History of Dect, description and application of the standard. Cordless DECT phone: technical specifications and reviews

History of the DECT standard

Back in the early 1980s, when cordless analog phones began to reach European markets from the Far East, engineers realized that telephony would become better thanks to the transition from an analogue to a digital standard.

By the end of 1987, two technologies emerged seeking to accomplish this task: English standard CT2 and Swedish CT3. But for the sake of unity, the European Telecommunications Standards Institute (ETSI) decided to create a single standard that would take the best of the previous two. So in 1992 a new one was officially published wireless standard– DECT ( English Digital Enhanced Cordless Telecommunications) - digital standard wireless communication. This radio access standard efficiently utilizes radio frequency bandwidth and opens up new wireless communications applications for the home, office and private local commercial areas.

DECT system architecture

Rice. 1 DECT system architecture

The controller is designed to interface the DECT system with external networks, for example, a city telephone exchange. In some cases, special devices are used for these purposes - protocol converters.

BS– the DECT base station provides the required radio coverage. The DECT BS is connected to the controller via one or two pairs of wires. It is a transceiver that provides simultaneous operation on 4 to 12 frequency channels, operating on two spatially separated antennas. DECT BS are available in two versions – for indoor and outdoor placement.

UD– the access devices are a DECT telephone or a fixed subscriber terminal, sometimes referred to as a “radio socket”.

Basic technical specifications DECT standard

Table 1 Main technical characteristics of the DECT standard

The DECT radio interface is based on the following technologies:

1) Multi Carrier – radio access using multiple carriers;

2) Time Division Multiple Accesses – the principle of multiple access with time division of channels;

3) Time Division Duplex - duplex communication with time division.

DECT technology uses 10 frequency channels (MC - Multi Carrier) in the range 1880-1900 MHz. The time spectrum for DECT is divided into time frames repeating every 10 ms. The frame consists of 24 time slots, each of which is individually accessible (TDMA - Time Division Multiple Access), the slots can be used either for transmission or reception. To facilitate implementations of the basic DECT standard, a 10 ms time frame is divided into two halves (TDD - Time Division Duplex); The first 12 time slots are used to transmit the fixed part (“downlink”), and the remaining 12 are used to transmit the wearable portion (“uplink”) (Fig. 2).

Rice. 2. Time frame DECT technology

Dynamic allocation and channel selection in DECT technology

Instead of frequency network scheduling, the Continuous Dynamic Channel Selection and Allocation (CDCS/CDCA) mechanism is used. The essence of this mechanism is that channels are selected dynamically from the entire set of channels based on the following indicators: signal transmission quality and interference level. Moreover, the channel is not assigned to the connection for the entire time; it can change as needed. This happens as follows:

Each DECT base station continuously scans all 120 frequency channels, measures the received signal strength (RSSI - Received Signal Strength Indicator) (low signal strength values ​​indicate free channels without interference, and high values ​​indicate busy or interference channels) and selects the channel with the minimum interference level. In this frequency channel The DECT base station emits service information, which, among other things, contains data:

1) To synchronize a DECT phone;

2) About the system identifier;

3) About the possibility of the system;

4) About free channels;

5) Paging.

Analyzing this information, the DECT phone finds its base station and registers with it. When you leave the coverage area of ​​one DECT base station, it searches for the next one. Thus, the phone is always assigned to one or another base station of its own or a friendly system. Next, the phone, synchronously with the base station, begins to continuously scan all 120 channels and measure the signal strength in each of them. The channel numbers with the lowest RSSI are stored in memory. There are at least two such channels in memory at the same time.

If it is necessary to organize an outgoing connection, the phone sends a request to the DECT base station to which it is currently assigned, offering to establish communication in one of the free channels, from the point of view of the phone. If this channel is rejected by the base station, then the phone offers the next one from the free list. After the base station agrees to establish a connection via one of the proposed channels, signaling and other service information is exchanged, and then a connection is established and a conversation is made.

Incoming communications are organized in a similar way. The DECT radiotelephone continuously analyzes the “paging” message for the presence of “its” incoming call. After recognizing an incoming call, it sends a request to establish communication in one of the free channels. Thus, the selection of the channel to establish a connection occurs dynamically and only at the initiative and control of the DECT handset. This mechanism is called continuous dynamic channel selection (CDCS).

The channel in which the conversation takes place is not dedicated for the entire duration of the connection. For one reason or another (for example, deterioration in communication quality when the handset is moved to the “shadow” zone), the radiotelephone may change it. In this case, the DECT radiotelephone selects a channel from the list of free ones and offers it to the base station. When coordinated with the DECT base station, a transition to new channel. The transition can also occur at the initiative of the base station. At the same time, she informs the radiotelephone receiver about her desire to switch to a new channel, then everything happens as described above, i.e. the selection of a new channel is carried out by the radiotelephone. This mechanism is called continuous dynamic channel allocation (CDCA).

Handover in DECT standard

Thanks to Continuous Dynamic Channel Selection and Allocation and DECT's non-disruptive handover capabilities, a speaker can avoid an interfering connection by establishing a second connection on a newly selected channel, either with the same base station (intra-cell handover) or with another base station ( handover between cells). These two radio connections are temporarily maintained in parallel, transmitting identical voice information, and at the same time analyzing the quality of the connections. After some time has passed, the base station determines which radio connection has better quality and releases another channel. If the speaker moves from one cell to another, the received base station signal strength, measured by the wearable's dynamic channel selection and allocation, will gradually decrease. The signal power of the DECT base station serving the cell in the direction in which the speaker is moving will gradually increase. At the moment when the signal of the new base station becomes stronger than the signal of the old base station, handover occurs without interrupting communication to the new BS. This process remains unnoticed by the user, because there is no interruption of communication.

