only between subscribers who have contacted it), the probability of a “busy” situation is not high (significantly less than when even several subscribers are hard-attached to one channel ).

Radiotelephones can work both in the system, being in the coverage area of ​​the base (base) station and through it communicating with any subscriber of the telephone network (including the trunking subscriber), and individually with each other, being both inside and outside the area base radios. In the first case, the direct connection of subscribers will provide a more efficient connection (the connection time usually does not exceed 0.3–0.5 s). The possibility of direct communication of subscribers without the participation of a base station is the main, global difference between trunking systems and cellular ones.

The first mobile radio systems appeared in the United States in the late 1930s. These were single-channel conventional systems designed primarily for radio communications in the police and army. During the Second World War, the first multi-channel systems with "manual" channel switching were created.

A significant drawback of conventional systems is their vulnerability to unauthorized use of frequency resources. Any radio amateur who is knowledgeable in radio engineering is able to assemble a device to tune in to the frequencies used by this system and thus become an unauthorized user. In addition, in these systems, it is not easy to disconnect subscribers who create an excessive load of endless non-business "talks". The connection of subscriber terminals with the public switched telephone network (PSTN) is not implemented in all conventional systems.

The main idea of ​​trunking communication is that when a request is received from a subscriber to establish a connection, the system automatically determines free channels and assigns one of them to a given pair or group of subscribers. The problem of automating channel selection was partially solved in the so-called pseudo-trunking systems, which include SmarTrunk / SmarTrunk II, popular in Russia, from SmarTrunk System and ArcNet from Motorola. Their radio stations do not have a dedicated control channel (control channel) and, in search of a free one, scan a dedicated frequency range. Most of these systems (with the exception of ArcNet) are single-zone.

At the end of the 70s. The radio communication market was replenished with the first analog trunking systems with a dedicated control channel. Such systems implement the transmission of speech information on the principle of "one channel - one carrier", the frequency spacing of the channels is usually 25 or 12.5 kHz. Theoretically, with a sufficient number of frequency channels, they are able to serve tens of thousands of subscribers. However, the real values ​​of the allocated frequency resource limit the number of subscribers of the analog trunking network to 3-5 thousand people.

In addition, these systems still do not solve the problem of protecting the network from unauthorized access. Systems based on analog standards provide communication with PSTN subscriber terminals, but such terminals are quite expensive ($1500-2000). A significant drawback of these systems is also a limited number of user groups. And although the implementation of the dynamic group reconfiguration function allows you to get around this limitation, the game is not always worth the candle: the complexity of the equipment leads to a significant increase in the cost of the infrastructure.

In the early 90s. Trunking systems began to appear using digital voice signal transmission technologies. Today, digital standards such as APCO25, TETRA and PRISM (the digital version of EDACS) have received the greatest popularity. They allow you to significantly increase the capacity of the system - up to several thousand subscribers. In addition, they practically solve the problem of data protection and confidentiality of negotiations, since it is impossible to become an unauthorized user of a digital system or listen to a channel.

Many modern trunking communication systems (Fig. 1) - both analog and digital - are capable of transmitting data over a voice communication channel, i.e., performing the functions of a wireless modem. At the same time, in analog standards, the data transfer rate does not exceed 4800 bps, and in digital standards it reaches higher values ​​- from 9600 bps to 28 kbps (TETRA). Unlike analog, digital trunking systems allow you to send text messages through control channels (paging). The text of the message is displayed on the display of the subscriber terminal.

Currently, three different areas of application of mobile radio communication systems can be distinguished: public (police, fire brigade, ambulance, etc.); - type PS (Public Safety); private, such as PMR (Private Mobile Radio); commercial public networks SMR (Shared Mobile Radio).

Picture 1.
Mobile communication technologies (*technologies based on TDMA)

Systems of the first type are usually designed for a relatively small number of subscribers (as a rule, no more than 500-1000). They are characterized by increased requirements for reliability and confidentiality, as well as the presence of special features like Emergency Call. The cost of subscriber terminals of PS systems is quite high. The previously mentioned Public Safety/PMR networks include SmartNet, EDACS/PRISM, systems based on the APCO25 standard, and networks based on the digital TETRA standard currently under development.

Commercial systems of the SMR type are distinguished by a large capacity (the number of subscribers can reach tens of thousands), the possibility of providing additional information services, as well as the moderate cost of subscriber terminals. Among them are networks based on SmartZone, MPT1327, LTR/ESAS protocols and GeoNet systems. Note that most existing analog SMR systems have restrictions on frequency reuse and channel switching, as well as automatic identification of subscribers when they move from one area to another, etc.

Unlike conventional and trunked radio communication systems, mobile telephone cellular communication is intended primarily to provide one-on-one personal mobile voice communication in duplex mode. The first generation of cellular technology, which appeared in the early 1980s, used analog standards. The most widely distributed in the world (including Russia) are the North American AMPS standard, the British TACS and the Scandinavian NMT-450.

The use of digital technologies made it possible to understand that two different types of mobile voice communications - cellular and trunking - have much in common (territorial organization of the system, infrastructure, organization of access to the PSTN, etc.). However, analog trunking system technologies are unable to provide the level of service provided by mobile telephony.

In the mid 90s. Motorola decided to realize the idea of ​​an integrated system that combines the capabilities of group and dispatch radio communications, mobile cellular telephone communications, as well as the transmission of alphanumeric messages (paging) and data. The proposed system was supposed to provide a modern level of service for all types of communication. All this was implemented in iDEN (integrated Digital Enhanced Network) technology.

System Services

Mobile dispatch radio communication based on iDEN technology provides all types of services provided by modern digital trunking systems:

• group call (group call) for mobile subscribers and dispatchers in half-duplex communication mode. One button press is enough to make a call; the connection establishment time does not exceed 0.5 s. In this case, only one voice communication channel is used - regardless of the number of subscribers in the group. The number of possible groups in iDEN is large enough (65,535) to eliminate the need for dynamic group reconfiguration. All configurations can be created in advance: if necessary, subscribers simply go to the appropriate groups. Group members can be located at a distance of tens and hundreds of kilometers from each other (of course, within the coverage area of ​​the system);
• a personal call (private call) in half-duplex mode, when only two subscribers participate in a conversation and complete confidentiality of negotiations is ensured. Note that in the group and individual call mode, the caller's name or digital identifier appears on the display of the subscriber terminal of the called subscriber;
• call signaling (call alert) - the transmission of a special signal to the subscriber (or group), indicating the need to establish radio communication. If at this moment the subscriber is outside the system zone or the subscriber terminal is disconnected, the call is stored in the system. At the moment when the subscriber becomes available, he receives a sound signal, and the caller ID appears on the terminal screen. Only then does the caller receive an acknowledgment of receiving the call.

In addition to the services typical of conventional trunked communication, the iDEN system provides a number of features of modern mobile telephone systems:

• mobile telephone communication between subscribers, including through the PSTN (both incoming and outgoing in duplex mode). The iDEN system provides the functions of local telephony (mini-ATS, UPATS), voice mail (voice mail), long-distance and international communication;
• sending text messages. Subscribers can receive alphanumeric messages displayed on the screen of the subscriber terminal, which is capable of storing up to 16 messages of 140 characters. At the same time, both group and individual mailing of messages is provided. Receiving text messages is possible simultaneously with a mobile phone session;
• data transfer. iDEN portable (wearable) terminals have built-in modems and can be connected to a PC via an RS-232C adapter. In the circuit switching mode, the data transfer rate is up to 9600 bps, and in the packet mode - up to 64 kbps. To improve the reliability of data transmission, the system uses a forward error correction scheme. The data transfer function allows mobile subscribers to receive and send faxes and e-mails, exchange data with office computers and provide access to the Internet. In burst mode, the standard TCP/IP network protocol is supported.

Note that adding a data transfer function to an existing iDEN system does not require the installation of additional equipment at base stations (BS). It is only necessary to install additional blocks of the central system management infrastructure and install the appropriate software on the base stations and the central system.

Subscriber terminals

Although the iDEN system provides several types of communication, this does not mean that the subscriber needs to "subscribe" to all types of services and, accordingly, purchase a fully functional subscriber terminal from the operator. The user can always choose a model that matches the package of services he is interested in. The cost of iDEN portable subscriber terminals and digital cellular phones is approximately the same.

