Trunking system

Trunking systems(English) trunking- bundling) - radial-area communication systems that automatically distribute communication channels between subscribers. The term “trunking” refers to a method of subscriber access to a common dedicated channel bundle, in which a free channel is allocated to the subscriber for the duration of the communication session.

Includes ground infrastructure (fixed equipment) and subscriber stations. The main element of the ground infrastructure of a trunking radio communication network is base station(BS), which includes several repeaters with the corresponding antenna equipment and a controller that controls the operation of the BS, switches the repeater channels, and provides access to the public telephone network (PSTN) or other fixed-line network. A trunked radio communication network may contain one BS (single-zone network) or several base stations (multi-zone network). A multi-zone network usually contains an inter-zone switch connected to all BSs via dedicated lines, which processes all types of inter-zone calls.

Modern trunking systems, as a rule, provide Various types calls (group, individual, broadcast), allow priority calls, provide the ability to transfer data and direct communication mode between subscriber stations (without using the BS channel).

Classification

analog (SmarTrunk II, Smartlink, EDACS, LTR, MPT 1327)
digital (EDACS, APCO-25, TETRA, TETRAPOL)

To organize access to the system:

without control channel (SmarTrunk II)
with distributed control channel (LTR, Smartlink)
with dedicated control channel (MPT 1327)

By channel retention method:

with channel retention for the entire conversation session (SmarTrunk II, MPT 1327)
with channel holding for the duration of one transmission (LTR, Smartlink)

According to the radio network configuration:

single-zone systems (SmarTrunk I)
multi-zone systems (MPT 1327, LTR, Smartlink, TETRA, APCO-25, EDACS, TETRAPOL)

According to the method of organizing the radio channel:

half-duplex (SmarTrunk II, MPT 1327, LTR, Smartlink, TETRA, APCO-25, TETRAPOL)
duplex (TETRA, APCO-25, TETRAPOL)

Trunk connection

In the modern business world, more and more attention is paid to mobile communications: pagers, cellular and satellite communications devices, personal communicators and similar devices. Indeed, in order to be competitive, modern companies need to constantly maintain communication with their customers, and, just as importantly, between the employees of their organization. Recently, some mobile operators have been offering so-called “corporate” tariffs (for example, the MTS corporate program), which are specifically designed to create a “virtual telephone network” for company employees. However, such programs are not the cheapest solution to the communication problem, but, fortunately, not the only possible one.

For a company that decides to “connect” its mobile employees, there is an alternative solution - the use of trunk communication. Perhaps many readers are seeing the phrase “trunk connection” for the first time. Indeed, trunk communication systems are now receiving less attention than even paging systems. To some extent, this is due to the fact that trunk communication systems are intended primarily for use by large organizations, and not by mass users. Despite this, this technology has its merits and deserves to be considered in this article.

So, what is hidden behind the term “trunk system”? Paradoxically, we use it every day without even thinking about it. It is on the principle of trunking that the operation of modern automatic telephone exchanges is based. Let's see what happens when you try to call, say, your friend from your home phone. You pick up the phone, wait for the "line free" signal, then dial the number and wait for an answer. All other actions are performed by the PBX: it selects one of the free communication channels and switches (links) your telephone set with the telephone set of a friend. At the end of the conversation, the line that was used is released and becomes available for use by other people. As you might guess, the number of communication lines is limited and is certainly less than necessary to connect all telephone sets in the city. Thus, the PBX controls the distribution of a limited number of lines among a large number of subscribers. It is assumed that a situation where all subscribers suddenly decide to contact each other at the same time will not arise. Therefore, it is necessary to correctly calculate the minimum required number of communication channels so that problems associated with their shortage do not arise during the work process. This issue is effectively solved using the mathematical theory of queuing systems.

Rice. 1. In trunk telephony, the subscriber simply dials a number, and the PBX allocates a free line through which a conversation can be carried out.

What is a trunked radio system?

Trunk radio systems are mobile radio communication systems that are based on the same principles as conventional telephone networks. In other words, in a trunk radio communication system there is a limited number of radio channels (usually from two to twenty), which are allocated by the central controller for negotiations as needed.

Fig.2. In trunked radio systems, the subscriber requests permission to talk, and a central controller (consisting of several repeaters) allocates a channel on which the conversation can be carried out.

In conventional radio communication systems, the user has to manually retune to a free radio channel; in trunk communication systems, this work is undertaken by the central controller, which itself allocates a free channel to two radio stations. Thus, the user just needs to dial the number of the called subscriber, and the system will do the rest itself. The trunk system can be defined as follows: Automatic and dynamic distribution does not large number channels among a large number of radio users.

