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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING What is DECT. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Telephony The creation of the DECT standard (Digital Enhanced Cordless Telecommunications - a flexible digital standard for wireless communication) can be considered June 1992, when it was approved by the European Standards Institute ETSI. Now it is adopted in more than 100 countries around the world. DECT standard equipment has a wide range of applications: from single-cell (Fig. 1) home cordless phones to multi-cell wireless PBXs (Fig. 2; subscriber terminals in the foreground, followed by two base stations and a controller) and wireless access systems. The standard defines a radio access technology for transmitting information in digital form over a distance of up to 10 km. It supports voice and facsimile communication, data transmission.
The standard is based on the use of modern digital methods for processing and transmitting information. These are, first of all, time division multiple access TDMA (Time Division Muptiple Access), which provides a low level of radio interference and a large system capacity (up to 100 thousand users), adaptive differential pulse-code modulation ADPCM (Adaptive Differential Pulse Code Modulation), which allows to obtain high voice quality, dynamic channel selection / dynamic channel selection DCS / DCA (Dynamic Channel Selection / Allocation), providing the ability to select the best available radio channel in any mode of operation of the wireless device and providing stable communication, protected from unauthorized listening. The DECT standard contains protocols that define interaction with networks such as ISDN and GSM, as well as a protocol that ensures the compatibility of wireless subscriber terminals produced by different manufacturers. The advantage of the systems operating in this standard is that, according to the decision of the State Committee for Radio Frequencies of the Russian Federation of August 26.08.97, 39 (Protocol 7/XNUMX), they can work without issuing a special permit for the use of allocated frequencies. DECT systems provide support for a subscriber within a given network, while synchronization of all radio ports of base stations operating within this network is important. To transmit information about the location of the subscriber, a two-tone multi-frequency code (DTMF) is used. When using radio access technology that provides mobility, it is necessary to solve the problems of security of transmitted information. The DECT standard provides in this regard the necessary measures to counteract possible attempts of unauthorized access and listening. For example, it introduces efficient registration and authentication protocols and improved encoding of transmitted information. Let's explain what "registration" means. This is the process by which the system accepts a particular mobile DECT handset for service. The network operator, or service provider, informs the user of the DECT handset (hereinafter referred to as the subscriber radio unit (EPR)), the secret registration key (PIN code), which must be entered both in the base radio unit (BRB) and in the EPIRB before start of the registration procedure.The PIN code can usually be applied only once, which minimizes the risk of unauthorized access.After radio communication is established, it is determined that the same key was used by both parties.Then, by exchanging identification information, both parties calculate a secret authentication key that is used every time a connection is established. A mobile DECT handset can be registered at several base stations. At each registration session, the EPIRB calculates a new authentication key associated with the network to which it registers. New keys and 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 a special list). Authentication of the DECT handset can be carried out as a standard procedure every time a connection is established. This happens as follows: BRB sends a random number to the handset, which is called a "request". In the EPIRB, a "response" is calculated, consisting of a combination of the authentication key with the received random number, and transmitted to the base station. It also calculates the expected "response" and compares it with the received one. Depending on the result of the comparison, either the connection is continued or the connection is disconnected. If someone wants to eavesdrop on information transmitted over the air interface, then in order to steal the authentication key, he needs to know the algorithm for determining the key from the "request" and "response". Determining the "reverse" algorithm is a time-consuming and rather expensive procedure. Ensuring the secrecy of transmitted information is possible by encrypting data during transmission. In this case, the same encryption key can be used as in the EPIRB authentication. The receiving party uses it to decrypt the information. This process is part of the DECT standard (though not mandatory). For DECT standard systems, the range 1880 ... 1900 MHz is allocated. Ten of its frequencies are used, each of which includes 12 duplex channels with time division. The interval between adjacent frequencies is 1,728 MHz. The transmission time is divided into 10-ms frames consisting of 24 timeslots. Each subscriber is allocated one frame for transmission and one frame for reception (duplex, but not frequency, as in analog systems, but temporary). Average radiated power - 10 mW (peak - 250 mW). Unlike cellular systems, where the decision to allocate a channel is made by the base station and the channel is allocated for the entire duration of the connection, in DECT the channel is selected by the handset, and during the conversation this channel changes almost continuously. This mode of operation is called continuous dynamic channel selection - CDSC. The BRB continuously transmits on at least one channel a signal containing the service information necessary to establish a connection, thus acting as a beacon for the EPIRB. The transmission may be part of an active connection, or it may be idle. The EPIRBs connected to the beacon transmission will analyze the transmitted information and determine if they have access rights to the