The i370/r370 portable terminals are capable of operating as both trunked radios and mobile phones. They are equipped with a multi-line LCD display, which displays lists of available groups (subscribers) and alphanumeric messages. The advanced i600 multifunctional terminal is smaller and lighter and has longer battery life.

The latest model of the i1000 handheld terminal has even smaller weight and size: its weight without batteries is 120 g, dimensions are 120x60x30 mm.

The i470/r470 models have a built-in modem, making them suitable for data and fax communications. In addition, these terminals support additional functions of the iDEN system, such as simultaneous operation in several groups, communication in isolated BS mode (in case of communication failure with the central infrastructure of the system), Emergency Call, etc.

Models r370 and 470, which meet the requirements of US military standards, have a shock-resistant case and are not afraid of moisture. The signal output power of portable terminals of all types is 600 mW.

The family of mobile user terminals iDEN consists of three models - m100, m370 and m470. The first one works only in dispatch radio mode, the other two are equipped with a handset and support mobile telephony. In addition, the m470 has a built-in modem and provides the same special features as the i470/r470 terminals. All types of mobile terminals have an output power of 3W.

The iDEN system also provides desktop dispatch stations based on m100/m370/m470 mobile terminals. They have an external antenna, a tabletop microphone, and an AC power supply.

Air interface and voice coding

The iDEN technology is based on the TDMA (Time Division Multiple Access) standard, according to which 6 digitized speech signals are simultaneously transmitted over each 25 kHz wide frequency channel. iDEN technology does not require all frequency channels to be contiguous.

The time interval of 90 ms is divided into 6 time slots of 15 ms each, in each of which one voice signal is transmitted (Fig. 2). The use of radio signal modulation using the M16-QAM (Quadrature Amplitude Modulation) method provides a total data transfer rate over one frequency channel of 64 kbps (the transmission rate in the voice channel is 7.2 kbps). Adequate reproduction of the human voice and other sounds at such a low transmission rate is achieved through the use of an improved coding scheme using the VSELP algorithm.

Figure 2.
iDEN frequency channel capacity

Frequency range

The system based on iDEN technology operates in the standard for America and Asia trunking band 806-825/851-870 MHz. Note that recently in Russia a part of this range, namely 815-820/860-865 MHz, is also reserved for trunked radio communication systems (Fig. 3).

Figure 3
The frequency range allocated for the iDEN system in Russia: mobile terminals (MT) 806-821 MHz; base stations (BS) 851-866 MHz

When developing iDEN technology, Motorola wanted to achieve the most efficient use of the frequency resource, at least not inferior to existing implementations of the CDMA standard. Since iDEN provides for the simultaneous transmission of six voice signals on each 25 kHz wide frequency channel, 240 such channels can be placed in 1 MHz of spectrum. For comparison, with a bandwidth of 1 MHz, analog and digital trunking communication systems can support no more than 80, analog cellular communication systems - from 30 to 40, and systems in the GSM standard - 40 voice channels (Fig. 4).

Figure 4
Comparison of spectrum utilization efficiency. In 1 MHz spectrum it is possible to place voice channels (GC): analog trunking systems - 40/80; analog cellular systems - 33-40; GSM - 40; TETRA - 160; IDEN-240

Structure of the iDEN system

The system based on iDEN technology consists of two main components: BS and central infrastructure. (Fig. 5). The iDEN infrastructure is organized to maximize the functionality of the BS, so the most important functional element is the EBTS Enhanced Base Transceiver System base station. The EBTS includes an integrated node controller (iSC), up to 20 base radio stations (BR) of the omni type or 24 sector BRs, an amplifier and radio signal transmitters, a synchronizing receiver, and BS antennas.

Figure 5
The structure of the system based on iDEN technology: * provide telephone communication; ** provide radio communication; *** provided by the system operator; DACS (Digital Access Crossconnect Switch) - digital access switch; IWF (Interworking Function) - data transfer interface with PSTN; VMS (Voice Mail System) - voice mail

EBTS provides interaction between the system and subscriber devices, supports the transmission of voice traffic on several frequency channels, and also performs a number of control functions, such as separation of radio and telephone traffic, synchronization of the BS and subscriber terminals, radio signal level control, etc. Multifunctionality of EBTS allows you to significantly reduce the load on the components of the central infrastructure, primarily on the MSC (Mobile Switching Center). The EBTS transmitter supports a maximum of 144 voice channels per system node.

The main function of the BSC (Base Site Controller) is communication management when user terminals move from one coverage area to another (handover). Each BSC is capable of supporting up to 30 zones, performing the full range of actions for concentrating traffic from node stations and distributing it to the appropriate zones.

The XCDR transcoder converts VSELP audio to and from PCM digital format.

The MPS (Metro Packet Switch) packet switch consists of a switch and a packet duplicator. It transmits voice packets arriving in dispatch radio mode and control information from EBTS to DAP and vice versa.

Dispatch Application Processor (DAP) manages group and personal calls, call signaling and other functions. With a large number of system subscribers, it is possible to create clusters of four DAPs.

The HLR/VLR (Home Location Register/Visited Location Register) subscriber location registration units serve mobile telephony. The HLR stores complete information about all subscriber terminals registered in various geographical segments of the system. VLR contains information about the movement of subscriber devices and provides the system with the information necessary to perform roaming. Note that in the iDEN system there is no roaming in the sense in which it is understood in cellular systems, since not PSTN, but dedicated E1 channels are used to connect geographically distant segments of the system.

The MSC (Mobile Switching Center) provides an interface between the PSTN and iDEN mobile phones, performing the typical functions of such a switch, and also manages the transfer when subscribers move from an area controlled by one BSC to an area controlled by another. If the iDEN network covers a large area, several MSCs can be installed in it. The functions of the MSC of the iDEN system are completely identical to the functions of the GSM cellular network switch.

The main control module of the system is OMC (Operation Maitenance Center), which provides system configuration, emergency management, collection of statistical data on system operation and a number of other management functions.

The Short Message Service (SMS) supports all text messaging functions, including text notifications of the presence of messages for a given subscriber (voice mail).

iDEN MicroLite

Motorola is currently finalizing the iDEN MicroLite system, which is a "small" iDEN-based system designed to serve hundreds to thousands of subscribers. While maintaining all iDEN technological solutions, using the same subscriber equipment and base stations, this system differs, first of all, in the maximum number of frequency channels (there are 40 of them).

The main technological difference between iDEN MicroLite and iDEN is the organization of the central infrastructure of the system. In the iDEN MicroLite system, it is implemented on a single Compact PCI standard computer platform (a variant of the PCI platform for industrial computers) running under the Neutrino real-time OS from QNX Labs.

The first version of iDEN MicroLite will provide two types of communication - group (individual) radio communication and mobile telephone communication. Future releases will add short message and dial-up/packet data services to the system. The maximum number of base stations that the central infrastructure of the first version of the system can support is 5, in the future it will be increased to 8-10.

If it is necessary to migrate from iDEN MicroLite to a complete iDEN system, a new installation of the central infrastructure of the system is required, however, by modifying the appropriate software, subscriber terminals and existing BS equipment can be used.

Deliveries of the iDEN MicroLite system will begin in the II quarter of 1999. The technical study of the projects of the iDEN MicroLite systems is expected from the III quarter of 1998.

Applications for iDEN

iDEN technology is focused on the creation of systems such as SMR (Shared Mobile Radio), i.e. commercial networks that provide integrated services to organizations and individuals. In order to provide communication between individual departments and groups of employees, a so-called "fleet" is created for each corporate user of the system - a virtual private network within the organization's network. Different groups can be created within the fleet, corresponding to the company's divisions (the maximum number of groups in one fleet is 255). The possibility of accidental or deliberate intrusion of subscribers into foreign fleets is absolutely excluded. Fleet members can be located in different geographical regions, move from one city to another.

Thus, an organization can build its own mobile telecommunications system, which is completely equivalent to the network of this organization. At the same time, she does not need to purchase equipment and build antennas, as well as spend several months installing and debugging the system. All you need to do is to become a corporate user of the already existing iDEN system.

Where and when

The first commercial system based on iDEN technology, deployed in the US by NEXTEL in mid-1994, is now nationwide. It has about 4500 BS and about 2 million subscribers. In the southwestern US, there is another network based on iDEN technology, operated by the energy company Southern Co. In addition, in the southwestern provinces of Canada, Clearnet also provides communication services in the iDEN network, which consists of 320 BS.