Fig.3. Load diagram of a five-channel trunk system. The bottom graph shows cases of call blocking when all five channels of the system are busy.

Areas of application of trunk radio communication systems.

Now, knowing the basic principles of operation of trunk systems, let's talk a little about their areas of application and the advantages of using them. Areas of application - large commercial and government organizations, for example, traffic inspection services, various repair services, companies specializing in the field of industrial mountaineering (maintenance of high-rise buildings) and so on. A trunk communication system can be deployed both in a large city and in a remote, sparsely populated area, which is especially important in the conditions of our country. Trunk systems effectively use the frequency band allocated to them and provide high level confidentiality (there are even tools that allow you to encode speech during its transmission), reliable, provide a large number of service functions. Finally, perhaps their greatest advantage is that the organization itself can become the owner of a trunk radio communication system, freeing itself from subscription fees and traffic fees.

Types of trunk radio communications.

It's time to understand the types of trunk communication. Various companies and organizations have developed a huge number of trunk communication formats, many of which are incompatible with each other. In the USA, the most popular formats are Privacy Plus, developed by Motorola, Logic Trunked Radio - LTR, manufactured by E.F. Johnson, as well as SmarTrunk II from SmarTrunk Systems, formerly known as Selectone. Also noteworthy is Motorola's iDEN project, which offers a digital trunk communication format. In Europe, the MPT1327 standard, developed in England for public radio networks, has become widespread. Now this standard has become popular in Asia, Australia, and Latin American countries. Currently, work is underway in Europe to create a new European protocol for digital trunk systems - TETRA (Trans European Trunked Radio).

In Russia, the most well-known protocols are SmarTrunk II, MPT1327, LTR.

If we classify trunk systems according to the number of subscribers, we can distinguish three groups:

small, in which the number of subscribers does not exceed 300 people. When building such systems, the SmarTrunk II protocol is used;
medium-sized, the number of subscribers does not exceed 3,000 people. Most often, when creating such systems, the LTR standard is used;
large, with a number of subscribers exceeding 3000 people. In this case, the MPT 1327 protocol is most often used.

MPT1327 and TETRA are classified as open protocols, while LTR, SmartNet, etc. - to the class of closed, “branded” ones, however, both of them work according to two basic principles, which we will consider in the next part of the article.

Comparison of trunking methods.

Currently, there are two methods for managing trunk systems. The first is distributed control, the second is control over a dedicated channel.

The dedicated channel method has several disadvantages compared to the distributed control method. One of them is that when using a dedicated channel, all requests are carried out with its participation, therefore, it is necessary to somehow avoid collisions when transmitting data. Another disadvantage is that a system with a dedicated channel must process requests sequentially, and as the load increases and the number of available channels decreases, the number of requests grows exponentially, so that mobile devices are forced to fight each other for a single channel.

One of the advantages of the distributed control method is that access can be obtained via any currently free channel. Repeaters determine an open channel and transmit this information in a data stream that exists together with voice information. This means that each repeater maintains its own data flow and processes all requests on its own channel. Collision processing is performed by mobile devices, which ensures parallel processing of calls.

Another advantage of the distributed control method is that the voice data is transmitted over all channels, whereas in the dedicated channel method the control channel generally cannot be used in this way. The figure shows the locking speed of a five-channel system compared to the locking speed of a four-channel system (one channel is used for control). It can be seen that the blocking time of the five-channel system is significantly less.

Fig.4. Blocking time comparison.

Typically, in trunk systems, idle time (the time between two adjacent transmissions) is not used in negotiations. The channel is held only for the duration of the transmission, and the time between transmissions can be used by other people making calls. And only during telephone conversations the channel is held permanently.

Some trunking systems take advantage of downtime in negotiations during busy periods. This allows the called party to almost always answer the call without fear of being blocked. The obvious disadvantage of this approach is an increase in the total transmission time and, consequently, an increase in the probability of blocking and waiting time for other subscribers.

Access priority is a setting that determines who gets access to a busy system first. Most systems with a dedicated control channel use a method that allows all mobile devices to try to gain access, but denies the channel to devices with a lower priority level. In distributed control systems, all mobile devices have the same priorities, and no device can access the system while the channel is busy. When a channel becomes free, the device that first tries to occupy the line gets access to it.

Mobile devices used in trunk communication systems must be programmed to operate on a specific frequency (usually 800 or 900 MHz); many functions (eg channel selection, channel check before transmission) are performed automatically.

Each repeater can have up to 250 ID codes associated with it. The ID code and home repeater number form the address of the mobile device on the network. Thus, in a system containing 20 repeaters, the maximum number of subscribers is 5000. The ID code can be assigned to either one mobile device or several at once.

Block diagram of a base station for a trunk radio communication system.

