Range dect. Description of the DECT standard


DECT telephone

  • The range of radio frequencies used for reception/transmission is 1880-1900 MHz in Europe, 1920-1930 MHz in the USA.
  • The operating range (20 MHz) is divided into 10 radio channels, each 1728 KHz.
  • The maximum power of the station and handsets in accordance with the standard is 10 mW.

Health effects

DECT phones, like any other radio-emitting devices, are not absolutely safe for consumer health. All spectra of wave action in different modes work.

German institution for protection against harmful radiation(German) Bundesamt für Strahlenschutz), based on the fact that DECT phones have a detrimental effect on human health, put forward a requirement for wireless manufacturers wired phones produce telephones with an adjustable communication function between the handset and its station. These phones are called Eco-DECT.

see also

Notes


Wikimedia Foundation. 2010.

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

DECT – digital standard wireless communication. 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 work?

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 phone base supports simultaneous work with several handsets, then handsets of different phones can be connected to one through one base phone number. There are exceptions: there are phones that are compatible only with handsets from the same manufacturer, and some budget models 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 is 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 a wireless headset to your phone.

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 - on the case telephone base control buttons will be placed: play/pause, volume control and rewind and forward, as in portable players.

Related terms asked:

DECT - Digital wireless communication standard. 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 is a system for sending and receiving short messages in cellular network, which allows owners of cell (and sometimes wired) phones to exchange text messages.

______________________________________________________________________

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.

1 Brief overview of the standard

DECT (Digital Enhanced Cordless Telecommunication) is a wireless communication technology at frequencies of 1880-1900 MHz with GMSK modulation (BT = 0.5), used in modern radiotelephones. The DECT standard is not only widely used in Europe, but is also the most popular wireless telephone standard in the world due to the ease of deployment of DECT networks, a wide range of user services and high quality communications. According to estimates in 1999, DECT has been adopted in more than 100 countries, and the number of DECT subscriber devices in the world is approaching 50 million. In Europe, DECT has almost completely replaced cordless phones CT2, CT3 standards; on other continents, DECT successfully competes with the American PACS standard and the Japanese PHS. The DECT standard in Russia for home use does not require licensing (obtaining a frequency solution from the SCRF, permission from Roskomnadzor).

The range of radio frequencies used for reception/transmission is 1880-1900 MHz in Europe, 1920-1930 MHz in the USA. The operating range (20 MHz) is divided into 10 radio channels, each 1728 KHz wide. The maximum power of the station and handsets in accordance with the standard is 10 mW.

DECT refers to packet radio communication systems with frequency-time division of channels (information is transmitted over a radio channel in the form of packets organized into frames) and is based on technologies:

TDMA - Time division multiple access;

FDMA - Frequency division multiple access;

TDD - Time division duplex (duplex channel with time division);

Information is exchanged frame by frame using time division in each frame. Each frame with a duration of 10 ms is divided into 24 time intervals (TI, or slots in English slot), with the first 12 TIs (0-11) used for transmitting packets in the PF-IF direction, and the next 12 TIs (12-23) for transmission packets in the opposite direction, IF-PF. Duplex communication channels form sequences of two packets of one frame with an interval between them of 12 VI. Transmission and reception of information in DECT are carried out on the same frequency (duplex with time division of channels). 16 DECT frames are combined into a multiframe. All DECT frames are numbered, frame numbers are used to encrypt messages and are transmitted over the Q broadcast channel.

Transfer connection mobile subscriber from one base radio station to another during a conversation is absolutely invisible to the subscriber (handover mode). 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 implementation of wireless communication (according to the DECT standard) occurs both within the framework of analog telephony and IP telephony. Corporate radiotelephones operating on the Voice over IP principle are one of the most popular and fastest growing segments of the IP telephony market.

2 Structure of DECT systems

Below is standard scheme structures of DECT systems:

The controller is designed to interface the DECT system with external networks, for example, a city and/or private branch exchange. In this case, the CCS, as a rule, ensures the conversion of signaling protocols between the PBX and the DECT system. In some cases, they are used for these purposes special devices– protocol converters. In addition, the DCS converts speech information ADPCM ⇔ PCM when pairing via digital interfaces and ADPCM ⇔ analog signal when pairing via analog interfaces;

BS - Base station (in foreign literature they are called Radio Fixed Part) provide the required radio coverage. The BS is connected to the controller via one or two pairs of wires. The base station is a transceiver that provides simultaneous operation on 4 to 12 channels, operating on two spatially separated antennas. BS are performed in two
options - for indoor and outdoor placement;

UD - Access devices are a mobile handset or
a fixed subscriber terminal, which is sometimes called a “radio socket”;

To increase coverage area base station A repeater may also be used.