In Latin America, iDEN networks already exist in Bogota (Colombia) and Buenos Aires (Argentina). They are being built in Sao Paulo and Rio de Janeiro (Brazil), as well as in Mexico City (Mexico). Rollouts of iDEN-based systems are planned for the near future in Peru, Venezuela and Chile, as well as system expansions in Colombia and Argentina.

In Asia, iDEN systems are operated in several countries: such systems have been operating in Tokyo and Osaka (Japan) for more than two years, and in Singapore for about a year. There are systems in China, South Korea and the Philippines. Construction is underway in Indonesia. In the Middle East, a nationwide iDEN network has been deployed in Israel, and the construction of such systems has begun in Morocco and Jordan.

Each of these systems is designed to serve tens of thousands of subscribers.

The modular principle of the system organization provides its various implementations. For example, iDEN could initially be deployed as a purely trunked system and then added mobile telephony, text messaging and data capabilities as needed. According to the developers of the system, today iDEN is one of the few technologies tested in commercial operation that provide the entire range of mobile communication services.

Andrey Aleksandrovich Denisov is Motorola's manager for the iDEN system in the region of Eastern Europe and the former USSR. He can be contacted at: [email protected] and fax 785-0160

In almost every mobile phone shop, the windows of which are full of mobile phones, there is a security guard with an obligatory bulky walkie-talkie. Here you involuntarily ask yourself: “Why does this person not use a simple mobile phone for the service?”

Today, along with the usual cellular communication, there are so-called professional mobile radio systems (PMR) (Professional Mobile Radio-PMR), or trunked mobile radio. They occupy their sector of the mobile communication equipment market for corporate users, various departments and social services, performing the functions that these users need.

Trunked mobile radio communication (from the English. trunking- provision of free channels, trunk- trunk line) - a system of two-way mobile radio communication, which uses the range of ultrashort waves. In practice, the PMR system is designed similarly to a cellular one: user terminals and base stations (BS), equipment for increasing the communication range - repeaters and a controller that controls the operation of the station, processes repeater channels (switches them) and provides access to the city telephone network. Trunking networks can be single-zone (contain one BS) or multi-zone (several BS). There are analog and digital trunking systems.

Better than cell phone?

What is the difference between a trunked connection and a cellular one, if, apart from the difference between the user terminal (walkie-talkie / telephone), everything is arranged the same?

Cellular communication is positioned as a "phone in your pocket", and trunking is designed to solve a narrow range of professional tasks. Cellular communication, for example, provides a variety of multimedia services, but an oilman on duty on a drilling platform in the Baltic Sea, or an EMERCOM rescuer, is unlikely to hope to be able to download Madonna's new album. Trunking communication is chosen by such organizations as the Ministry of Emergency Situations, security agencies, taxi companies, etc. For ordinary office workers, the “cellular phone + corporate tariff plan” option is quite suitable.

The communications system used by professionals should support features such as:

Implementation of instant communication (0.2-0.5 sec) within a group of subscribers, which can be set in advance;

Ability to redistribute group members during a communication session;

Call priority system (the mobile operator does not distinguish between subscribers);

Preservation of communication even in case of failure of the base station;

Broadcast signal transmission to network subscribers;

The ability to quickly reconfigure the network.

These requirements are not feasible in cellular communication systems, but are fully supported by trunking systems. It should be noted that the participants of the mobile communication market do not sit idly by and offer the service Push To Talk with the possibility of establishing a group call and fast connection establishment. However, the innovation in any case does not meet the requirements of professionals. You can read more about Push-To-Talk here.

We offer a comparison table on the example of two versions of TETRA - a popular standard for digital trunked radio communication, and GSM networks.

Modes and functionality, communication standards TETRA (Rl) TETRA (R2) GSM Group call + + +/- Broadcast call + + - Emergency call + + +/- Priority call + + +/- Priority access + + - Duplex + + + Delayed call + + - Delayed entry into communication + + - Direct mode (without base station) + + - Mode - "receive only" - + - Possibility of expanding the communication zone - + - Zone selection + + - Status messages + + - Transmission of short text messages + + + Calling the dispatcher + + - Provision of broadband upon request of the subscriber + + - Possibilities of encrypting the signal and the radio interface + + +/- Simultaneous transmission of voice and data + + + High-speed data transmission - + + Selective listening of subscribers by the dispatcher + + - Remote listening to the acoustic environment + + - Dynamic rearrangement + + - From steampunk to cyberpunk

Professional analog communications have existed almost since the beginning of the 20th century, and during this time it has changed a lot, having come to digital technologies with impressive baggage.

Everyone knows that radio communication began in 1895, when A. Popov (and only a year later G. Marconi) created the first receiver. From 1897 to 1915 G. Marconi organizes the first communication companies and expands the production of equipment; radio communication regulations appear, including on the distribution of frequencies between various services. Professional radio communication was born in the period from 1915 to the 1950s.

In the first half of the 20th century, the possibility of communicating at different wavelengths was explored. Until 1920, communication was carried out using waves ranging in length from hundreds of meters to tens of kilometers. In 1922, the property of short waves to propagate to any distance, refracted in the upper layers of the atmosphere and reflected from them, became known - an ideal means for long-distance communication. The 1930s were the time for meter waves; and the 1940s - decimeter and centimeter, propagating rectilinearly for 40-50 km within line of sight. The popularization of radio communications directly depended on the achievements of technology. Before the advent of miniature semiconductors, receivers remained bulky and at best fit in a suitcase, which imposed certain limitations.

The history of professional radio networks is usually divided into stages. First step networks are considered conventional type (from the English. conventional- usual, traditional). Their poor capabilities are as follows: simplex mode of operation (pressed the button - asked a question - released the button - received an answer - pressed the button - ...), making individual and group calls (up to several tens of subscribers) In conventional systems, the communication channel (frequency) is strictly assigned to a certain group of subscribers. At the same time, high communication efficiency is guaranteed (you only need to adjust the frequency), but it causes a low network bandwidth (there are few frequencies).

Second phase- Trunking networks. Such networks made it possible to serve up to several hundred subscribers and made it possible to use the radio frequency resource more efficiently. Such communication systems have become systems with the general access of subscribers to the frequency range, in contrast to conventional systems. This provides increased throughput and greater coverage area.

Multi-zone trunking networks have become third stage. The service area in them has increased even more due to several base stations. The number of subscribers served has become practically unlimited, a system of call priorities has appeared, the possibility of a duplex call mode (you do not need to press a button, the connection is similar to a telephone connection, adjusted for a much higher call speed), access to public telephone networks, data transfer.

Simplex, half duplex and duplex

No, these are not the names of the sequels to the comedy "Duplex", which starred Hollywood stars Ben Stiller and Drew Barrymore. The header contains the names of the three basic wireless radio modes.

1. Simplex communication uses one frequency - for reception and transmission. Only exchange of replicas is possible. Due to the limitations that physics imposes, it will be possible to use this most economical type of wireless radio communications at a distance of no more than 5 km. For a stable signal, an open area is highly desirable. Communication is via user terminals.

2. Half-duplex communication also uses two frequencies, but you will have to communicate, as in simplex mode. The base station (BS) constantly receives subscriber signals on one frequency, and then broadcasts what it has received on another frequency. The radio uses the frequency on which the BS broadcasts for reception, and must contain an RF switch. The principle of half-duplex is the basis of low-cost networks that connect dozens of subscribers in various parts of the city and open areas.

3. Duplex communication uses two frequencies - one for reception, the other for transmission, and is designed to carry on a familiar dialogue. Naturally, base stations are involved for relaying signals. Analog duplex systems require two channels (4 radio frequencies) to connect subscribers. The terminal is equipped with an overall duplex filter, whose role is to give the receiver and transmitter simultaneous access to the antenna. Digital duplex is implemented differently and does not require a cumbersome filter - the subscriber's device receives or transmits at any time. For example, in the TETRA standard, switching occurs 18 times per second.

Modern digital trunking networks (CFTS) are the top of the evolutionary chain of professional communication. In addition to the opportunities available to users of analog systems, reliable protection against unauthorized access is added (besides, listening to conversations using analog devices becomes impossible) and packet data transmission (Internet access). The subscriber's device is identified using various identification mechanisms or SIM cards. In fact, digital trunking systems are universal communication networks that ensure the confidentiality of subscriber contacts and are capable of simultaneously transmitting large data streams over communication channels, whether it be telemetry data or video information (in the latest editions of the standards, such capabilities are provided).