Figure 5 shows a block diagram of a base station in the case of using one channel.

The repeater consists of a repeater designed to receive signals from subscriber radio stations, amplify and transmit them, and a trunk channel controller that performs control functions.

Duplex filter is a device that allows you to use one antenna for reception and transmission. In principle, nothing prevents you from using two different antennas for reception and transmission, but in this case a situation may arise when reception is possible in some places, but transmission is not possible, or vice versa. In addition, the power emitted by the transmitter affects the receiver, so if you have two antennas, they must be installed at a sufficient distance from each other.

The power supply is for the repeater. As a rule, it allows for the possibility of switching to battery when the power is turned off.

The considered scheme is quite simple and effective, but in real conditions one trunk channel is not enough. Therefore, systems containing two or more channels are used. The figure shows a diagram of a system containing four independent channels. As you can see, the main difference from the previous version is in the antenna-feeder path, where two more devices appear: the receiving distribution panel and the combiner.

The receive distribution panel provides the same input signal to each repeater in the system, as if the repeater were connected directly to the antenna.

A combiner is a device that allows you to combine the outputs of a certain number of transmitters without interference with each other.

The source is also included separately uninterruptible power supply, which simply must be present in the system, because the lack of communication in emergency circumstances can lead to unpredictable consequences.

The considered system is easy to expand, that is, if designed correctly, the number of channels can be increased quite painlessly.

Review of radiotelephone models.

Currently, equipment for base stations and subscriber devices for trunk radio communication systems are produced by a large number of companies. Of these, the most famous are Motorola, Nokia, Ericsson, SmarTrunk Systems and others. As an example, let's look at several models of radiotelephones produced by Nokia.

Nokia H85.

Nokia H85 is a lightweight (weighs only 345 g with battery, output power 1 W in duplex mode), convenient radiotelephone for use in MPT 1327 systems. The device has a large, high-contrast alphanumeric display (contains 3 lines of 10 characters each) with voltage level indicators fields and battery charge. Access to numerous functions and settings of the device is carried out using the menu. H85 supports individual and group calls, calls over the public telephone network. The radiotelephone's memory can store up to 99 names and numbers of subscribers. There is also one programmable button that can be assigned to either your most frequently dialed number or an emergency number.

This device comes with a wide range of accessories, including a cigarette lighter charger and a dashboard holder. There are two types chargers: tabletop and portable.

Nokia R40.

Nokia R40 is a universal half-duplex radio station for users of trunk systems (weight 1.8 kg, output power 10 (15) W). The radio complies with MPT 1327 and MPT 1343 specifications, and the R40 also supports the MAP 27 data interface.

The radio station can be used both in a car and in a desktop version. The CU 43 alphanumeric console has 22 keys and a three-line, 100-character LCD display and allows you to make all possible types of calls on the radio network. Additionally, the console allows you to receive and transmit status messages and data. The on-screen menu is used to control the station. You can store up to 43 names and numbers of subscribers in memory.

The CU 45 communicator has a built-in digital LCD display, microphone and loudspeaker. Control is carried out using four function keys.

Peripheral devices, such as a data modem, can be connected to the radio via the MAP 27 interface.

Nokia R72.

Nokia R72 is a radiotelephone for working in MPT 1327/1343 networks (weight 1.8 kg, output power 10 W in full-duplex mode and 15 W in half-duplex mode). In addition to voice communication, a radiotelephone provides the ability to transmit and receive encoded messages and data.

The phone is convenient to use in the car. When the charging cable is connected to the cigarette lighter socket, the battery is automatically charged. The phone has a memory for 97 names and numbers of subscribers, and also allows you to program up to nine speed dial numbers. In addition, the phone has a number of other features, including the transmission of tones to connect to telephone network equipment, the use of an encrypted sequence number (ESN) and lock codes to protect against unauthorized access.

You may have noticed that the R72 looks exactly the same as the famous Nokia 720 - a mobile phone for use in NMT 450 networks. And the names of these two devices show that they have a lot in common.

Conclusion.

Having defined the main purpose of trunk radio communications, reviewed and compared its standards, studied the principles of constructing a central controller and, finally, familiarized ourselves with some models of radio stations, we have gained a general understanding of what trunk radio communications systems are. It should be noted that at present they continue to actively develop, new standards and equipment are being developed. The number of trunk communication systems designed and put into operation is growing every year. They certainly have a future.

For those who are interested in the topic discussed, I provide links to some resources on the Internet dedicated to trunk communication issues. At the end of the article there is also a glossary of terms used when describing trunk radio communication systems.