Standard characteristics of modern DECT systems

3 MC/TDMA/TDD principle

The DECT air interface is based on multi-carrier radio access methodology, time division multiple access and time division duplex (MC/TDMA/TDD). In the DECT standard in a dedicated
10 frequency channels (MC - Multi Carrier) are used in the range 1880-1990 MHz. The time spectrum for DECT is divided into time frames (frames) repeated every 10 ms (Fig. 3). The frame consists of 24 time slots, each of which is available individually (TDMA - Time Division Multiple Access).

In the basic DECT voice service, two time slots - separated by 5 ms - form a duplex pair to provide 32 kBit/s connections (ADPCM - Adaptive Differential Pulse Code Modulation - G.726). For implementation basic standard
6 DECT time frame of 10 ms is divided into two halves (TDD - Time Division Duplex) - the first 12 time slots are used for BS transmission (“downlink”), and the remaining 12 are used for EPIRB transmission (“uplink”). The TDMA structure provides up to 12 simultaneous DECT voice connections (full duplex) per BS, which provides significant cost advantages compared to
technologies that allow only one connection per BS (for example, CT2). Thanks to an advanced radio protocol, DECT can offer different frequency bands, combining multiple channels into a single carrier.

3.1 Use of radio spectrum

When using the MC/TDMA/TDD principle for the basic DECT specification (10 frequency and 12 time values), the DECT device is always available shared resource of 120 duplex channels. At high density installation of DECT base stations (for example, at a distance of 25 m in an ideal hexagonal coverage model) taking into account the low coefficient reuse channel (C/I = 10 dB), it is possible to achieve traffic capacity for basic DECT technology of up to approximately 10,000 Erlang/km2 without the need for frequency planning. Installation of DECT equipment is simplified as only radio coverage and traffic requirements need to be considered.

3.2 Dynamic selection and dynamic selection channel

Instead of frequency scheduling, a mechanism is used for continuous dynamic channel selection and allocation (CDCS/CDCA). The essence of this mechanism is that channels are selected dynamically from the entire set of channels based on indicators such as signal 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 BS continuously scans the receiving timeslots of all 120 channels, measures
level received signal(RSSI - Received Signal Strength Indicator) and selects the channel with minimum level(free channel without interference). In this channel, the BS emits service information, which, among others, contains the following data:
- for synchronizing the EPIRB;
- about the system identifier;
- about the capabilities of the system;
- about free channels;
- paging.
Analyzing this information, the ARB finds its BS and registers with it. When leaving the coverage area of ​​one BS, a search for the next one occurs. Thus, the EPIRB is always registered to one or another BS of its own or a friendly system. Next, the EPIRB is synchronized with the BS
begins to continuously scan all 120 receiving timeslots 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 outgoing communication, the EPIRB sends a request to the BS, in which it this moment is registered, offering to establish a connection in one of the free ones, with
point of view of EPIRB, channels. If this channel is rejected by the BS, then the EPIRB offers the next one from the free list. After the BS 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 takes place. Incoming communications are organized in a similar way. EPIRB continuously
analyzes the "paging" message for the presence of "its" incoming call. After recognizing an incoming call, the EPIRB sends a request to establish communication in one of the free channels. Thus, the choice of channel for establishing a connection occurs
dynamically and only on the initiative and under the control of the ARB. This mechanism is called continuous dynamic channel selection (CDCS). The channel on which the conversation occurs is not permanently allocated for the entire duration of the connection. For one reason or another (for example, deterioration in communication quality when the EPIRB moves to the “shadow” zone), the EPIRB may change the channel. In this case, the EPIRB selects a channel from
free list and offers it to BS. If the BS agrees, a transition to new channel. The transition can also occur at the initiative of the BS. At the same time, BS about his desire to switch tothe new channel is reported by the EPIRB, then everything happens as described above, i.e. choosing a new one
channel is carried out by the EPIRB.
If during the connection process a new channel is requested from the same BS, then the transition is called “intercell handover”, and if from another BS - then “intracell handover”. This mechanism is called continuous dynamic channel allocation (CDCA). Handover in the DECT system occurs in a soft way. This means that during a handover between the EPIRB and the system, two channels operate simultaneously: “old” and “new”. At some point in time, information between the EPIRB and the system is transmitted simultaneously through both channels. Only after a successful transition to the “new” channel does the “old” one deactivate. It should be noted that handover occurs not only when the quality of communication deteriorates or when the connection is broken, but also when the EPIRB finds the best one from its point of view
channel. Thus, the best free channel is always used for the connection.
The CDCS/CDCA mechanism significantly distinguishes DBCT from cellular systems communications: channels are controlled not by a central controller, but by mobile terminals. Unique DECT capability for dynamic channel selection and allocation
guarantees that only the best channel is used. This DECT capability allows multiple systems to coexist in the same frequency band, while maintaining high quality and secure communications in each. In addition, this mechanism is significantly
increases system traffic capacity by minimizing multi-path channels. This is especially important for office applications, where the radio signal is repeatedly reflected from the walls of the room. The MC/TDMA/TDD method, together with the CDCS/CDCA mechanism, provides high capacity to DECT systems even in conditions of high traffic and difficult interference conditions.
At the same time, high quality services are achieved without the use of frequency planning.