There are a large number of different standards for trunked mobile radio systems that differ in many ways. In our country, as well as throughout the world, analog systems of various versions and standards are still common. However, due to their obsolescence, they are not as interesting to consider as their digital counterparts. The five most popular and recognized in many countries of the world should be considered in more detail.

EDACS (Enhanced Digital Access Communication System)

Firm Ericsson(Sweden) earlier than others (until I bought it Sony in the 1980s) took care of the problem of the obsolescence of analog technologies and the insufficient degree of security of negotiations in such systems and began to develop the corporate closed standard EDACS (Enhanced Digital Access Communication System). Initially, the standard provided for the transmission of voice over analog protocols, later the standard was modified and a digital version of the system appeared called EDACS Aegis. EDACS systems operate on 138-174 MHz, 403-423 MHz, 450-470 MHz and 806-870 MHz; the network can be extended to more than 16,000 subscribers. In Russia, this standard is not very popular in this regard due to its closeness and rapid obsolescence (in fact, this is a digital standard for transmitting analog signals). All rights belong to the developer, and you will not be allowed to release equipment just like that. In addition, Ericsson has stopped deliveries of equipment for the deployment of new networks of this standard and is only engaged in supporting existing ones.

iDEN technology ( integrated Digital Enhanced Network) is a closed corporate standard, the development of which was started by the company Motorola in the early 1990s. In 1994 in the USA the company NEXTEL based on this technology, the first network of commercial applications has been deployed. Today, such networks are deployed in many countries of North and North South America and Asia. Today iDEN subscribers are more than 3,000,000 people (90% of them are in the USA). iDEN has gained such popularity due to the fact that it is a kind of compromise between trunking and cellular systems (it provides the ability to send messages, fax, data transfer via TCP / IP at speeds up to 36 kbps, low cost). Each organization using the iDEN standard can create up to 10,000 virtual networks, each of which can have up to 65,500 subscribers. iDEN uses the frequency range 805-821/855-866 MHz. There are no iDEN systems in Russia - most likely due to the inconvenience of using such a frequency range when solving problems that professional communication systems are designed for. It is noteworthy that the company Motorola various iDEN-devices are produced with the functions of modern mobile phones. For example, Motorola ic502 is a CDMA/iDEN phone with GPS and Motorola i290 with an MP3 player.

Tetrapol PAS (Tetrapol)

Developed by a French company Matra Communication. The creation of this closed standard was started in 1987 by Matra Communications by order of the French gendarmerie. The Tetrapol communication network has been operating in half of France since 1994 and serves more than 15,000 subscribers. Tetrapol communication systems operate starting from 70 MHz and have a performance ceiling of 520 MHz, which does not contribute to popularization in other countries where other frequency ranges can traditionally be assigned to such systems. Experimental zones for the functioning of the Tetrapol network have been created in Russia.

TETRA (Terrestrial Trunked Radio)

TETRA- an open standard for professional radio communications, developed since 1994 ETSI(European Telecommunications Standards Institute - European Telecommunications Standards Institute). TETRA stands for Terrestrial Trunked Radio - "Terrestrial Trunked Radio". Initially, until the standard gained popularity outside of Europe, TETRA stood for Trans European Trunked Radio- "Trans-European Trunking Radio". In Europe, the TETRA standard PMR operates in the frequency ranges 380-385/390-395 MHz, 410-430/450-470 MHz. In Asia - 806-870 MHz.

In the specifications, TETRA is listed as an open standard, which means that anyone who wishes to produce communication equipment may not think about compatibility problems with equipment from other companies and about the division of copyright. To produce products that support this standard, you must join the organization MoU TETRA- Memorandum on the promotion of the TETRA standard. Nokia, Motorola, RohdeSchwarz and other major communication equipment companies support this standard. TETRA networks are deployed almost throughout Europe, in Asia, Africa and South America. TETRA Release 2- a new version of the standard, which allows for tight integration with third-generation mobile networks and significantly increases the data transfer rate. The project to deploy networks of this standard in Russia is called "Tetrarus". Says a lot at least the fact that "within the framework of the Federal target program" Development of Sochi as a mountain climatic resort until 2014 " TETRA standard radio communication will operate in the venues of sports competitions and throughout the Krasnodar Territory.”

APCO Project 25 (APCO 25)

The open standard APCO 25 was created by the organization AssociationofPublicSafetyCommunicationsOfficials-international-Association of representatives of communication services of public security bodies. The standard was created and improved (construction of the radio interface, encryption protocols, speech coding methods) in the period from 1989 to 1995. One of the main advantages of APCO 25 is that it allows you to work in any of the frequency bands available for mobile radio systems: 138-174, 406-512 or 746-869 MHz. Up to two million people and up to 65 thousand groups can be combined into one network. Since 2003, a similar network has been operating in St. Petersburg for several hundred subscribers for the purposes of the Russian Ministry of Internal Affairs.

Trunking can be used not only for communication:

Latest Trunking System JRC Trunked Radio System with automatic vehicle location function based on GPS and MPT 1327/1343 standards. In addition to, in fact, providing communications between subscribers, the standard provides automatic transmission of data on the location and status of each machine to the terminal in the control center.

An example of two ways to organize a trunking network:

The characteristics of the standards are more fully reflected in the table:

Functionality, digital trunking standards APCO 25 EDACS IDEN TETRA Tetrapol Individual, group, broadcast calls + + + + + PSTN access + + + + + Full duplex subscriber terminals - + + + - Data transmission and database access + + + + + Direct mode + + ? + + Automatic registration of mobile subscribers + + + + + Personal call + - + + + Access to IP networks + + + + + Transmission of status messages + + + + + Transmission of short messages + - + + + Transmission of subscriber location data from GPS receiver? +? + + Fax + - + + + Possibility to set open channel? - - + + Multiple access using the list of subscribers + - + + + Signal relay mode + ? ? ++ Mode "dual surveillance" ? - ? + + Access/call priority + + - + + Dynamic rearrangement + + - + + Selective listening + + - + + Remote listening? - - + + Identification of the calling party + + - + + Call authorized by the dispatcher + + - + + Key transfer over the air (OTAR) + - - + + Simulation of subscriber activity - - - - + Remote disconnection of the subscriber + ? - + + Authentication of subscribers + ? - + +

In Russia, along with the introduction, successful use and development of digital networks of various trunking standards, analog systems based on the old MRT1327. And this is by no means bad. Digital trunking is convenient where you need not only operational communication, but also data transmission and telephony. Often, simplex voice communication and messaging functions are quite enough for customers. Using analog systems saves time and money.

In general, the situation with professional mobile radio communications resembles the transition from the use of cellular networks of the second generation of the standard GSM to standards 3G. Cellular networks, despite their growth rates, will not be able to completely replace professional radio networks in the near future due to the fact that they perform other functions.

Section 4 Mobile trunking systems

Lecture #23

What is a "trunk"? Let's try to figure out what is hidden behind this "fashionable" word? Here is the translation given by the English-Russian Dictionary of Radio Electronics, 1987 edition:

Trunk (trunk) - connecting line; trunk communication line; link

Trunking (trunking) - group formation

The electronic dictionary "PROMT" of 1999 is more "educated":

Trunking - provision of free channels

Trunked radio system - radio system with automatic redistribution of channels

As can be seen from the translation, there is nothing special behind the word "trunk". It's just "auto channel provisioning".

Trunking principles have been used for over 70 years in telephony. Any automatic telephone exchange, mini PBX, cellular communication uses trunking as the basis of its work. We all use trunking almost daily. Although not many of us realize that when we pick up the phone and dial a number... we are using trunking. After all, it would be an unaffordable luxury to allocate a separate line to each telephone subscriber, especially long-distance. All of us are allocated a line for the conversation only for the duration of the communication session. The rest of the time (free from our conversations) other users are served on it.

Imagine a situation where residents of, say, one of the districts of Tashkent would simultaneously decide to call their friends. What would happen in this case? But nothing. They simply could not do this, since the number of telephone lines (between automatic telephone exchanges) is limited and a quite certain number of subscribers can conduct communication sessions at the same time (how many specifically is a topic for a separate conversation).

Now imagine that all telephone sets have been replaced with radio stations, and wire lines with radio frequency channels. As you probably already guessed, we received a trunk - a radio communication system with automatic provision of a free channel.