Links.

http://members.dingoblue.net.au/~activemedia/trnklinks.htm - Collection of links to resources dedicated to trunk systems.

http://www.sotovik.ru/analit.htm - The library on Sotovik contains a very large amount of materials on mobile communications, including a section dedicated to trunk systems.

Glossary.

Base station- a group of repeaters connected to the same data bus and located in one place.

Home repeater- all radio stations in a trunk radio communication system have one of the repeaters located at the base station as a “home” one. The radio station monitors this repeater to receive calls and receive information about which repeaters are available.

Duplex- a mode in which you can speak and listen at the same time (that is, receive and transmit).

Controller (Central Controller)- a computer that ensures the joint operation of all repeaters. Each repeater contains a controller. They are connected to each other using a data bus.

Mobile device- a transceiver installed in a car or a portable radio station.

Repeater- a device that receives and relays a radio signal. If you are using a five-channel trunk system, you will need five repeaters. One repeater can handle only one conversation at a time.

Simplex- a mode in which either transmission or reception is possible.

Trunking- Automatic and dynamic distribution of a small number of channels among a large number of radio users.

Control channel- one of the radio channels that is used to communicate with everyone mobile devices and for distributing official information.

iDEN subscriber terminals, like the GSM system, use SIM cards. In terms of interconnection, GSM signaling control algorithms are used, which greatly simplifies roaming with cellular networks. Professional (industrial) terminals (R370, R470, R765, R765IS) and commercial “i” series are produced. There are dual mode models iDEN/GSM, iDEN/CDMA. Some terminals have the “Direct Connect” function, which allows you to connect network subscribers directly, bypassing base stations, in local areas in the SMR (Specialized Mobile Radio) frequency range of 800 MHz. Currently, two companies Motorola and RIM are engaged in the production of terminals. In 2010, the Android terminal with a touch screen i1 was presented.

It's a miracle - it works like a radio.

In 2005, a further development of the standard was introduced, which allows, by combining the time slots of four physical channels, to obtain a data transfer rate of up to 100 kBit/s. The upgrade is called WiDEN (Wideband Integrated Digital Enhanced Network).
As of 2010, along with systems

In almost every cellular communication store, the windows of which are bursting with mobile phones, there is a security guard with the obligatory bulky walkie-talkie. Here you involuntarily ask the question: “Why doesn’t this person use a simple mobile phone for service?”

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

Trunking mobile radio communication (from English. trunking- provision of free channels, trunk- trunk line) is a two-way mobile radio communication system that uses the ultrashort wave range. In practice, the PMR system is structured similarly to a cellular one: user terminals and base stations (BS), equipment to increase 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 communication systems.

Better than a cell phone?

How does trunking communication differ from cellular communication, if, apart from the difference between the user terminal (walkie-talkie/phone), everything is arranged the same?

Cellular communications are positioned as a “phone in your pocket,” while trunking is designed to solve a narrow range of professional tasks. Cellular communications, for example, provide a variety of multimedia services, but an oil worker on duty on a drilling platform in the Baltic Sea or a rescue worker from the Ministry of Emergency Situations are unlikely to rely on the opportunity to download Madonna’s new album. Trunking communications are chosen by organizations such as the Ministry of Emergency Situations, security agencies, taxi companies, etc. For ordinary office workers, the “cell phone + corporate tariff plan” option is quite suitable.

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

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

Ability to redistribute group members during a communication session;

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

Maintaining communication even if the base station fails;

Transmission of a broadcast signal to network subscribers;

Ability to quickly reconfigure the network.

These requirements are impossible to meet in cellular communication systems, but are fully supported by trunking systems. It is worth noting that mobile communications market participants do not sit idly by and offer the service Push-To-Talk with the ability to establish a group call and quickly establish a connection. 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 using the example of two versions of TETRA - a popular standard for digital trunked radio communications, 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 connection + + - Direct communication mode (without base station) + + - “Receive only” mode - + - Possibility of expanding the communication area - + - Zone selection + + - Status messages + + - Transmission of short text messages+ + + Calling the dispatcher + + - Providing a wide bandwidth upon the subscriber’s request + + - Signal and radio interface encryption capabilities + + +/- Simultaneous transmission of voice and data + + + High-speed data transmission - + + Selective listening of subscribers by the dispatcher + + - Remote listening acoustic environment + + - Dynamic regrouping + + - 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, coming 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 launches equipment production; Radio communication regulations are appearing, including on the distribution of frequencies between various services. Professional radio communications began in the period from 1915 to the 1950s.