Application of diversity antennas in DECT

However, handover does not occur quickly enough to counteract fast fading situations. To combat fast interference fading (FIF), the DECT standard provides a spatial diversity reception mechanism. BIZs arise as a result of the interference of several beams at the receiving point, which moves relative to the base station. As a result, the path difference between these beams changes and, as a consequence, the level of the total signal undergoes fluctuations that can reach 30 dB or more. When using two spatially separated antennas, the difference in the path of the rays from each of them at the reception point will be different. Two switched antennas spaced apart in the horizontal plane are connected to each base station, and the antenna spacing is office systems approximately equal to λ (wavelength), and in WLL (Wireless Local Loop) systems (fixed radio access systems) – 10λ. Therefore, the effectiveness of this method in office systems affects small distances. In WLL systems, the speakers are stationary and the cause of fading is the effect of refraction on the path difference between the direct and reflected beams. It is known from theory that when antennas are separated by 10λ or more, the total signals received by each antenna are practically uncorrelated.

Cordless phones gained popularity in the 90s, replacing traditional wired phones for many. Users liked the freedom they gained by getting rid of a leash. This opportunity was also appreciated by those who like to be outdoors - in their yards, garages or gardens. This way, they won’t miss important calls without losing reception or sound quality.

How does a cordless phone work?

The operating principle of a radiotelephone is quite simple. It consists of a handset that is powered by a battery and communicates with a base connected to an electrical and telephone socket. DECT (Digital Enhanced Cordless Telecommunication) is a wireless communication technology at frequencies of 1880-1900 MHz with GMSK modulation.

Today, home phones are not as common as they once were due to the dominance of cellular networks, but there are still many benefits that make having a land line worth it. The voice quality of such devices far exceeds even the best mobile models, and they provide higher reliability of services.

Headset jack

This feature comes in handy when working from home, hosting webinars, for conference calls, or even when transcribing and wanting to keep your hands free to type while listening to a call. The headset will be useful when moving around the house, if you need to do something else during a conversation, if the handset has a mount that allows you to place it on your belt. The headset is usually inexpensive, just make sure the plug size matches the jack on your phone.

Backup battery

This is probably one of the most popular features that only a DECT phone can provide, and also one of the most important. Wireless systems may stop working if the power goes out, but with battery backup the device will continue to function even then. Regardless of whether this is possible, it is a good idea to have a regular corded phone in reserve in case of an emergency and battery failure.

Caller ID

This is a very popular and frequently used feature that allows you to find out who is calling and what is their phone number. Some models have a voice notification, but mostly the information is simply displayed on the LCD screen.

Call waiting

This feature is very useful if the subscriber is currently talking but is waiting for another call. This is useful for those working from home, as they can talk to one person and take work calls without the frustration of a busy line, which in many cases can lead to the loss of clients. The DECT phone alerts you to a call waiting on the line with an audible signal. You can choose to put the current call on hold, answer it (which is done automatically when a proper switch occurs), or forward the incoming call to voicemail while continuing the current conversation.

Panasonic KX-TGE233B

If you require a high-level communication device, you can purchase a Panasonic KX-TGE233B DECT phone. Large buttons will not be a problem even for people with low vision, and an improved noise reduction system will make it possible to understand calls from very noisy places. The built-in equalizer allows you to customize the sound individually. Three handsets will eliminate the need to carry your phone around the house - it will always be at hand. The battery provides power to the device even during a power outage. A greater reception range is noted by those who like to spend a long time in their garden or garden. The answering machine is digital and user friendly.

Gigaset S820A-DUO

One of best devices priced under $250 are the Gigaset DECT 6.0 S820A-DUO phones. The handset between charges provides 20 hours of talk time and 250 hours of work time. Availability of Bluetooth and fast sync allows you to exchange data with your phone. The answering machine is designed for 55 minutes of recording. There is a 2.4-inch screen. According to reviews, incoming call blocking and call management functions are supported.

Philips D4552B/05

The Philips D4552B/05 radiotelephone is distinguished by the ability to bar certain incoming and outgoing calls, the presence of an alarm clock, support for up to 4 handsets, automatic caller ID, the ability to hold a call, and maintain lists of missed and received calls. Display - white backlit, 1.8 inches diagonal. Reviews note the sound high quality, the presence of 10 polyphonic melodies. The Philips cordless telephone can store 30 minutes of messages in memory and provides up to 16 hours of talk time. The operating range of the device is up to 50 m indoors and up to 300 m outdoors.

Conclusion

Wireless systems have come a long way since their introduction in the 90s and continue to increase their functionality and technology every year. Most models offer a wide range of useful features, including selecting the number of connected handsets. Regardless of each user's specific needs, there is always a DECT phone that can meet them.

DECT is a unique standard. Conceived for telephony, it almost immediately began to be used in data transmission systems. Originally intended for Europe, it has spread throughout the world. DECT competes with cellular communication standards, radio relay technologies, penetrates home multimedia systems, becomes a means of primary access to public telephone networks, and is included in the pool of third-generation cellular telephony standards IMT-2000. The DECT systems market still remains one of the most dynamic in the world.

Historically, DECT was intended to free telephone users from connecting cords. With the development of integrated semiconductor technologies, phones began to be equipped with transceiver units: the first radio extenders- ordinary analog telephones, in which the cord is replaced by a radio path. This generation of phones is called ST-0 (Cordless Telephone). Such devices still exist today. Their main purpose is to allow the owner to move freely within a radius of tens to hundreds of meters from the point of connection to the telephone network. The main disadvantages of these devices are the relatively high radiation power (up to 1 W), mutual interference, and absolute openness to eavesdropping and unauthorized connection to the radio path. In the 80s, systems of the ST-1 standard appeared in Europe - the same analog radio extenders, but with the rudiments of the functions of modern wireless communications, such as roaming and moving between cells without breaking the connection.

However, a real breakthrough was the appearance of the digital specification ST-2 table. 1.5). This specification was developed in the UK in 1989 (Common Air Interface, CAI/CT-2, MPT 1375 standard). In 1992, ETSI adopted CAI/CT-2 as European standard. Based on ST-2, the Telepoint system was created, which has become quite widespread throughout the world. It actually implemented microcellular architecture for the first time.