3.3 Antenna diversity

Handover in DECT is a mechanism for avoiding channels that are susceptible to interference or channels with low signal strength. However, handover does not occur quickly enough to counteract fast fading situations. To combat fast
Interference fading (IFF) The DECT standard provides a mechanism for spatial diversity reception. BIZs arise as a result of the interference of several beams at the receiving point, which moves relative to the BS. Resulting in
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 a point
reception will be different. In office and WLL systems, two switched antennas spaced apart in the horizontal plane are connected to each BS, and the antenna spacing in office systems is approximately equal to λ (wavelength), and in WLL systems -
10 λ. Therefore, the effectiveness of this method in office systems affects small distances. In WLL systems, the EPIRBs 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 the antennas are separated by 10 λ or more, the total signals received by each antenna are practically uncorrelated. Switching antennas and selecting a working channel occurs under the control of the EPIRB.

4 Security of DECT systems

Currently, more and more attention is being paid to the problems of security of communication systems against unauthorized access. The DECT standard provides measures to protect the accessibility of telecommunications systems inherent in wireless communications. The list of standard services and procedures for ensuring security in DECT standard systems includes:

ARB registration;
- EPIRB authentication;
- BS authentication;
- mutual authentication of EPIRB and BS;
- user authentication;
- data encryption.

4.1 Registration of the ARB

Registration is the process by which the system allows a specific EPIRB 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 KBS and the ARB
before the start of the registration procedure. Before the handset initiates the actual registration procedure, it must also know the identifier of the BS into which it must register (for security reasons, the registration procedure can be organized even for a system with
one BS). The time for the procedure 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. An exchange takes place identification information, and both parties calculate a secret authentication key that is used for authentication each time communication is established. The authentication secret key is not transmitted over the air.
The EPIRB 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 a network to which they have access rights (the network identification information is contained in the list). During the authentication process at any level, a cryptographic “request-response” procedure is used to find out whether the authenticated party knows the authentication
key.

4.2 EPIRB Authentication

Authentication of the EPIRB allows you to prevent its unauthorized use (for example, in order to avoid paying for services) or exclude the possibility of connecting a stolen or unregistered EPIRB. Authentication occurs at the initiative of the BS with each attempt to establish a connection (incoming and outgoing), as well as during a communication session. First, the BS generates and
transmits a request containing some constant or relatively rarely changing parameter (64 bits), and random number(64 bits) generated for this session. Then in the BS and ARB using the same algorithms using authentication
key K, the so-called authentication response (32 bits) is calculated. This calculated (expected) response in the BS is compared with the one received from the EPIRB, and if the results match, it is considered that the authentication of the EPIRB was successful.