SOME EXPLANATIONS

Trunk systems DO NOT regulate:

access to the telephone network;

the use of duplex (“I speak and listen” at the same time, as in telephony);

huge range;

the highest service;

free access;

and much more...

They simply allow you to communicate with each other without thinking about technical subtleties and physical problems. You are talking - the equipment is working. Works so you can talk.

More scientifically, the essence of trunk communication is that the subscriber is not assigned to a specific channel, but has equal access to all channels in the system. And which one to use for a communication session is decided by special control equipment. At the request of the subscriber, the system automatically provides the subscriber with a free channel.

ABOUT TERMINOLOGY

In Russian publications, the words “trunking” and “trunking systems” have become established. Let's leave these turns on the conscience of translators and linguists. In our opinion, the words "trunk" and "trunk systems" are more harmonious in pronunciation and easier to write. As a rule, their use does not cause ambiguous understanding. Therefore, in what follows, we will mainly use “our” formulations.

MYTHS AND REALITY

Ten considerations to cool the ardor of optimists and lift the spirits of pessimists regarding the "miracles" of trunking:

Trunk is not a miracle, but a process of development of radio communications.

A trunk does not replace a cell phone, it does not replace a pager ... a trunk does not replace anything at all, but complements.

Trunked means: convenient, flexible, expandable, versatile, reliable, complex, expensive...

Trunk systems are used to communicate between radios and once again radios, and not between radios and telephone lines.

Trunk systems can do a lot, but not all.

There are many trunk systems, and which one to choose depends on the tasks.

If the trunk system does not solve the task, then this is the wrong task.

If you could not choose a suitable trunk system, then you do not need a trunk system.

There are many suppliers, but little money - do not pay twice.

Don't flatter yourself! Entrust the choice to specialists.

But seriously, what are the advantages of trunk systems in comparison with traditional, so-called "ordinary" communication networks, with cellular telephony, with personal radio call systems (paging)?

It is rather difficult to answer this question unambiguously. As with any system, there are both advantages and disadvantages.

Perhaps the main advantage of trunk systems is the ability to integrate different services with different needs within the same network with minimal (compared to other radio systems) material costs.

BENEFITS OF TRUNK NETWORKS

Compared to cellular systems:

the ability to communicate simultaneously with several subscribers (group calls);

high speed of establishing a connection (0.2–1 sec);

queuing to system resources when busy and automatic connection after the possibility of access;

access to the system based on the established priorities and emergency provision of a communication channel to a subscriber with a higher priority;

lower costs for deployment and operation of systems.

Compared to "conventional" radio systems:

saving frequency resources;

higher level of service - individual calls, priorities, integration with other networks;

the possibility of digital data transmission;

communication coverage of large areas due to multi-zone configuration.

Compared to paging networks:

two-way communication;

the ability to transmit short messages (similar to paging) over trunk channels using existing equipment.

This is not a complete list of available benefits. Still, the trunk is not a panacea for all ills. Along with trunk systems, there are a number of users who, for various reasons, need a cell phone, someone needs a pager, and a number of users get by (and will get by) with “normal” communication systems.

It must be clearly understood that the trunk is not a universal solution to the entire set of radio communication tasks. In any, even the most “trunk” state, there are still a number of problems that are solved by other communication systems that have nothing to do with trunk ones.

The disadvantages of trunk systems include:

low profitability with a small number of subscribers;

relatively high cost of equipment (compared to "conventional" radio communication systems);

the need for inter-zone communication lines (wire, radio frequency, radio relay, fiber optic) and, as a result, the complexity and cost of deployment *;

the need for professional service.

* It is worth noting that in order to cover large areas, most radio communication systems require multi-zone implementation and, of course, inter-zone communication lines.

CLASSIFICATION OF TRUNK SYSTEMS

Trunked systems can be classified according to many criteria, for example, by the format of the transmitted data (analogue, digital), by the types of protocols (LTR, MPT 1327, SmarTrunk II), by the number of serviced zones (single or multi-zone), by the methods of presenting the radio channel (" transmission trunking” or “message trunking”), by methods of controlling base stations (centralized or distributed), by types of control channels (dedicated or distributed), etc.

We will not dwell on a detailed classification of trunk systems, especially since there is no single and generally accepted methodology in this area. We will try to characterize modern trunk systems, describe their capabilities, note the most important points that you should pay attention to when choosing.

Architecture of trunked systems

Trunking systems are called radial-zone systems of terrestrial mobile radio communication, which automatically distribute communication channels of repeaters between subscribers. This is a fairly general definition, but it contains a set of features that unite all trunking systems, from the simplest SmarTrunk to modern TETRA. The term "trunking" comes from the English Trunking, which can be translated as "bundling".

Single zone systems

Figure 67 Structural diagram of a single-zone trunking system

The main architectural principles of trunking systems are easily seen in the generalized block diagram of a single-zone trunking system, shown in fig. 67. The infrastructure of the trunking system is represented by a base station (BS), which, in addition to radio frequency equipment (repeaters, radio signal combiner, antennas), also includes a switch, a control device and interfaces of various external networks.

Repeater. Repeater (RT) - a set of transceiver equipment serving one pair of carrier frequencies. Until recently, in the vast majority of TSSs, one pair of carriers meant one traffic channel (CT). Currently, with the advent of the TETRA standard and the EDACS ProtoCALL system, which provide for temporary consolidation, one RT can provide two or four CTs.

Antennas. The most important principle in building trunked systems is to create radio coverage areas as large as possible. Therefore, base station antennas are usually placed on high masts or structures and have a circular radiation pattern. Of course, when the base station is located at the edge of the zone, directional antennas are used. The base station can have both a single transceiver antenna and separate antennas for receiving and transmitting. In some cases, multiple receive antennas may be placed on a single mast to combat multipath fading.

The radio signal combining device allows the use of the same antenna equipment for the simultaneous operation of receivers and transmitters on several frequency channels. Transponders of trunking systems operate only in duplex mode, and the frequency spacing between receiving and transmitting (duplex spacing), depending on the operating range, ranges from 3 MHz to 45 MHz.

The switch in a single-zone trunked system handles all of its traffic, including the connection of mobile subscribers to the public switched telephone network (PSTN) and all data calls.

The control device ensures the interaction of all nodes of the base station. It also handles calls, performs caller authentication (friend or foe verification), call queuing, and time billing database entries. In some systems, the control device regulates the maximum allowed duration of a connection to the telephone network. As a rule, two throttling options are used: reducing the duration of connections during predefined peak hours, or adaptively changing the duration of a connection depending on the current load.

The PSTN interface is implemented in trunked systems in various ways. In low-cost systems (such as SmarTrunk), the connection can be made over two-wire switched lines. More modern TSNs have DID (Direct Inward Dialing) direct dialing equipment as part of the interface to the PSTN, which provides access to trunked network subscribers using standard PBX numbering. A number of systems use a digital PCM connection to PBX equipment.

One of the main problems in registering and using trunking systems in Russia is the problem of their interface with the PSTN. When outgoing calls from trunked subscribers to the telephone network, the difficulty lies in the fact that some trunking systems cannot dial a number in ten-day mode over subscriber lines in electromechanical exchanges. Thus, it is necessary to use an additional tone-to-decade converter.

Incoming communication from PSTN subscribers to radio subscribers is also problematic, but for a number of reasons. Most trunked networks interface with the telephone network via two-wire subscriber lines or E&M type lines. In this case, after dialing the PSTN number, additional dialing of the radio subscriber's number is required. However, after the subscriber line is fully dialed and the loop is closed by the trunking system control device, the telephone connection is considered established, and further dialing in the pulse mode is difficult, and in some cases impossible. The “click” detector used in the SmarTrunk II system does not guarantee the correctness of the pulse dialing, since the quality of the “click pulses” coming from the subscriber line depends on its electrical characteristics, length, etc.

To get out of this situation, in the laboratory of IVP, together with specialists from ELTA-R, a telephone interface (TI) ELTA 200 was developed for interfacing trunking communication systems of various types with the PSTN. Such an interface allows pairing trunking communication systems and PSTN via digital channels (2.048 Mbit s), three-wire connecting lines with ten-day dialing, as well as via four-wire PM channels with signaling systems of various types when interfaced with private telephone networks.

Connection with the PSTN is traditional for TSN, but recently the number of applications involving PD has been increasing, and therefore the presence of an interface to the SCP also becomes mandatory.