Table 1.5.

Main characteristics of communication systems of ST-2 (Tangara RD), STZ and DECT standards

ST-2 systems were predicted to have a great future, but after a sharp decline in prices for cellular services in the second half of the 90s . years of the last century, interest in them fell. In the early 90s, systems based on the ST-3 standard from Ericsson appeared. They were installed in a number of countries, but soon the attention of the European telecommunications community switched to a new specification called Digital European Cordless Telecommunications - DECT.

In 1988, the new standard was approved by the Conference of European Postal and Telecommunication Administrations (CEPT). The range 1880-1900 MHz was allocated for it. In 1992, ETSI publishes the first DECT specifications - ETS 300 175 and 176 (DECT Common Interface (CI) and DECT Approval Test Specification respectively). These documents laid down the basic foundations of DECT.

In fact, DECT is a set of specifications that define radio interfaces for various types of communication networks and equipment. DECT CI contains requirements, protocols and messages that ensure the interaction of communication networks and terminal equipment. The organization of the networks themselves and the design of the equipment are not included in the standard.

The most important task of DECT is to ensure compatibility between equipment from different manufacturers. For this purpose, a number of interaction profiles for various systems have been developed. In 1994, the first of these, the unified access profile GAP (Generic Access Profile) - ETS 300 444, appeared. It defines the operation of DECT terminal devices (telephones, base stations, PBXs) for all voice communications applications with a vocal tract bandwidth of 3, 1 kHz. Later, profiles appeared for the interaction of DECT and GSM, DECT and ISDN, the interaction of subscribers with limited mobility with public networks (Cordless Terminal Mobility, CTM), with subscriber radio access facilities (Radio Local Loop, RLL), etc.

Initially, DECT was focused on telephony - radio extenders, wireless private branch exchanges, providing radio access to public telephone networks. But the standard turned out to be so successful that it began to be used in data transmission systems and wireless subscriber access to public communication networks. DECT has found use in multimedia applications and home radio networks, Internet access and fax communications.

DECT systems and devices are common in all countries of the world. There are more than 200 different DECT products on the global market. It is no coincidence that the abbreviation DECT now stands for Digital Enhanced (instead of European) Cordless Telecommunication.

In the 20 MHz wide range (1880-1900 MHz), 10 carrier frequencies are allocated with an interval of 1.728 MHz. DECT uses time division multiple access technology - TDMA (Time Division Multiple Access). The time spectrum is divided into separate frames of 10 ms each (Fig. 1.14). Each frame is divided into 24 time slots: 12 slots for reception (from the point of view of the wearable terminal) and 12 for transmission. Thus, on each of the 10 carrier frequencies, 12 duplex channels are formed - 120 in total. Duplex is ensured by time division (with an interval of 5 ms) of reception/transmission (TDD, Time Division Duplex). The 32-bit sequence “101010...” is used for synchronization. DECT provides speech compression using Adaptive Differential Pulse Code Modulation (ADPCM) technology at 32 kbit/s (ITU-T recommendation G.726). Therefore, the information part of each slot is 320 bits. When transmitting data, it is possible to combine time slots. The radio path uses frequency modulation with a Gaussian filter (GFSK).

Figure 1.14 - Information transmissiontions in the systemDECT

DECT base stations (BS) and subscriber terminals (AT) constantly scan all available channels (up to 120). In this case, the signal strength on each channel is measured, which is entered into the RSSI (Recieved Signal Strength Indication) list. If a channel is busy or very noisy (for example, interference from another DECT device), the RSSI indicator for it is high. The BS selects the channel with the lowest RSSI value for constant transmission of service information about subscriber calls, station identifier, system capabilities, etc. This information plays the role of reference signals for AT - using them, the subscriber device determines whether it has the right to access that or another BS, whether it provides the services required by the subscriber, whether there is free capacity in the system, and selects the base station with the highest quality signal.

In DECT, the communication channel is always chosen by AT. When a connection request is made from the base station (incoming connection), the AT is notified and selects a radio channel. Service information is transmitted to the BS and analyzed by the AT constantly, therefore the AT is always synchronized with the closest available BS. When establishing a new connection, AT selects the channel with the lowest RSSI value - this ensures that the new connection occurs on the clearest channel available. This DCS (Dinamic Channel Selection) procedure allows you to get rid of frequency planning - the most important property of DECT.

Since AT constantly (even when a connection is established) analyzes available channels, they can be dynamically switched during a communication session. Such switching is possible both to another channel of the same BS, and to another BS. This procedure is called “handover”. During handover, AT establishes a new connection, and communication is maintained on both channels for some time. Then the best one is selected. Automatic switching between channels of different BS occurs almost unnoticed by the user and is completely initiated by AT. This is especially important for building microcellular systems that allow a subscriber to move from cell to cell without interrupting the connection. Note that, although the choice of channels is always up to AT, DECT provides the ability to notify the subscriber terminal from the BS side about low communication quality, which can initiate handover.

It is important that in the radio path of DECT equipment the signal power is very low - from 10 to 250 mW. Moreover, 10 mW is practically the nominal power for microcellular systems with a cell radius of 30-50 m inside a building and up to 300-400 m in open space. Transmitters with a power of up to 250 mW are used for radio coverage of large areas (up to 5 km with a directional antenna). Such low power makes DECT devices the safest for health. It is not for nothing that in European medical institutions the use of radiotelephony systems only of this standard is allowed.

In addition, with a power of 10 mW, it is possible to locate base stations at a distance of 25 m. As a result, a record density of simultaneous connections is achieved - up to 10,000 Earl/km 2 (about 100 thousand subscribers), provided that the BS is located according to a hexagonal pattern in one plane (on one floor). This is the best indicator in terms of the efficiency of using the radio spectrum (in terms of 1 MHz band). Compare - 500 Earl/MHz/km 2 for DECT versus 100 Earl/MHz/km 2 in the most capacious cellular networks GSM-1800 (DCS 1800).