4.3 BS authentication
BS authentication eliminates the possibility of unauthorized use of the station. This procedure ensures the protection of service information (for example, user data) stored in the EPIRB and updated upon command from the BS. In addition, the threat of redirecting subscriber calls and user data for the purpose of intercepting them is blocked. The BS authentication algorithm is similar to the sequence of actions for EPIRB authentication. Mutual authentication can be accomplished in two ways:
- With the direct method, two authentication procedures are carried out sequentially
EPIRB and BS;
- The indirect method in one case involves a combination of two procedures - EPIRB authentication and data encryption (since knowledge of the authentication key K is required to encrypt information), and in the other - data encryption using the static key SCK (Static Cipher Key), known to both stations.

4.4 User authentication

User authentication allows you to find out whether the user of the EPIRB knows his personal identifier. The procedure is initiated by the BS at the beginning of the call and can be activated during the communication session. After the user manually enters his personal identifier UPI (User Personal Identity), and the authentication key K is calculated in the ARB with its help, a procedure similar to the sequence of actions during ARB authentication occurs.

4.5 Authentication key

In all described procedures, the authentication response is calculated from the authentication request and the authentication key K in accordance with the standard algorithm (DSAA-DECT Standard Authentication Algorithm) or any other algorithm that meets the communication security requirements. The DSAA algorithm is confidential and is supplied under contract to ETSI. The use of a different algorithm will limit the capabilities of subscriber stations, as difficulties will arise when roaming in networks common use DECT.

The authentication key K is derived from one of the three values ​​or combinations thereof given below.

1. Subscriber authentication key UAK (User Authentication Key) up to 128 bits long. UAK is a unique value contained in the user's registration data. It is stored in the subscriber station ROM or in the DAM (DECT Authentication Module) card.
2. AC Authentication Code (Authentication Code) 16-32 bits long. It can be stored in the subscriber station's ROM or entered manually when required for the authentication procedure. It should be noted that there is no fundamental difference between the UAK and AC parameters. The latter is usually used in cases where fairly frequent changes of the authentication key are required.
3. Personal user identifier UPI (User Personal Identity) 16-32 bits long. The UPI is not recorded in the memory devices of the subscriber station, but is entered manually when required for the authentication procedure. The UPI ID is always used in conjunction with the UAK key.

4.6 Data encryption

Data encryption provides cryptographic protection user data and control information transmitted over radio channels between the BS and the EPIRB.

In EPIRB and BS it is used shared key encryption SK (Cipher Key), on the basis of which the encryption sequence KSS (Key Stream Segments) is formed, superimposed on the data stream on the transmitting side and removed on the receiving side. KSS is calculated in accordance with the standard DCS encryption algorithm (DECT Standard Cipher) or any other algorithm that meets the cryptographic strength requirements. The DSC algorithm is confidential and is supplied under contract to ETSI.

Depending on the conditions of use of DECT systems, two types of encryption keys can be used: calculated - DCK (Derivation Cipher Key) - and static - SCK (Static Cipher Key). Static SCK keys are entered manually by the subscriber, and calculated DCKs are updated at the beginning of each authentication procedure and are derived from the authentication key K. Up to 8 keys can be stored in the subscriber station ROM.

A static key is commonly used in home communication systems. In this case, the SCK is unique for each subscriber/base station pair forming the home communication system. It is recommended to change the SCK once every 31 days (frame number repetition period), otherwise the risk of information disclosure increases significantly.

5 Organization of DECT protocols

5.1 DECT protocol architecture

The DECT protocol architecture includes:
- physical layer (PHL Layer);
- media access layer (MAC Layer);
- data link control layer (DLC layer);
- network layer (NWK. layer);
- Application profiles

5.2 Physical layer

The first layer, PHL, provides the medium for communication between the EPIRB and the BS and is described in the ETS 300 174-2 standard. This standard defines the parameters of the DECT radio path. In particular, the standard defines the frequency range, radiated power, modulation method, TDMA time division structure, etc. It is the PHL level that is responsible for the MC/TDMA/TDD mechanism.
To ensure high-speed data transmission (up to 2 Mbit/s), the basic ETS 300 175 standard was supplemented with a high-speed transmission method based on phase modulation. Two modulation schemes are used: 4-level (π /4-DQPSK) and 8-level (π /8-D8PSK). High-level modulation (4- and 8-level) is used only to modulate the information channel (user data), and frequency shift keying is used to modulate the synchronization and control channels. This ensures that new high-level modulation systems are compatible with existing systems. Each timeslot (Fig. 4) contains a guard interval of 25 μs, 32 synchronization bits (SYN), 64 control bits (C) and data bits (B). Since synchronization bits are present in each physical channel, synchronization can occur before each physical channel. Bits C and B form 2 logical channels, respectively, for control and transmission of user data (as in ISDN).