The maintenance and operation terminal (TOE terminal) is located, as a rule, at the base station of a single-zone network. The terminal is designed to monitor the state of the system, diagnose faults, record billing information, and make changes to the subscriber database. The vast majority of manufactured and developed trunking systems have the ability to remotely connect the TOE terminal via the PSTN or SCP.

Dispatching console. Optional, but very characteristic elements of the infrastructure of a trunking system are dispatch consoles. The fact is that trunking systems are used primarily by those consumers whose work is not complete without a dispatcher. These are law enforcement services, emergency medical services, fire protection, transport companies, municipal services.

Dispatch consoles can be included in the system via subscriber radio channels, or connected via dedicated lines directly to the base station switch. It should be noted that several independent communication networks can be organized within one trunking system, each of which can have its own dispatcher console. Users of each of these networks will not notice the work of their neighbors, and, no less important, will not be able to interfere with the work of other networks.

The subscriber equipment of trunking systems includes a wide range of devices. As a rule, half-duplex radio stations are the most numerous, because. they are the most suitable for working in closed groups. For the most part, these are radios with a limited number of functions that do not have a numeric keypad. Their users, as a rule, have the ability to communicate only with subscribers within their workgroup, as well as send emergency calls to the dispatcher. However, this is quite enough for most consumers of communication services of trunking systems. There are also half-duplex radio stations with a wide range of functions and a numeric keypad, but they, being somewhat more expensive, are intended for a narrower privileged circle of subscribers.

Trunked systems, especially those designed for commercial use, also use duplex radios, which are more like cell phones, but have much more functionality than the latter. Duplex radios of trunked systems provide users with a full connection to the PSTN. As for group work in the radio network, it is performed in half-duplex mode. In corporate trunking networks, duplex radios are used primarily for senior management personnel.

Both half-duplex and full-duplex trunked radios are available not only in portable, but also in automotive versions. As a rule, the output power of car radio transmitters is 3-5 times higher than that of portable radios.

A relatively new class of devices for trunking systems are data terminals. In analog trunking systems, data terminals are specialized radio modems that support the appropriate radio interface protocol. For digital systems, it is more typical to embed a data interface in subscriber radio stations of various classes. The composition of the automobile data transmission terminal sometimes includes a satellite navigation receiver of the GPS (Global Positioning System), designed to determine the current coordinates and then transfer them to the dispatcher on the console.

Trunking systems also use fixed radio stations, mainly for connecting dispatcher consoles. The output power of fixed radio transmitters is approximately the same as that of car radios.

Multizone systems

Early standards for trunked systems did not provide for any mechanisms for the interaction of different service areas. Meanwhile, customer requirements have increased significantly, and although equipment for single-zone systems is still being manufactured and successfully sold, all newly developed trunked systems and standards are multi-zone.

The architecture of multi-zone trunking systems can be based on two different principles. In the event that the determining factor is the cost of equipment, distributed inter-zone switching is used. The structure of such a system is shown in fig. 2. Each base station in such a system has its own connection to the PSTN. This is already quite enough to organize a multi-zone system - if necessary, a call from one zone to another is made through the PSTN interface, including the dialing procedure. In addition, base stations can be directly connected using physical leased communication lines (most often, small-channel radio-relay lines are used).

Each BS in such a system has its own connection to the PSTN. If it is necessary to call from one area to another, it is made through the PSTN interface, including the dialing procedure. In addition, BSs can be directly connected using physical leased lines.

The use of distributed inter-zone switching is reasonable only for systems with a small number of zones and with low requirements for the efficiency of inter-zone calls (especially in the case of connection via switched PSTN channels). High QoS systems use a CC architecture. The structure of a multizone TSS with a CC is shown in Fig. . 68.

The main element of this scheme is the interzonal switch. It handles all kinds of inter-zone calls, ie. all interzonal traffic passes through one switch connected to the BS via leased lines. This provides fast call processing, the ability to connect centralized DPs. Information about the location of the subscribers of the system with the Central Committee is stored in a single place, so it is easier to protect it. In addition, the interzone switch also performs the functions of a centralized interface to the PSTN and SCP, which allows, if necessary, to fully control both the voice traffic of the TS and the traffic of all PD applications associated with external SCPs, such as the Internet. Thus, the system with the CC has a higher controllability.

Figure 68 Structural diagram of a trunking network with distributed inter-area switching

Figure 69 Structural diagram of a trunking network with centralized inter-zone switching

So, we can single out several important architectural features inherent in trunking systems.

First, it is a limited (and therefore inexpensive) infrastructure. In multi-zone trunking systems, it is more developed, but still cannot be compared with the power of cellular network infrastructure.

Secondly, this is a large spatial coverage of base station service areas, which is explained by the need to maintain group work over vast territories and the requirements to minimize the cost of the system. In cellular networks, where investments in infrastructure quickly pay off and traffic is constantly growing, base stations are placed more and more densely, and the radius of coverage areas (cells) decreases. When deploying trunked systems, things are a little different - the amount of funding is usually limited, and to achieve a high return on investment, it is necessary to serve as large an area as possible with one set of base station equipment.

Thirdly, a wide range of subscriber equipment allows trunking systems to cover almost the entire range of needs of a corporate consumer in mobile communications. The ability to service dissimilar devices in a single system is another way to minimize costs.

Fourthly, trunking systems allow organizing independent dedicated communication networks (or, as they say in recent times, private virtual networks) on the basis of their channels. This means that multiple organizations can work together to deploy a single system instead of installing separate systems. At the same time, a tangible saving of the radio frequency resource is achieved, as well as a reduction in the cost of infrastructure.

All of the above indicates the strength of the position of trunking systems in the corporate sector of the market of systems and mobile communications.

Classification of trunking systems

The following features can be used to classify trunking communication systems.

Voice information transmission method

According to the method of voice information transmission, trunking systems are divided into analog and digital. Speech transmission in the radio channel of analog systems is carried out using frequency modulation, and the frequency grid step is usually 12.5 kHz or 25 kHz.

To transmit speech in digital systems, various types of vocoders are used that convert an analog speech signal into a digital stream at a rate of no more than 4.8 Kbps.

Number of zones

Depending on the number of base stations and the overall architecture, single-zone and multi-zone systems are distinguished. The former have only one base station, the latter - several base stations with the possibility of roaming.

Method of combining base stations in multi-zone systems

Base stations in trunked systems can be combined using a single switch (centralized switching systems), as well as connected to each other directly or through public networks (distributed switching systems).

Multiple access type

The vast majority of trunked systems, including digital systems, use frequency division multiple access (FDMA). For FDMA systems, the relationship "one carrier - one channel" is valid.

Single-zone TETRA systems use Time Division Multiple Access (TDMA). At the same time, TETRA multizone systems use a combination of FDMA and TDMA.

Channel search and assignment method

According to the method of searching and assigning a channel, systems with decentralized and centralized control are distinguished.

In systems with decentralized control, the procedure for searching for a free channel is performed by subscriber radio stations. In these systems, the base station repeaters are usually not connected to each other and operate independently. A feature of systems with decentralized control is a relatively long time for establishing a connection between subscribers, which grows with an increase in the number of repeaters. This dependence is caused by the fact that subscriber radio stations are forced to continuously sequentially scan channels in search of a ringing signal (the latter can come from any repeater) or a free channel (if the subscriber himself sends a call). The most characteristic representatives of this class are the SmarTrunk protocol systems.

In systems with centralized control, the search and assignment of a free channel is performed at the base station. To ensure the normal functioning of such systems, two types of channels are organized: working (Traffic Channels) and a control channel (Control Channel). All communication requests are sent over the control channel. On the same channel, the base station notifies the subscriber devices about the assignment of the working channel, the rejection of the request, or the queuing of the request.

Control channel type

In all trunked systems, the control channels are digital. There are systems with a dedicated frequency control channel and systems with a distributed control channel. In systems of the first type, data transmission in the control channel is carried out at a speed of up to 9.6 Kbps, and protocols such as ALOHA are used to resolve conflicts.

All MPT1327 protocol trunking systems, Motorola systems (Startsite, Smartnet, Smartzone), Ericsson EDACS and some others have a dedicated control channel.

In systems with a distributed control channel, information about the state of the system and incoming calls is distributed among low-speed data sub-channels, co-located with all working channels. Thus, in each frequency channel of the system, not only speech is transmitted, but also data of the control channel. To organize such a partial channel in analog systems, a subtonal frequency range of 0 - 300 Hz is usually used. The most characteristic representatives of this class are LTR protocol systems.