What is the DECT standard, and how do DECT phones differ from conventional cordless phones that came before?

DECT is a digital wireless communication standard. It provides its users with stable, high-quality communications protected from unauthorized access. Compared to cordless telephones that came before, DECT ensures the security of cordless telephones through encryption and frequent changes of the encryption code during a call. Also, the quality of communication is significantly higher and does not interfere with televisions and radios. In addition, DECT offers the use of multiple handsets with one base, and handsets can be used simultaneously different manufacturers.

At what distance from the base will the handset operate?

Everything will depend on where the base station is located. The approximate distance at which the handset will work is 30-50 meters indoors. In a straight line (without partitions or walls) between the base and the handset, communication can remain at a distance of up to 300 meters.

How secure are DECT phones from the point of view of outsiders connecting to the line?

Initially, DECT technology was positioned as the most secure against unauthorized connection due to the constant change of signal encryption codes. However, according to unverified data, the ability to listen to a conversation from such a phone still remains, however, this requires very rare and expensive equipment, and its use by ordinary attackers is not justified.

Do DECT phones have caller ID?

Most models have a caller ID function. According to technology, there are two options for implementing this function: Caller ID and Caller ID.

What is the difference between Caller ID and Caller ID number identification modes?

Caller ID is accepted in the CIS countries, and Caller ID is a combination of two standards adopted in other countries (FSK and DTMF). Caller ID identifies the subscriber before the connection, Caller ID determines the number after the connection. If Caller ID is working, when calling, you hear first rare long beeps, then a click and again rare long beeps. This means that the identifier has worked, the connection has already been established, and the caller will be billed for this “conversation,” even if no one picked up the phone. If CallerID works, the caller hears only rare long beeps. Such an identifier does not reveal itself in any way, and the tariffication of the conversation begins from the moment the handset is picked up, and not from the moment the number is identified.

Can I use DECT phones with Caller ID in Russia?

Yes you can. But you need a special adapter that can convert the Russian signal telephone line into a foreign standard that is understandable to such a device.

Yes, such a service is provided; for this, the phone must support the function of sending SMS, and the PBX must be digital, not analog.

If I buy 2 different DECT phones, will they work from the same base?

DECT phones are mostly compatible with each other, even among different manufacturers. If the telephone base supports simultaneous operation of several handsets, then the handsets different phones through one base you can connect to one phone number. There are exceptions: there are phones that are only compatible with handsets from the same manufacturer, and some budget models are not compatible with others at all.

Is it possible to call from handset to handset without a base (what is the walkie-talkie function)?

Yes, some handsets have a walkie-talkie function, it allows you to use two handsets as walkie-talkies and talk on them bypassing the “base” (i.e. directly). You can even go far from the base, for example, into the forest to pick mushrooms, but the distance between the tubes rarely reaches 100 meters. In an open field it increases to 200-250 meters. Required condition operation of handsets in this mode - the handsets must be “registered” on the same base (they must understand the same base).

Is it possible to replace the battery in the handset yourself?

Yes, you can. To do this, you need to buy a battery that is suitable for the type. If the battery in the handset is connected by wires, then it is important to remember the location of the wires by color (there are only two of them, and it is enough to remember where the red one is connected). If the battery is a AA battery, then you just need to observe the polarity ("+" and "-").

Is it possible to use a Handsfree headset with a DECT phone?

Yes, it's possible. But you should remember that, as a rule, Dect phones use a headset with a jack 2,5 mm. Some modern models, support Bluetooth, this allows you to connect to your phone wireless headset.

Do Dect phones have an answering machine?

Yes, many modern Dect phones have an answering machine function. The answering machine is digital and the information left by callers is stored in the phone's memory. Typically, devices store up to 20 minutes of recorded information. It is quite easy to determine support for this function - control buttons will be placed on the body of the telephone base: play/pause, volume control and rewind and forward, as in portable players.

Related terms asked:

DECT- Digital standard wireless communication. Provides stable, high-quality communication protected from unauthorized access. The DECT standard supports voice, fax and data communications.

Repeater - Repeater (repeater - i.e. repeater) is a device that allows you to increase the range of DECT radio tubes by 1.5-2 times.

SMS - sending and receiving system short messages V cellular network, which allows owners of cell (and sometimes wired) phones to exchange text messages.

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Battery - there are several types of batteries: nickel-cadmium (NiCd), nickel-metal hydride (NiMH), lithium-ion (Li-Ion). The type of battery installed in the device determines not only its operating time without recharging, but also the service life of the battery itself. See NiCd, NiMH, LiIon.

DESCRIPTION OF THE DECT STANDARD

The DECT standard for wireless telephony was published in 1992 by the European Telecommunications Standards Institute (ETSI). In 1993, at the CeBIT exhibition in Germany, several telecommunications industry corporations presented the first wireless communication and data transmission systems based on the DECT (Digital European Cordless Telecommunications) standard. These were a wireless private PBX manufactured by Ericsson for departmental communication networks, wireless subscriber access equipment from Siemens and a wireless LAN from Olivetti. Three years later, at CeBIT -96, there were already 5 presenters European manufacturers telecommunications equipment presented their wireless telephone systems DECT - standard: Alcatel, Ericsson, Nokia, Philips, Siemens.

The main areas of application of the DECT standard are microcellular communication systems for business (wireless private PBXs for medium and large organizations, distributed industries, factories, etc.), subscriber access devices to the public telecommunications network as an alternative to the standard wired connection(Wireless Local Loop), single-cell radiotelephones/radio PBX for home, for small offices.
As of December 1996, the DECT standard has been approved for use in 24 countries, with more than 12 countries on the way. Among those who have approved this standard are not only European countries, but also Australia, Indonesia, and South Africa. Among the countries where the use of the DECT system requires permission from the relevant authorities are Argentina, Brazil, Singapore, Thailand. This standard promises to become the most widely used standard for digital wireless telecommunications systems. The approval of the DECT standard in many countries outside Europe led to the standard changing its name to Digital Enhanced Cordless Telecommunications.