In the first half of the 20th century, the possibilities of communication at different wavelengths were explored. Before 1920, communications were carried out using waves ranging in length from hundreds of meters to tens of kilometers. In 1922, the property of short waves to propagate over 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 became the time of meter waves; and the 1940s - decimeter and centimeter, spreading rectilinearly over 40-50 km within line of sight. The popularization of radio communications directly depended on technological advances. Before the advent of miniature semiconductors, receivers remained bulky and, at best, fit into a suitcase, which imposed certain limitations.

The history of professional radio networks is usually divided into stages. The first stage networks are considered to be of a conventional type (from the English. conventional- ordinary, traditional). Their limited capabilities are as follows: simplex operating mode (pressed a button - asked a question - released the button - received an answer - pressed the button - ...), making individual and group calls (up to several dozen subscribers) In conventional systems, the communication channel (frequency) is strictly assigned to a specific group of subscribers. This guarantees high communication efficiency (you only need to adjust the frequency), but causes low network throughput (there are few frequencies).

Second phase- trunking networks. Such networks made it possible to serve up to several hundred subscribers and allowed for more efficient use of radio frequency resources. Such communication systems have become systems with shared access subscribers to the frequency range, in contrast to conventional systems. This provides increased throughput and a large coverage area.

Multi-zone trunking networks third stage. The service area in them has increased even more due to several base stations. The number of subscribers served has become almost unlimited, a call priority system has appeared, the possibility of a duplex call mode (no 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, and 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 heading contains the names of the three basic wireless radio communication modes.

1. Simplex communication uses one frequency for reception and transmission. Only exchange of replicas is possible. Due to the limitations imposed by physics, you can use this most economical type of wireless radio communications at a distance of no more than 5 km. An open area is highly desirable for a stable signal. Communication is carried out through user terminals.

2. Half-duplex communication also uses two frequencies, but you will have to communicate as in simplex mode. A 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 a radio frequency switch. The half-duplex principle underlies low-cost networks that connect dozens of subscribers in different parts of the city and open areas.

3. Duplex communication uses two frequencies - one for reception, the other for transmission and is intended to conduct a familiar dialogue. Naturally, base stations are used to relay signals. Analog duplex systems require two channels (4 radio frequencies) to connect subscribers. The terminal is equipped with a large 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 - at every moment of time the subscriber’s device is receiving or transmitting. For example, in the TETRA standard, switching occurs 18 times per second.

Modern digital trunking networks (DH) are the top of the evolutionary chain of professional communication. In addition to the capabilities available to users of analog systems, reliable protection against unauthorized access is added (in addition, 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 essence, digital trunking systems are universal communication networks that ensure the confidentiality of subscriber contacts, and are capable of simultaneous transmission of large data streams over communication channels, be it telemetry data or video information (the latest editions of the standards provide for such capabilities).

There are a large number of different standards for trunked mobile radio communication systems, differing in many ways. In our country, as well as throughout the world, analog systems are still common different versions and standards. 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 are worth considering in more detail.

EDACS (Enhanced Digital Access Communication System)

Firm Ericsson(Sweden) before others (until it was bought Sony in the 1980s) became concerned about the problem of obsolescence of analog technologies and the insufficient degree of security of communications in such systems and began developing the corporate closed standard EDACS (Enhanced Digital Access Communication System). Initially, the standard provided for speech transmission via analog protocols, later the standard was modified and a digital version of the system appeared called EDACS Aegis. EDACS systems operate at frequencies of 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 due to its closed nature and rapid obsolescence (in fact, it is digital standard for transmitting analog signals). All rights belong to the developer, and you will not be allowed to release the equipment just like that. In addition, Ericsson has stopped supplying equipment to deploy new networks of this standard and is only 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 The first commercial application network was deployed based on this technology. Today, similar networks are deployed in many countries in North and North America and Asia. Today, iDEN has more than 3,000,000 subscribers (90% of them 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, and transmit data via TCP/IP at speeds of up to 36 kbit/s, 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 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 for which professional communication systems are designed. 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 at the request of the French gendarmerie. The Tetrapol standard communication network has been operating on half of France since 1994 and serves more than 15,000 subscribers. Tetrapol standard communication systems operate starting at a frequency of 70 MHz and have a performance ceiling of 520 MHz, which does not contribute to their popularity in other countries, where such systems may traditionally be allocated other frequency ranges. 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 Institute of Telecommunications Standards). TETRA stands for 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 wants to produce communications equipment does not have to worry about problems of compatibility with equipment from other companies and the division of copyrights. To produce products that support this standard, you must join the organization MOU TETRA- Memorandum on promotion of the TETRA standard. Nokia, Motorola, RohdeSchwarz and other large communications equipment companies support this standard. TETRA networks are deployed throughout almost all of Europe, Asia, Africa and South America. TETRA Release 2 - a new version standard, which allows for tight integration with mobile networks third generation and significantly increase data transfer speed. Network Deployment Project this standard in Russia it is called “Tetrarus”. The fact that “within the framework of the Federal Target Program “Development of Sochi as a mountain climatic resort until 2014” says a lot. TETRA radio communications will operate at the venues of sporting competitions and throughout the Krasnodar Territory.”