5.3 Media access level

The medium access layer is responsible for establishing a radio channel between the EPIRB and the BS. The main functions of this level are:
- establishing connections;
- alarm provision;
- handover control.
It is the MAC level that is responsible for “soft” handover and the CDCS/CDCA mechanism. In addition, the MAC layer provides a channel for transmitting paging information and signaling.


5.4 Data link control layer

The DLC layer is responsible for the reliable transmission of control information over the physical channel. At this level the following tasks are solved:
- protection of transmitted data from errors;
- physical connection quality management;
- control of the channel selection procedure at the MAC level.
At the MAC and DLC levels, so-called protocol data units are used, consisting of:
- title;
- fields MAC data level;
-DLC level data fields;
- cyclic check code (CRC).
The message header determines the type of message and the type of DECT system (home, office or public). In addition, the system ID, information about supported system functions, and paging information are transmitted.


5.5 Network layer

This level is responsible for signaling and carries out:
- MAC and DLC level management;
- call management;
- mobility management (external handover, roaming, etc.);
- transfer of information with/without establishing a connection;
- provision of further education.
To ensure internal handover, the participation of the third level is not required, because Only the second level is responsible for this. This is the main (fundamental) difference between DECT and GSM.

DECT is a digital radio access standard that makes very efficient use of radio frequency bandwidth and opens up all new cordless communication applications for the home, office and private local commercial areas (airports, train stations, shopping centers, banks, exchanges, etc.). It provides its users with stable, high-quality communications protected from unauthorized access.

The DECT standard supports voice, fax and data communications. In addition, it is designed taking into account modern telecommunications trends, such as the convergence of fixed and mobile networks, the integration of voice and data communications, multimedia services and simultaneous services from several operators.

DECT has strengthened its position as a global standard wireless access- it has been accepted (as of January 1999) in more than 100 countries around the world.

BENEFITS OF DECT

  • Wired line quality - 32k ADPCM
  • The most high speed data transmission among all TDMA standards
  • Possibility of creating various systems based on DECT:
    • home cordless multi-tube systems that are also suitable for small offices,
    • microcellular wireless corporate systems(office and institutional PBXs with radio access),
    • public microcellular systems (STM),
    • fixed radio access systems (WLL), etc.
  • Coexistence of different uncoordinated DECT systems in a common frequency range without the need for frequency planning
  • Compatibility of equipment from different manufacturers (if GAP is available)
  • Ensuring transition from cell to cell without breaking the connection (handover)
  • Possibility of servicing one handset per different networks(private and public)
  • Ensuring high traffic - up to 10,000 Earl/km.
  • Compatible with other radio systems
  • No control channel - immunity to radio interference
  • Low level radiation - health safety

Modern PBXs provide functions that provide working together mobile phones with PBX, and these functions are performed as if the user mobile phone is an internal user of the PBX. This will allow you to achieve true mobility in your work and will be especially useful in such areas as real estate agencies, insurance companies, law firms, delivery services, construction business.

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 fulfill this task: the English CT2 standard and the Swedish CT3 standard. 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) is a digital wireless communication standard. 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 specifications DECT standard

Table 1 Main technical characteristics of the DECT standard

Operating spectrum (DECT frequency)

1880..1900 MHz

Number of carrier frequencies

Frequency spacing

1.728 MHz

Access method

MC/TDMA/TDD

Number of channels per frequency

24 (12 duplex channels)

Frame duration

10 ms

Data transfer rate

1.152 Mb/s

Modulation type

GMSK (BT = 0.5)

ADPCM

Acceptable signal-to-noise ratio

12 dB

Transmitter power

10 mW (average), 240 mW (peak)

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 of 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 others, 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 a new channel occurs. 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 signal strength of the base station, 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. The moment the signal from the new base station becomes stronger signal 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 in office systems is 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.


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