In Russia, the Ministry of Communications has opened the range of 1800-1900 MHz for DECT systems. Channels for voice/data transmission in the DECT standard are formed through the use of 10 carrier frequencies, time division technology with multiple access and time duplexing (Multy Carrier/Time-Division Multiple Access/Time-Division Duplexing). Capacity of DECT systems (an indicator that takes into account the intensity of subscriber traffic, the width of the used frequency range and coverage area - Erlang/MHz/sq.km) higher than others digital systems mobile communications and is 500 Erlang/MHz/sq.km (for comparison: GSM - 10, DCS-1800 - 100). Theoretically, one DECT base radio can simultaneously provide 120 voice/data channels to wireless subscribers. Therefore, DECT equipment is suitable for organizing mobile communications where many subscribers are concentrated in a small area. This is important for both business applications and telecom operators. Connecting subscribers to communication networks using DECT equipment (solving the last mile issue) may turn out to be more cost-effective than a standard cable connection, and in some cases, the only possible one. Such systems are quicker to deploy, easier to expand, easier to manage, and more reliable in operation.

The subscriber is connected using a fixed access unit (FAU), usually installed outside a home or office, performs the functions of a transceiver, being a device that converts DECT radio signals into signals of an ordinary analog telephone line, and allows the user to connect ordinary analog equipment, for example telephone sets, faxes and modems. WLL equipment gives full access to the entire telephone network service.

Among single-cell radio PBXs for home and small offices, the most famous are Gigaset systems manufactured by Siemens, which are a system of one base unit for 1-2 external city lines and 4-8 radiotelephones.

DECT technology
The standard is based on digital radio data transmission between radio base stations and radiotelephones using time division multiple access technology, TDMA (Time-Division Multiple Access). Full duplex communication is provided using TDD (Time-Division Duplexing).
The range of radio frequencies used for reception/transmission is 1880-1900 MHz. The operating range (20 MHz) is divided into 10 radio channels, each 1.728 MHz. Information exchange is carried out by personnel; using time division, 24 time slots are created in each frame; 24 slots provide 12 duplex channels for voice reception/transmission. When establishing a conversation connection, 2 of the 24 time slots in each frame are used: one for voice transmission, the other for reception.
The DECT radiotelephone continuously polls the radio base stations, selecting the best available channel for communication (the so-called Continuous Dynamic Channel Selection (CDCS) process). Thanks to CDCS, the mobile subscriber does not notice the transition from the coverage area of ​​one radio base station to another; such a transition is carried out without loss of speech quality. The CDSC process is characterized by the fact that the search best channel occurs not only at the moment the connection is established, but continues during the conversation. A DECT radiotelephone spends most of its time monitoring available channels rather than receiving/transmitting speech. The transfer of a mobile subscriber's connection from one radio base station to another when moving from one microcell to another during a conversation is completely invisible to the subscriber. This property is very important, because due to the small size of microcells, there can be several such transitions during one conversation. The CDSC process allows the same time slots on the same carrier frequencies to be used to connect different subscribers in non-overlapping microcells.

The DECT standard allows you to organize data transmission, which makes it possible to build wireless LANs based on subscriber access equipment.
An interesting direction in the production of PBXs and the development of the DECT standard is the integration of DECT equipment into PBXs. BRs can connect directly to such PBXs without additional radio modules, and in addition to wired analog and digital subscribers, the PBX can also support wireless subscribers. This saves money when organizing a departmental microcellular communication network. In addition, due to the fact that usually serious PBXs support many different types of trunk lines, situations arise when such PBXs can be connected to those lines that are not supported by wireless DECT systems. An example of such a PBX is the Ericsson BusinessPhone 250.

ABBREVIATIONS USED:
CDCS - Continuous Dynamic Channel Selection
DECT - Digital European Wireless Telephony standard.
FAU - Fixed Access Unit.
HDSL - High Speed ​​Digital Subscriber Line.
ISDN - Integrated Service Digital Network.
TDD - Time Division Duplex.
TDMA - Time Division Multiple Access.
WLL - Wireless subscriber access devices to public networks
ADC - Analog to Digital Conversion
AdPCM - Adaptive Differential Pulse Code Modulation.
LCD - Liquid crystal display
PCM - Pulse Code Modulation

Basic principles of operation of DECT systems
MC/TDMA/TDD principle
The DECT air interface is based on a multi-carrier, time-sharing multiple access, time-division duplex (MC/TDMA/TDD) radio access methodology. Selection base frequency DECT uses 10 frequency channels (MC - Multi Carrier) in the range 1880-1920 MHz. The time spectrum for DECT is divided into time frames repeating every 10 ms. The frame consists of 24 time slots, each of which is individually accessible (TDMA - Time Division Multiple Access), the slots can be used either for transmission or reception. In a basic DECT voice service, two time slots - separated by 5 ms - are paired to provide supporting capacity typically for full duplex 32 kbit/s connections (ADPCM - Adaptive Differential Pulse Code Modulation - G.726 coded speech). To facilitate implementations of the basic DECT standard, a 10 ms time frame is divided into two halves (TDD - Time Division Duplex); The first 12 time slots are used to transmit the fixed part (downlink) and the remaining 12 are used to transmit the wearable part (uplink).
The TDMA structure provides up to 12 simultaneous DECT voice connections ( full duplex) per transceiver, which provides significant cost advantages over technologies that only allow one connection per transceiver (such as CT2). Thanks to an advanced radio protocol, DECT can offer different frequency bands, combining multiple channels into a single carrier. For data transmission purposes, error-proof pure speeds of n x 24 kbit/s up to a maximum of 552 kbit/s are achieved, while ensuring, as stipulated by the DECT standard, complete security.