APCO Project 25 (APCO 25)

Open standard APCO 25 created by the organization AssociationofPublicSafetyCommunicationsOfficials-international-Association of representatives of public security communications services. 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 operation 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 united into one network. Since 2003, a similar network with several hundred subscribers has been operating in St. Petersburg for the purposes of the Russian Ministry of Internal Affairs.

Trunking can be used not only for communication:

Latest system trunking JRC Trunked Radio System with automatic vehicle location function based on GPS and MPT 1327/1343 standards. In addition to, in fact, ensuring communications between subscribers, the standard provides automatic transmission 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 reflected more fully 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 access to databases + + + + + Direct communication mode + + ? + + Automatic registration mobile subscribers+ + + + + Personal call + - + + + Access to IP networks + + + + + Transmission of status messages + + + + + Transmission of short messages + - + + + Transmission of subscriber location data from a GPS receiver ? + ? + + Fax + - + + + Is it possible to set up an open channel? - - + + Multiple access using a subscriber list + - + + + Signal relay mode + ? ? + + “Dual surveillance” mode ? - ? + + Access/call priority + + - + + Dynamic regrouping + + - + + Selective listening + + - + + Remote listening? - - + + Identification of the calling party + + - + + Call authorized by the dispatcher + + - + + Transfer of keys over the radio (OTAR) + - - + + Simulation of subscriber activity - - - - + Remote disconnection of the subscriber + ? - + + Subscriber authentication + ? - + +

In Russia, simultaneously with the introduction, successful use and development of digital networks of various trunking standards, analogue systems based on the old MRT1327. And this is by no means a bad thing. Digital trunking is convenient where not only operational communication is needed, but also data transfer and telephony. Often, customers find that simplex is quite sufficient voice communication and messaging functions. Using analog systems saves time and money.

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

Section 4 Mobile systems trunking communication

Lecture No. 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 1987 edition of the English-Russian Dictionary of Radio Electronics:

Trunk – connecting line; trunk communication line; link

Trunking (trunking) - group formation

Electronic dictionary"PROMT" 1999 is more "educated":

Trunking – provision of free channels

Trunked radio system - radio system with automatic channel redistribution

As can be seen from the translation, there is nothing special hidden behind the word “trunk”. Just “automatic channel provisioning”.

Trunk principles have been used for over 70 years in telephony. Any automatic telephone exchange, mini PBX, cellular uses trunking as the basis of its work. We all use trunking almost every day. 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 assign each telephone subscriber a separate line, especially long-distance. All of us are allocated a line to conduct a conversation only for the duration of the communication session. The rest of the time (free from our conversations) other users are served through it.

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

Now imagine that all telephone sets are 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.

A FEW EXPLANATIONS

Trunking systems do NOT regulate:

access to the telephone network;

using duplex (“talking and listening” at the same time, as in telephony);

enormous range;

highest service;

free access;

and much more...

They simply allow you to communicate with each other without thinking about technicalities and physical problems. You are talking - the equipment is working. It 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. When a subscriber requests, 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 phrases to the conscience of translators and linguists. In our opinion, the words “trunk” and “trunk systems” are more harmonious in pronunciation and easier to spell. As a rule, their use is not controversial. Therefore, in the future we will mainly use “our” formulations.

MYTHS AND REALITY

Ten thoughts to cool the ardor of optimists and raise the spirits of pessimists regarding the “miracles” of trunk communication:

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

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

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

Trunk systems serve to communicate between radios and more radios, rather than between radios and telephone lines.

Trunk systems can do a lot, but not everything.

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

If the trunk system does not solve the task, then it 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 - don’t pay twice.

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

But seriously, what are the advantages of trunk systems compared to traditional, so-called “regular” communication networks, with cellular telephony, with personal radio call systems (paging)?

It is quite difficult to answer this question unequivocally. Like 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 one network with minimal (compared to other radio systems) material costs.

ADVANTAGES OF TRUNK NETWORKS

Compared to cellular systems:

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

high speed of connection establishment (0.2–1 sec);

organization of queues to system resources when busy and automatic connection after access becomes possible;

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

lower costs for deploying and operating systems.

Compared to “conventional” radio communication systems:

saving frequency resources;

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

ability to transmit digital data;

communication coverage of large areas thanks to a multi-zone configuration.

Compared to personal radio calling (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 advantages. And yet 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, for some a pager is enough, and a number of users make do (and will make do) with “regular” communication systems.