Channel hold method

Trunking systems allow subscribers to hold the communication channel throughout the conversation, or only for the duration of the transfer. The first method, also called message trunking (Message Trunking), is the most traditional for communication systems, and is necessarily used in all cases of duplex communication or connection with the PSTN.

The second method, which involves holding the channel only for the duration of the transmission, is called transmission trunking. It can only be implemented using half-duplex radios. In the latter, the transmitter is turned on only for the time the subscriber pronounces the phrases of the conversation. In the pauses between the end of the phrases of one subscriber and the beginning of the response phrases of the other, the transmitters of both radio stations are turned off. Some trunking systems make effective use of such pauses, releasing the working channel immediately after the end of the user's radio station transmitter. The working channel will be re-assigned for the response cue, and replicas of the same conversation will most likely be transmitted on different channels.

The trade-off for some improvement in overall system efficiency with transmission trunking is a reduction in the comfort of conversations, especially during busy hours. The working channels for continuing the started conversation during such periods will be provided with a delay of up to several seconds, which will lead to fragmentation and fragmentation of the conversation.

IDEN: one for all...

The idea of ​​an integrated system that combines the capabilities of group and dispatch radio communications, mobile telephone communications, as well as the transmission of alphanumeric messages (paging) and wireless data transmission, has been embodied in iDEN technology.

Foreword

VimpelCom JSC, operator of the BeeLine network, announced the introduction of iDEN/GSM international interstandard roaming. The new technology will significantly expand the possibilities of providing roaming services to BeeLine subscribers around the world. Starting from mid-September 2000, the "Beeline GSM" network started providing international automatic roaming services in Argentina, Peru, the Arab Emirates and Turkmenistan. The total number of countries where this service has become available to BeeLine GSM subscribers has reached 87. Roaming operates with 171 operators. In the near future it is planned to introduce roaming in Brazil, Mexico and Macedonia. Thus, the company came out on top among operators in the Moscow region in terms of the number of countries with which roaming agreements are in effect. A noticeable increase in the number of new countries where this service became available to Bee Line GSM subscribers is associated with the launch of iDEN/GSM international interstandard roaming between Bee Line GSM networks and the US operator Nextel, which owns iDEN standard networks in several regions of the world. In a number of countries, such as Argentina, Brazil, Peru, Mexico, GSM networks are either not widespread or are completely absent. At the same time, for example, networks built on the basis of iDEN digital trunking technology have been widely developed in Latin America.

At present, three different areas of application of mobile radio communication systems have been clearly identified: these are Public Safety systems (police, fire brigade, ambulance, etc.), private, i.e. systems owned and operated by organizations (PMR, Private Mobile Radio), and commercial public systems (SMR, Shared Mobile Radio).
The systems of the first type are characterized by a relatively small number of subscribers (no more than 500-1000), increased requirements for ensuring reliability and confidentiality, and the presence of special functions such as Emergency Call. A distinctive feature of such systems is the high cost of subscriber terminals, which in digital systems can reach $4,000. The Public Safety/PMR category includes the Smartnet/Smartzone, EDACS and APCO25 standards, as well as the European digital standard TETRA currently under development.
Commercial systems of the SMR type are characterized by a much larger capacity (the number of subscribers of digital systems can reach tens of thousands), the possibility of providing additional information services, as well as the moderate cost of subscriber terminals. The SMR category includes the MPT1327, LTR/ESAS and GeoNet standards.
It should be noted that most of the existing analog SMR systems have limited capabilities for frequency reuse and channel switching, automatic identification of subscribers when they move from one site to another, etc., and also do not provide full mobile telephony services at the modern level. .

Mobile telephone (cellular) communication

Mobile telephony is designed primarily to provide one-to-one personal mobile voice communication in duplex mode. Cellular communication technologies have gone through approximately the same development path as trunking systems. The first generation of cellular technology, which appeared in the early 1980s, was based on analog standards. The most widespread in the world are the North American AMPS standard, the British TACS and the Scandinavian NMT-450. All analog cellular standards provide good voice quality. Their main disadvantage, as with analog trunked systems, is their limited capacity. In addition, in analog cellular systems, the problem of unauthorized access to the system also remains.
In the early 90s, the transition to digital cellular communication standards began everywhere. The most widespread in the world is the Western European GSM standard, which is currently adopted in more than a hundred countries. Among other digital standards, D-AMPS has gained wide recognition, and in recent years, CDMA has become increasingly popular. It should be noted that in mobile telephony, the use of digital technologies does not always provide higher sound quality compared to analog systems. For example, it is generally accepted that the sound quality in GSM systems is somewhat lower than in analog systems. The main advantages of digital standards for mobile telephony are the large capacity of the system, complete confidentiality of negotiations and resistance to various types of radio interference.
Both digital and most analog mobile telephony standards also provide text messaging and data capabilities.
So, mobile radio communications and cellular communications are focused on providing, respectively, group half-duplex and personal duplex mobile communications (Fig. 1). However, with the development of digital technologies by the mid-90s, it became obvious that these two initially different types of mobile voice communications have a lot in common in terms of the territorial organization of the system, infrastructure, access to the PSTN (public switched telephone network), etc. At the same time, existing SMR technologies were not able to provide the same level of service that mobile telephone systems provided.

In this regard, the idea arose to develop an integrated system that combines the capabilities of group and dispatch radio communications, mobile telephone communications, as well as the transmission of alphanumeric messages (paging) and wireless data transmission. The proposed system was supposed to provide a modern level of service for all types of communication. It was this idea that was implemented in the iDEN (integrated Digital Enhanced Network) technology developed by Motorola in the mid-90s.

Integrated Services

iDEN technology is an integrated system (Fig. 2) that provides users with all major types of mobile communications

Rice. 2. Geographic structure of groups in iDEN

Mobile radio

iDEN provides features that are typical for modern digital trunked radio systems, namely:

• group call - a system subscriber (both a mobile and a dispatcher located in the office) can make a group call in the half-duplex communication mode. The call is made with one button press, and the connection establishment time does not exceed 0.5 sec. It is important to note that with such a call, only one voice communication channel is used in each cell of the system, regardless of the number of subscribers in the group. The number of possible groups in iDEN is practically unlimited, more precisely, it is equal to 2550000, which eliminates the need to have a dynamic group reconfiguration function in the system. All possible group configurations can be programmed in advance, and if necessary, subscribers simply move to the appropriate groups. Group members can be in different cities at a distance of tens and hundreds of kilometers (of course, within the system coverage area) and talk in group call mode as if they were on neighboring streets;
• private call - also a call in half-duplex mode, however, only two subscribers participate in the conversation, while ensuring complete secrecy of the negotiations. Both in group mode and in individual call mode, the name (or digital identifier) ​​of the caller appears on the display of the subscriber terminal of the called subscriber;
• call alert - used when it is necessary to call a subscriber (or a group of subscribers) who is either talking in mobile phone mode, or is outside the system coverage area, or has turned off his subscriber terminal. The call is stored in the system, and at the moment when the subscriber becomes available, he receives a sound signal, and the caller ID appears on the terminal screen. At the same time, the caller receives an acknowledgment of the receipt of the call.

Mobile telephony

The iDEN system provides all the features of modern mobile telephony systems: subscribers can call both other mobile phones and fixed PSTN phones, as well as receive calls from both. Telephone communication is full duplex. iDEN has features such as storing up to 100 numbers in the phone's memory and call by name, automatic dialing, standby mode, various call forwarding modes, caller identification. The following restrictions can be imposed on the subscriber: only incoming calls, only local calls, barring international calls, limiting the time of calls. The system also has voice mail.

Sending text messages

Subscribers of the iDEN system can receive alphanumeric messages, which then appear on the screen of the subscriber terminal. iDEN terminals can store up to 16 messages of 140 characters each. Messages are transmitted as in a conventional paging system: either through an operator or from a computer. The message can be sent both to one subscriber and to a group of subscribers.