Use of radio spectrum
When using the MC/TDMA/TDD principle for basic DECT (frequency and time measurements), a total spectrum of 120 duplex channels is available to the DECT device at any time. By adding a third dimension (space) - provided that DECT capacity is limited by interference from adjacent cells and a C/I (Carrier-to-Interface) ratio of 10 dB is achieved - a very low channel reuse ratio can be achieved. Various channels communications in adjacent cells can use the same channel (frequency/time slot combination). Therefore, when high density By installing DECT base stations (e.g. at a distance of 25 m in an ideal hexagonal coverage pattern), it is possible to achieve traffic capacity for basic DECT technology of up to approximately 10,000 Erlang/sq.km/floor (see Note 1) without the need for frequency planning . Installation of DECT equipment is simplified as only coverage and traffic requirements need to be considered.
Note 1.1 Erlang is equal to the average traffic load caused by one DECT voice connection - using one frequency/time slot pair - 100% of the time.
Continuous signal transmission
The DECT base station (base radio unit - RRB) constantly transmits a signal over at least one channel, thus acting as a beacon for connection with mobile DECT handsets (subscriber radio units - EPIRB). The transmission may be part of an active connection, or it may be idle. The BRB beacon transmission contains service information - in a multi-frame multiplex structure - about the identification of the base station, system capabilities, BRU status and paging information for establishing incoming communications. EPIRBs connected to the beacon transmission will analyze the transmitted information and determine whether the EPIRB has access rights to the system (only those EPIRBs with access rights can establish communication), whether the system capabilities correspond to the services required by the EPIRB and - in that case , if communication is necessary, does the EPIRB have free capacity to establish radio communication with the EPIRB.

Dynamic selection and dynamic channel highlighting

DECT defines continuous dynamic channel selection and dynamic channel allocation. All DECT equipment is required to scan its local radio environment regularly - at least once every 30 seconds. Scanning means receiving and measuring the strength of the local RF signal across all available channels. The scan runs as a background process and presents a list of free and busy channels (RSSI: Received Signal Strength Indication list), one for each time slot/carrier combination that will be used in the channel selection process. The free time slot is not used (temporarily) for transmission or reception. In the RSSI list, low signal strength values ​​indicate free channels without interference, and high values ​​indicate busy or interference channels. Using RSSI information, the DECT-ARB or DECT-BRB can select the optimal (with the least interference) channel for establishing a new communication line.
Channels with the most high values The RSSIs are continuously analyzed in the DECT EPIRB to verify that the transmission originates from a base station to which the wearable has access rights. The EPIRB will synchronize with the EPIRB having the strongest signal, as defined by the DECT standard. The channels with the lowest RSSI values ​​are used to establish radio communication with the EPIRB if the user of the EPIRB decides to establish communication, or in the event that the DECT mobile handset is signaled to incoming call through receiving a paging message.
In a DECT base station, channels with low RSSI values ​​are used when selecting a channel to establish a beacon transmission (idle transmission). A dynamic channel selection and allocation mechanism ensures that communication is always established on the clearest available channel.

Classification of DECT systems by architecture type
DECT systems vary in size and number of connected subscriber devices. There are three main types: Home System (home systems), Business System (enterprise level systems) and Enterprise System (large enterprise level systems).

1. The classic Home System consists of one base unit, usually connected via an ordinary analog line. It is usually installed in private apartments. Up to 8 mobile handsets can be connected. Within one base, they provide 2 connections between 2 handsets and 1 connection between one handset and an analog line at the same time. They have an advanced service, such as an answering machine or caller ID. Often integrated with a fax machine. A classic example of such a system: Siemens Gigaset - the most famous and widespread DECT system in Russia and Goodwin Lund - made in Russia and has proven itself well.

Home System
Similar systems are produced by LG and Sagem, which has released a fairly well-known cell phone that supports the DECT standard. All these systems have one unpleasant property in common - they are Home System, which means that these systems do not support the main function that distinguishes the Home System from the Business System - this is handover. Handover is the ability of a system to “hand over” a mobile subscriber from base station to base station. But this is not only a property of the base, but also a property of the mobile handset - the ability to move from base to base when the signal deteriorates. Please note that the ability of DECT handsets to register with many systems is a different function from the handover function. Yes, indeed, if you hang up on a mobile handset from Home System, or rather, disconnect the connection in the coverage area of ​​one base and establish a connection in the coverage area of ​​another base, the connection will be established. However, this is not a handover function. There are also sometimes problems with the fact that Home System manufacturers build their own characteristic functions into the systems, for example functions for accessing an answering machine or a list of callers, etc. These functions are naturally also unique to each individual manufacturer of DECT telecommunications equipment.

2. A standard Business System consists of one DECT controller (often the controller is integrated into the PBX and consists of many boards) and a fairly large number of base stations. Base stations are connected to the controller using different protocols and different lines. So, for example, Matra Telecom connects its base stations via ISDN-BRI. One such line provides a speed of 128 Kbps and, since DECT systems use ADPCM G.726 (32 Kbps) encoding, supports four voice channels. DeTeWe connects base stations using its own protocol, but similar to ISDN-BRI. It is possible to connect up to 2 such lines to the base station, providing a total of 8 simultaneous connections per base. Philips has developed 6 and 12 channel bases that connect to the controller via G.703. That is, 2 Mbit/s. The maximum distance of the BS from the controller is determined by the characteristics of the trunk interface and can vary from 1.7 km to 5 km in solutions offered by different manufacturers. The methods of power supply to base stations are also different. In some cases it is supplied remotely, and in others from a separate source.