It must be clearly understood that the trunk is not a universal solution to the entire set of radio communication problems. In any state, even the most “trunk” one, there still remains a number of problems that are solved by other communication systems that have nothing in common 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 interzonal communication lines (wired, radio frequency, radio relay, fiber optic) and, as a consequence, the complexity and cost of deployment*;

need for professional service.

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

CLASSIFICATION OF TRUNK SYSTEMS

Trunking systems can be classified according to many criteria, for example, by the format of transmitted data (analog, digital), by types of protocols (LTR, MPT 1327, SmarTrunk II), by the number of serviced zones (single or multi-zone), by 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 unified and generally accepted methodology in this area. We will try to characterize modern trunk systems, describe their capabilities, and note the most important points, which you should pay attention to when choosing.

Architecture of trunking systems

Trunking systems are radial-area land mobile radio communication systems that automatically distribute repeater communication channels between subscribers. This is a fairly general definition, but it contains a set of characteristics 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 "combining into a bundle."

Single zone systems

Figure 67 Block diagram of a single-zone trunking system

The basic architectural principles of trunking systems are easily visible 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 combining device, antennas), also includes a switch, a control device and interfaces of various external networks.

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

Antennas. The most important principle in building trunking 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 either a single transceiver antenna or separate antennas for reception and transmission. 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 simultaneous operation of receivers and transmitters on several frequency channels. Repeaters of trunking systems operate only in duplex mode, and the separation of reception and transmission frequencies (duplex separation), depending on the operating range, ranges from 3 MHz to 45 MHz.

The switch in a single-zone trunking 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 processes calls, performs caller authentication (friend or foe verification), maintains call queues and records in time-based billing databases. In some systems, the control device regulates the maximum permissible duration of the connection to the telephone network. Typically, two control options are used: reducing the duration of connections during predetermined peak hours, or adaptively changing the connection duration depending on the current load.

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

One of the main problems when registering and using trunking systems in Russia is the problem of interfacing them with the PSTN. When making outgoing calls to 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 PBXs. Thus, it is necessary to use an additional device for converting tone dialing to decade.

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

To overcome this situation, in the laboratory of the IVP company, together with specialists from the ELTA-R company, the ELTA 200 telephone interface (TI) was developed for interfacing trunk communication systems of various types with the PSTN. This interface allows you to interface trunking communication systems and PSTN via digital channels(2.048 Mbit s), three-wire connecting lines with ten-day dialing, as well as four-wire TC channels with various types of alarm systems when interfaced with departmental telephone networks.

Connection to the PSTN is traditional for TSS, but recently the number of applications that require PD has been increasing, and therefore the presence of an interface to the UPC is also becoming mandatory.

Terminal Maintenance and operation (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 tariff information, and make changes to the subscriber database. The vast majority of trunking systems produced and developed have the ability to remote connection TOE terminal via PSTN or SKP.

Dispatcher console. Optional, but very characteristic elements of the trunking system infrastructure are dispatch consoles. The fact is that trunking systems are used primarily by those consumers whose work cannot be done 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 within the framework of one trunking system several independent communication networks can be organized, each of which can have its own dispatch console. Users of each of these networks will not notice the work of their neighbors, and, equally important, will not be able to interfere with the work of other networks.

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

In trunking systems, especially those designed for commercial use, duplex radios are also used, more like Cell Phones, but with significantly greater functionality compared to the latter. Duplex radios of trunking systems provide users with a full connection to the PSTN. As for group work in a radio network, it is carried out in half-duplex mode. In corporate trunking networks, duplex radios are used primarily for senior management personnel.

Both half-duplex and full-duplex trunking radios are available not only in portable, but also in car versions. Typically, 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 integrate a data transmission interface into subscriber radio stations of various classes. A car data transmission terminal sometimes includes a satellite navigation receiver of the GPS (Global Positioning System) system, designed to determine current coordinates and subsequently transmit them to the dispatcher on the remote control.

In trunking systems, stationary radio stations are also used, mainly for connecting dispatch consoles. Transmitter output power stationary radio stations approximately the same as for car radios.

Multi-zone systems

Early trunking system standards did not provide any mechanisms for interaction between different service areas. Meanwhile, consumer demands have increased significantly, and although equipment for single-zone systems is still produced and successfully sold, all newly developed trunking systems and standards are multi-zone.

The architecture of multi-zone trunking systems can be built according to two different principles. In the event that the determining factor is the cost of equipment, distributed interzonal 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 it is necessary to call from one zone to another, it is made through the PSTN interface, including the dialing procedure telephone number. In addition, base stations can be directly connected using physical dedicated 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 make a call from one zone to another, it is made through the PSTN interface, including the procedure for dialing a telephone number. In addition, BSs can be directly connected using physical leased lines.