system, if the system capabilities match the services required by the EPIRB, and if the EPIRB has free capacity to establish radio contact with the EPIRB. All DECT equipment regularly scans all free radio channels at least once every 30 seconds. The scan measures the local RF signal. This process runs as a background process and creates a list of free and busy channels (RSSI list: Received Signal Strength Indication), one for each timeslot/carrier combination that will be needed when selecting a channel. The free time slot is not yet used for transmission or reception. In the RSSI list, low signal strength values indicate free channels with no interference, and high values indicate busy or noisy channels. With the help of the RSSI information, the EPIRB or RBR can select the optimal (least interference) channel for establishing a new link. The channels with the highest RSSI values are constantly analyzed in the EPIRB to verify that the transmission originates from the base station to which it has access rights. The operation of the EPIRB is synchronized with the BRB having the strongest signal, as defined by the DECT standard. The channels with the lowest RSSI values are used to establish a radio connection with the BRB if the EPIRB user decides to establish a connection, or in the case when an incoming call is signaled to the DECT mobile handset via the reception of a page message (not for voice transmission). In a DECT base station, when selecting a channel to establish a transmission to a beacon, channels with low RSSI values are used. The dynamic channel selection and allocation mechanism ensures that communication is always established on the cleanest available channel. Due to the use of CDCS in DECT systems, frequency planning is not required. The solution to this problem is shifted to the EPIRB, which makes the installation of such systems a simple procedure, and also allows, if necessary, to increase the total number of channels by simply adding new base stations. Now let's consider how the call is made from the subscriber (outgoing communication). It has already been said that the radio selection initiative in basic DECT applications always belongs to the EPIRB. Using dynamic channel selection, it selects the best available channel and communicates with the BRB over it. In order for the FRB to detect attempts to establish communication from the side of the EPIRB, their work is synchronized with the help of service information constantly transmitted by the base station. Based on this information, the EPIRB can determine exactly when it is possible to successfully access the FRB on the selected channel. In order for the EPIRB to use all ten DECT RF carriers, the EPI continuously scans its idle receiving channels sequentially for attempts by the EPIRB to establish communications. When an incoming call arrives at the EPIRB, the access network sends the appropriate identifier to it on the paging channel. The EPIRB, having received a paging message with its identifier, establishes a radio bearer to service the incoming call using the same procedure that is used when establishing an outgoing call. When a connection is established in DECT systems, a transition from cell to cell is ensured without breaking the connection. The process by which this occurs is called "handover" and is a mechanism for avoiding interference-prone or low-signal channels. The EPIRBs can move away from the interfering connection by establishing a second connection in parallel on the newly selected channel. First, both radio connections are maintained, identical voice information is transmitted over them, and the quality of the connection is analyzed. Then, after some time, the base station determines which connection has the best quality, while freeing up the other channel. And, finally, a little about the prospects for the development of the standard. DECT is strengthening its positions. Today, the market share of DECT-phones among all wireless phones for home is 53% in Europe. Among all the standards used to create wireless access networks (Wireless local loop - WLL), it is in the lead, occupying 32% of the total number of installed lines. Total sales in 2000 are expected to be about 30 million terminals. Promising for the development of DECT is the possibility of interoperability of this standard with GSM (since they are both based on digital transmission using TDMA technology). Combining the capabilities of DECT systems, which process heavy traffic, but impose a restriction on the speed of movement of the subscriber, and GSM, which have a lower capacity, but provide subscribers with the opportunity to travel in a car or train and provide roaming, made it possible to create a dual-mode subscriber terminal. It works in the DECT standard while the subscriber is in the radio coverage area of the DECT system in which he is registered. As soon as the subscriber leaves this area, the radiotelephone automatically switches to work in the GSM standard. The integration of the two standards should play a role in the personalization of communications. One of the possible applications of DECT - the organization of subscriber radio access - allows operators to minimize initial costs and gradually increase network capacity at the expense of revenues received from its operation, which will also contribute to the popularity of the standard. When implementing DECT systems, we recommend that you familiarize yourself with the documents regulating this activity. This is the decision of the State Committee for Radio Frequencies of Russia dated August 26.08.96, 39 (Minutes No. 7/1880) "On the use of the frequency band 1900-128 MHz for DECT technology wireless telephone communication equipment"; order of the Ministry of Communications of Russia No. 13.11.96 of November 27.04.98, 6 "On the procedure for introducing DECT equipment on Russian telecommunication networks"; decision of the State Committee for Radio Frequencies of Russia dated April 2, 1880 (Minutes No. 1900/134) "On the use of the radio frequency band 11.08.98-XNUMX MHz for DECT equipment"; order of the State Communications Committee of Russia No. XNUMX dated August XNUMX, XNUMX "On the procedure for introducing DECT equipment in Russian telecommunication networks."
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