Business System

3. The main difference between the Business System and the Enterprise System is the absence of roaming. Roaming is the ability of a system to transfer a call to another system and assign a class of service to the roaming subscriber. Typically, the transition of a mobile subscriber from system to system is not accompanied by the need to maintain a continuous connection. In a standard situation, different controllers are located at a considerable distance from each other, and the subscriber’s transition to a new system is accompanied by a disconnection. It would seem that a subscriber with a DECT mobile handset can be easily registered in the new system, but the following difficulties arise: calls arriving at the mobile subscriber number of the first system must be re-routed to the new system. However, it is rarely possible to solve this problem using ATS. Often the PBX itself does not support such functions, and often this service costs significant financial resources and the owner of the systems considers the purchase of this service inappropriate. Also, most often, customers are faced with the following problems: a large enterprise has installed a PBX that has several extensions, and currently, PBX extensions are usually installed with a connection to the main PBX via fiber optics, and there are no copper cables in the required direction. Often, the removal of the base station via optical fiber is installed at a distance of more than 10 km, and in this case, the removal of the base station via copper cable is in principle impossible. And there can be more than 5 such outlets. With such schemes, installing several DECT controllers and organizing roaming between them is in principle impossible. After all, each mobile handset will have to register its own separate number in each DECT controller, and then organize a complex search structure for this handset in different DECT controllers. This is the first thing. And secondly, in this case you will have to register the class of service of the original mobile handset (category, number, rights to access long-distance communication channels, etc.) to all “clones” of this mobile handset in all controllers, etc. It is much better when the microcellular communication system itself supports roaming. That is, it can, when necessary, begin to find the required handset among the specified controllers along prescribed routes and ensure that the class of service of the required handset is transferred to the desired controller. At the moment, there are few such systems. It would be correct to note that there are few customers for such systems on the world market and on the Russian market in particular. The rapid development of cellular communications and the reduction in the cost of its tariffs greatly hinder the active development of this area. At the moment, telecommunications equipment manufacturers are solving this problem by means of software PBXs with built-in integrated DECT controllers. Now the Avaya Communication company with the Definity PBX product software version above 8.4 and the Nortel Network company with the Meridian 1 product software version 25 have already solved this problem.

Classification of DECT systems according to the type of integration with the core PBX

Types of integration of DECT controllers with a core PBX differ in the type of interface with the PBX (digital or analog channels), the type of integration (external controller or integrated into the PBX) and the ability to switch calls.

When designing a system, it is necessary to decide what level of service and how many subscriber devices should be supported by the microcellular system. The following problems may occur:

1. Controllers connecting to the PBX via analog lines provide the DECT subscriber with the service of an analog device. This means that the subscriber is not allowed to see any caller IDs, last names, or other DECT things on the display. But this system provides such a subscriber with the full service of any analog PBX device, and this is not so little. These are various Call Forward, Call Park, Bridging, Call Pickup, etc. This is yours personal class service. However, such systems are poorly expandable and, of course, do not have roaming. Example: Goodwin Odense (Kirk).

2. Controllers that connect to the PBX via digital lines, but are essentially radio extenders, provide the DECT subscriber with almost the best service in case of successful integration. This subscriber is sent to the screen necessary information, such a subscriber has all the services of a modern digital telephone exchange. In such a system, each subscriber has his own class of service, etc. However, there are some problems. Such systems are not cheap, and if you purchase the entire line of equipment from one supplier, yes, indeed, everything will work perfectly, but it will be quite expensive, and the benefits of using open technology, which is the DECT standard, are lost. A huge advantage of such systems is the enormous expandability of the system. Example: Definity Dect R2 from Avaya Communication. This system is expandable to 16320 DECT subscribers and supports roaming starting from software version 8.4. A huge disadvantage is the problem of interfacing with third-party PBXs. Example: Goodwin WLL.

3. External microcellular systems that can independently switch channels are quite cheap, highly expandable, and quite mobile, but they have one huge drawback, which is that this system is a separate system. This means that practically the maximum service that a user of such a system can achieve is Basic Call. That is, such a connection will have all the features of an interstation junction. Even if you use “advanced” signaling such as ISDN QSIG, the maximum service that can be achieved is AutoCallBack. The main functions used in a modern PBX will not be available to a subscriber of a DECT microcellular system. There will be one class of service for the entire system. This means that DECT subscribers will be able to receive long-distance access with only one category and one caller ID. This means that it will not be possible to restrict long-distance access to any one DECT subscriber. Restrictions will be placed on the entire system, etc. An example of such a system is Goodwin Spree, Business Phone Ericsson or Merkator Nortel Network. Of course, only if these systems are used only as DECT controllers.

4. The best solution are controllers built into the PBX. In this case, the DECT subscriber receives the most accessible service, comparable to the service of system phones. The downside is low scalability and relative high cost. The advantage is the presence of roaming. Because Roaming is implemented using PBX facilities and via inter-exchange channels. A striking system that supports all of the above functions and is quite widely expandable is the Meridian DECT system. In this system, each DECT card, installed in a universal PBX slot, supports the installation of four base stations and can support 32 subscribers on one card. A total of 32 such cards can be installed in the PBX. Total: 1024 subscribers and 128 base stations. The system, starting from software version 25, can support roaming. The downside is the relative high cost and lack of monitoring. However, normal monitoring of DECT systems is usually implemented only in DECT switches.

Comparison of DECT technology with other wireless telephony technologies
CT0, CT1 and CT1+ are analog wireless telephony technologies intended primarily for use in residential areas (although some companies have developed small wireless PBXs based on CT1+). The main disadvantages of these technologies are the relatively low capacity and ease of eavesdropping on analogue radio transmissions.

ST2. This technology, developed in the UK, was one of the competitors of DECT technology. It uses FDMA (Frequency Division Multiple Access) technology, which means that each call occupies the entire radio channel. Although the capacity achieved by this technology is higher than that of analog wireless telephony, CT2 is also characterized by difficulties with capacity and mobility restrictions: call transfer from one base station to another is organized very inefficiently. CT2 is used mainly in Telepoint networks; Some manufacturers offer CT2-based wireless PBXs, but such systems have limited capacity and limited coverage (about 100 numbers are a practical maximum), and as a result, most manufacturers have now switched to DECT as the platform of choice for wireless PBX development.

STZ. A system developed (and owned) by ERICSSON as a "test bed" for DECT systems with which it has many similarities. BTS-based business wireless communications systems are sold in several countries.