Data transfer

iDEN portable terminals have built-in modems and can be connected to portable computers using an RS-232C adapter (serial interface). There is no need to have a modem in the computer. The circuit switching mode provides fax and data transmission speeds up to 9600 bps, and in packet mode - up to 32 kbps when using the entire frequency channel for data transmission. The advanced error correction scheme ensures reliable data transmission. The data transfer function allows mobile subscribers to receive and send faxes and e-mails, exchange data with computers in the office and work with the Internet. Packet data transfer mode supports TCP/IP protocol.
Note that adding data services to an existing iDEN system does not require the installation of any additional equipment at the base stations. It is only necessary to install additional blocks of the central infrastructure and install the software at the base stations and in the central infrastructure.

Air interface and voice coding in iDEN

The iDEN system is based on TDMA (Time Division Multiple Access) technology. In each frequency channel with a width of 25 kHz, 6 voice signals are transmitted simultaneously (Fig. 3). The transmission of a digitized voice signal both in radio mode and in cell phone mode is carried out as follows. Within the time interval of 90 ms, 6 time slots with a duration of 15 ms are allocated. In each of these slots, one voice signal is transmitted (it does not matter what type of communication it belongs to), and every sixteenth time slot is reserved for control signals. Due to the use of radio signal modulation using the M16-QAM (Quadrature Amplitude Modulation) method, a total data transfer rate over one frequency channel of 64 kbps is provided. In this case, the transmission rate of one voice signal is 7.2 kbps.
Adequate reproduction of the human voice and other sounds at such a low bit rate of the digitized voice signal is achieved through the use of an improved voice signal coding scheme based on the use of the VSELP algorithm. Without going into technical details, we note that the combination of TDMA / VSELP technologies provides higher voice quality than the GSM standard, and no worse than the latest versions of the CDMA standard.

Used spectrum

When developing the iDEN technology, the task was initially set to achieve the most efficient use of the frequency resource, at least not inferior to the existing implementations of the CDMA standard.
As already mentioned, the iDEN system provides the transmission of 6 voice signals simultaneously in each frequency channel with a width of 25 kHz. As a result, 240 voice channels can be accommodated in a 1 MHz spectrum. For comparison, we point out that analog and digital trunking communication systems provide no more than 80 voice channels per 1 MHz, analog cellular communication systems - 30-40 voice channels, the GSM standard - 40 voice channels (Fig. 4).

Rice. 4. Spectrum efficiency

The iDEN system operates in the standard American and Asian trunking band 806-825/851-870 MHz. Part of this range is reserved for trunked radio communication systems in the CIS countries.
Note that the iDEN system does not require all frequency channels to be contiguous.

IDEN Micro Lite

By mid-1999, Motorola plans to complete development of the iDEN MicroLite system, which is a smaller version of iDEN designed to serve systems with a few hundred to 5,000 subscribers. The maximum number of base stations is 8.
The iDEN MicroLite system is based on the same technological solutions as the iDEN system: the same M16-QAM radio signal modulation scheme, VSELP vocoder, TDMA voice channel time division technology with 6 voice channels in one 25 kHz frequency channel.
Subscriber terminals and base stations iDEN MicroLite and iDEN are identical.
The main technological difference between iDEN MicroLite and iDEN is the organization of the central infrastructure of the system. In the iDEN MicroLite system, the entire central infrastructure is implemented on two computers, one of which performs the DAP functions (see iDEN System Architecture), and the other - all other components of the central infrastructure (including the switch). The initial version of iDEN MicroLite provides two types of communication - group and individual radio communication and mobile telephone communication. Future versions will also support the transmission of short messages and data.
It should be noted that as the number of subscribers grows beyond the maximum capacity of iDEN MicroLite, it becomes necessary to migrate to a full iDEN system. With such a transition, it is necessary to install the central infrastructure of the iDEN system, however, subscriber terminals and existing base station equipment can be used with the necessary software modification.
Shipments of the iDEN MicroLite system will begin in the second half of 1999.

Subscriber equipment

All subscriber equipment for the iDEN system is represented by Motorola devices. Among them, there are both car and portable terminals that are in no way inferior to their counterparts from cellular communications: a multi-level menu, ringtone settings, a notebook, multilingual support and much more make them just as easy to use. etc. In 2000 released a model representing a new generation of iDEN devices - Motorola Timeport i2000. This device works not only in the "native" iDEN network, but also in GSM-900. Thus, the subscriber is provided with a wider range of services and can take advantage of both networks. For more information about iDEN subscriber units, please visit: http://www.motorola.com/LMPS/iDEN/product_features/phone.html

Areas of use

As already mentioned, iDEN refers to SMR (Shared Mobile Radio) type systems, that is, it is focused on the creation of commercial systems that provide integrated mobile communication service packages, both, first of all, to organizations and individuals. The system is primarily intended for use by organizations of various profiles and sizes interested in providing reliable mobile communication between individual departments and groups of employees.
For each corporate user of the system, a so-called “fleet” is created, which is nothing more than a virtual private network of this organization. One system can have up to 10,000 fleets, each fleet can have a maximum of 65-500 subscribers. Different groups can be created within the fleet, corresponding to the divisions of this company, the maximum possible number of groups in one fleet is 255. The possibility of both accidental and deliberate unauthorized intrusion of subscribers into other fleets is absolutely excluded. Fleet members can be in different cities, move from one city to another, and at the same time use all the services of both group and personal communications. As a result, the organization has at its disposal a mobile telecommunications system that is completely equivalent to the organization's own network. At the same time, she does not need to purchase equipment and build antennas, and spend several months installing and debugging the system. All that needs to be done is to become a corporate user of an existing iDEN system and configure the required groups.
We emphasize that the integrated services provided by the iDEN system cover almost the entire range of needs of companies and organizations in mobile communications. Communication within departments (groups) and between them is carried out in the radio mode, and for communication with external organizations (suppliers, customers) the mobile phone mode is used. Thus, firstly, instant individual and group communication within the organization is provided and, secondly, the cost of mobile communications is significantly reduced compared to the option when mobile phones are used for communication within organizations.
It is also possible to create combined private-commercial systems based on iDEN. In this case, the organization first creates a network for its internal needs, and then, due to excess capacity, begins to provide commercial communication services. There are also no problems with secrecy and confidentiality.
Due to the modular principle of the system organization, it is possible to create various implementations of it depending on the needs of the client. For example, iDEN can initially be deployed as a purely trunked system, and then mobile telephony, text messaging, and data capabilities can be added as needed.

Recognition of iDEN in the world

The first commercial system based on iDEN technology was deployed in the US by NEXTEL in mid-1994, and the network is now nationwide. It has about 5,500 sites and as of December 1998 had about 2.7 million subscribers. There is another network in the southwestern US, operated by the energy company Southern Co. In Canada, iDEN is operated by Clearnet. In Latin America, iDEN networks are available in Colombia and Argentina (two systems), with networks recently commercialized in Sao Paulo and Rio de Janeiro (Brazil) and Mexico City (Mexico). In addition, in 1998 iDEN deployments began in Peru, Chile and Venezuela.
In Asia, the iDEN system has been operating in Tokyo and Osaka (Japan) for more than two years, in April 1997 the system was launched in Singapore, there are such systems in China, South Korea and the Philippines. Three more systems are under construction in China and one in Indonesia. In the Middle East, there is a nationwide iDEN network in Israel.
Each of the above systems is designed to serve tens of thousands of subscribers. The total number of iDEN system subscribers in the world at the end of 1998 reached three million. iDEN is an open architecture. Motorola licenses the production of iDEN system components to various manufacturers.
Summing up, we note that today iDEN is the only commercially proven technology that provides the entire range of mobile services.

The article was prepared using materials from the site www.trunk.ru

Phil Petersen
Director for Europe and the Middle East
Motorola's iDEN group,

Andrey Denisov
Regional Manager for Eastern Europe and the former USSR
Motorola's iDEN group

Glossary

BCS (Base Site Controller) - base station controller;
DACS (Digital Access Crossconnect Switch)- digital channel switch;
DAP (Dispatch Application Processor)- trunking communication processor;
EBTS (Enhanced Base Transceiver System)- improved base station;
HLR/VLR (Home/Vehicle Location Register)- subscriber location recorder;
IWF (Interworking Function) - data/fax interface;
MDG (Mobile Data Gateway) - packet data gateway;
MPS (Metro Packet Switch) - packet switch;
MSC (Mobile Switching Center) - telephone switch;
OMC (Operations Maintenance Center)- control center;
SMS (Short Message Service) - text messaging system;
VMS (Voice Mail System) - voice mail;
XCDR Transcoder - block for converting speech packets from VSELP format to PCM format and vice versa