The use of distributed inter-zonal switching is advisable only for systems with a small number of zones and with low requirements for the speed of inter-zonal calls (especially in the case of connections via switched PSTN channels). On systems with high quality service uses an architecture with a central control center. The structure of a multi-zone TSS with a CC is shown in Fig. 68.

The main element of this scheme is the interzonal switch. It handles all types of inter-area calls i.e. all inter-zonal traffic passes through one switch connected to the BS via dedicated lines. This ensures fast call processing and the ability to connect centralized dispatch centers. Information about the location of subscribers of a centralized payment system is stored in a single place, so it is easier to protect. In addition, the interzonal switch also performs the functions of a centralized interface to the PSTN and UPC, which allows, if necessary, complete control of both the voice traffic of the vehicle and the traffic of all PD applications associated with external UPC, such as the Internet. Thus, a system with a central control system has higher controllability.

Figure 68 Block diagram of a trunking network with distributed interzonal switching

Figure 69 Block diagram of a trunking network with centralized interzonal switching

So, we can highlight several of the most important architectural features inherent in trunking systems.

First, there is 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 the large spatial coverage of base station service areas, which is explained by the need to support group work over large areas and the requirements to minimize system costs. 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 trunking systems, things are a little different - the amount of funding is usually limited, and to achieve high efficiency of capital investments, you need to serve as wide 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 corporate consumer needs in mobile communications. The ability to service devices of different functional purposes in a single system is another way to minimize costs.

Fourthly, trunking systems allow you to organize independent dedicated communication networks (or, as they say lately, 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, significant savings in radio frequency resources are 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 for mobile communication systems and equipment.

Classification of trunking systems

To classify trunking communication systems, the following characteristics can be used.

Voice transmission method

Based on the method of transmitting voice information, trunking systems are divided into analog and digital. Speech transmission in the radio channel of analogue 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, converting an analog speech signal into a digital stream at a speed 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 have several BSs with the possibility of roaming.

Method for combining base stations in multi-zone systems

Base stations in trunking systems can be combined using a single switch (centralized switching systems), and also connect to each other directly or through public networks (distributed switching systems).

Multiple access type

The vast majority of trunking systems, including digital systems, use frequency division multiple access (FDMA). For FDMA systems, the ratio “one carrier - one channel” is valid.

Single-zone TETRA systems use time multiplex access (TDMA). At the same time, multi-zone TETRA systems use a combination of FDMA and TDMA.

Method for searching and assigning a channel

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

In systems with decentralized control, the search procedure 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 the relatively big time establishing a connection between subscribers, growing with the 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 calling signal (the latter can come from any repeater) or a free channel (if the subscriber himself sends the call). The most typical representatives of this class are SmarTrunk protocol systems.

In systems with centralized control, the search and assignment of a free channel is carried out at the base station. To ensure the normal functioning of such systems, two types of channels are organized: traffic channels and a control channel. All requests for communication are sent through the control channel. Using the same channel, the base station notifies subscriber devices about the assignment of a working channel, the rejection of a request, or the placement of a request in a queue.

Control channel type

In all trunked systems, 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 speeds of up to 9.6 Kbit/s, and ALOHA type protocols are used to resolve conflicts.

All MRT1327 protocol trunking systems, Motorola systems (Startsite, Smartnet, Smartzone), Ericsson EDACS system 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 transmission subchannels, combined with all working channels. Thus, in each frequency channel of the system, not only speech is transmitted, but also control channel data. To organize such a partial channel in analog systems Typically the subtonal frequency range 0 - 300 Hz is used. The most typical representatives of this class are LTR protocol systems.

Channel hold method

Trunking systems allow subscribers to hold a communication channel throughout the entire conversation, or only for the duration of the transmission. The first method, also called message trunking, is the most traditional for communication systems, and is necessarily used in all cases of using duplex communication or connection to 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 while the subscriber is speaking phrases of the conversation. During the pauses between the end of one subscriber's phrases and the beginning of the other's response phrases, the transmitters of both radio stations are turned off. Some trunking systems effectively use such pauses, freeing up the working channel immediately after the end of operation of the subscriber radio station's transmitter. For a response replica, the assignment of the working channel will be made again, and replicas of the same conversation will most likely be transmitted over different channels.

The price for a slight increase in the efficiency of using the system as a whole when using transmission trunking is a decrease in the comfort of negotiations, especially during busy hours. During such periods, working channels for continuing an ongoing conversation will be provided with a delay of up to several seconds, which will lead to fragmentation and fragmentation of the conversation.

Trunking communication. Trunking systems