Establishing a connection
User-initiated handshake (outbound handshake)
The initiative to establish a radio channel in basic DECT applications always belongs to the EPIRB. The EPIRB selects (using dynamic channel selection) the best available channel and communicates on it with the EPIRB. To detect communications attempts by the EPIRB, the EPIB must receive on this channel when the EPIRB transmits its access request. To ensure that the EPIRB can use all 10 DECT RF carriers, the EPIRB continuously scans its idle receive channels sequentially for attempts by the EPIRB to communicate. The EPIRBs are synchronized with this sequence using service information constantly transmitted by the base station. Based on this information, the EPIRB can determine the exact moment when successful access to the EPIRB is possible on the selected channel.
Network initiated handshake (incoming handshake)
When an incoming call arrives at a DECT EPIRB, the access network informs the EPIRB by sending the corresponding identifier about this EPIRB via the paging channel. The EPIRB, having received a paging message with its identifier, establishes a radio channel to service the incoming call, using the same procedure as when establishing an outgoing connection.

Handover
Thanks to powerful dynamic channel selection and allocation and DECT capabilities that enable handover without interruption, EPIRBs can move away from an interfering connection by establishing a second connection - on a newly selected channel - either to the same base station (intra-cell handover) or to a different base station (handover between cells). These two radio connections are temporarily maintained in parallel, transmitting identical voice information, and at the same time analyzing the quality of the connections. After some time has passed, the base station determines which radio connection has better quality and releases the other channel. If a DECT EPIRB moves from one cell to another, the strength of the received EPIRB signal—measured by the wearable's dynamic channel selection and allocation—will gradually decrease. The signal power of the EPIRB serving the cell in the direction of which the EPIRB is moving will gradually increase. At the moment when the signal of the new BRU becomes stronger than the signal of the old BRU, a handover occurs without interrupting communication (as described above) to the new BRU. The handover without interruption of communication, initiated completely independently by the mobile DECT handset, remains unnoticed by the user.
Although handover is always initiated by the DECT EPIRB, there may be situations in which the EPIRB to EPIRB link does not provide the required quality. In this case, DECT provides notification protocols that allow the BRB to report the perceived quality of the connection to the EPIRB, which can then initiate a handover.

Diversity antennas
Handover in DECT is a mechanism for avoiding channels susceptible to interference or channels with low level signal. However, handover does not occur quickly enough to counteract fast fading situations. For this purpose, the DECT BRB can be equipped with diversity antennas. The standard provides a signaling protocol for monitoring the selection of the BRB antenna from a mobile DECT handset. Due to the fact that the radio link between the BRB and the EPIRB is of the nature of time division duplex (symmetry), the choice best antenna BRB improves not only the uplink quality, but also the downlink quality, at low speed.
Compatibility
The compatibility properties of radio access technology are mainly based on the ability to escape (handover) - in the frequency domain - from a noisy radio link without relying on information transmitted on the original (affected) channel. MC/TDMA/TDD, continuous dynamic channel selection and allocation and handover procedures in the DECT standard demonstrate excellent compatibility capabilities even in strong interference environments.

Security
The use of radio access technology that provides mobility involves significant security risks. The DECT standard provides measures to counteract the inherent security flaws inherent in cordless communications. Effective registration and authentication protocols have been introduced to prevent unauthorized access, and the concept of advanced encryption provides protection against eavesdropping.

Registration
Registration is the process by which the system allows a specific mobile DECT handset to be serviced. The network operator or service provider provides the ARB user with a secret registration key (PIN code), which must be entered into both the ARB and the ARB before the procedure begins. Before the handset initiates the actual registration procedure, it must also know the identification of the BRU in which it must register (for security reasons, the registration area can be limited to even one dedicated (low-power) BRU system). The procedure time is usually limited, and the registration key can only be used once; this is done specifically in order to minimize the risk of unauthorized use. Registration in DECT can be carried out “over the air”; after radio communication is established, it is verified on both sides that the same registration key is used. Identity information is exchanged and both parties calculate a secret authentication key that is used for authentication each time a connection is established. The authentication secret key is not transmitted over the air.
A mobile DECT handset can be registered at several base stations. At each registration session, the ARB calculates new key authentication, tied to the network in which it is registered. New keys and new network identification information are added to the list stored in the EPIRB, which is used during the connection process. Handsets can only connect to the network to which they have access rights (the network identification information is contained in the list).

Authentication
Handset authentication can be done as follows: standard procedure every time a connection is established. During an authentication session, the base station verifies the authentication key without transmitting it over the air.
The principle of non-disclosure of identification information over the air is as follows: the BRB sends a random number to the handset, which is called a “query”. The handset calculates the "answer" by combining the authentication key with the resulting random number and transmits the "answer" to the base station. The BRB also calculates the expected "response" and compares it with the received "response". As a result of the comparison, either continuation of the connection or disconnection occurs.
If someone is eavesdropping on the over-the-air interface, in order to steal the authentication key, they need to know the algorithm to discover the key from the "challenge" and "response". This "reverse" algorithm requires enormous computer power. Therefore, the cost of retrieving the key by eavesdropping on the authentication procedure is incredibly high.

Encryption
The authentication process uses an algorithm to calculate a "response" from a "challenge" and an authentication key in the handset and at the base station. It is a method of sending user identification information in encrypted form over the air to prevent identity theft. The same principle can be applied to user data (eg voice). During authentication, both parties also calculate the encryption key. This key is used to encrypt data transmitted over the air. The receiving party uses the same key to decrypt the information. In DECT, the encryption process is part of the standard (although optional).

DECT application profiles
Application profiles contain additional specifications that define how the DECT air interface should be used in specific applications. Standard messages and sub-protocols have been created from the base standard toolkit and tailored to specific applications in order to ensure maximum compatibility of DECT equipment from different manufacturers. In addition to the profiles, ETSI has also developed profile test specifications to allow for comprehensive testing of DECT equipment that claims to meet the profile requirements.