At the time when the DECT standard (Digital Enhanced Cordless Telecommunications - flexible standard for digital wireless communications) to read the June 1992 when it was approved by the European standards Institute ETSI. Now it is accepted in more than 100 countries.
The DECT standard equipment has a wide range of applications: from odnoslotovyj (Fig. 1) wireless home phones to mnogolistovyh wireless PBX (Fig. 2, at the forefront of user terminals, two base stations and controller) and wireless access systems. The standard defines technology radio access for transmission of information in digital form at a distance of 10 km. supports voice and facsimile communication, data transmission.
The standard is based on the use of modern digital processing techniques and information transfer. This is primarily multiple access temporary division TDMA (Time Division Muptiple Access) provides low level radio interference and the capacity of the system (up to 100 thousand users), adaptive differential pulse code modulation ADPCM (Adaptive Differential Pulse Code Modulation), allowing to obtain high quality voice transmission, dynamic channel selection/dynamic allocation of channel DCS/DCA (Dynamic Channel Selection/Allocation, which States that the choice of the best available radio channels in any operation mode of the wireless device and providing stable connection, protected from unauthorized eavesdropping.
The DECT standard provides protocols defining the interaction with these networks, as ISDN and GSM, as well as a Protocol for interoperability between wireless subscriber terminals, produced by different manufacturers. Dignity systems operating in the standard, is the fact that, according to the decision of the SCR The Russian Federation of 26.08.97 G. (39/7 without any response from the Protocol), they can work without registration a special permit for the use of the allocated frequencies.
In DECT systems and accompanied by a subscriber within the network, when this synchronization is important for all radioport base stations operating in the boundaries of the network. For transmitting location information of the subscriber is used dual tone multi-frequency code (DTMF).
In the application of the radio access technologies that provide mobility, have to solve the problems of security of the transmitted information. The DECT standard provides in this respect the necessary measures to counter possible the attempts of unauthorized access and eavesdropping. For example, it introduced effective protocols for registration and authentication, and advanced the encoding of the transmitted information.
Please explain what is "registration". This is the process by which the system takes specific DECT mobile handset to the service. The network operator, or the service provider informs the user DECT handset (we will call her subscriber radio unit (ARB), the secret key registration (PIN), which should to be entered in the base radio unit (BRB) and ARB prior to the procedure a residence permit. The PIN, as a rule, can be applied only once, which minimizes the risk of unauthorized access. After establishing radio communication with the two sides determined that was applied to the same key. Then, exchanging identification information, both sides calculate secret an authentication key that is used when establishing a connection.
Mobile DECT handset can be registered at multiple base stations. When each session registration ARB calculates a new authentication key that is bound to the network in which it is prescribed. New keys and identification information network added to the list stored in the ARB, which is used in the process connection. The tube can only connect to the network in which they have access rights (identification information of the network are contained in the special list).
Authentication DECT handset can be carried out as standard procedure when each communication. This occurs as follows: BRB sends tube a random number called a "request". In ARB calculated "response", the combination of the authentication key with the received random number, and transmitted to the base station. It also calculates the expected "answer" and compares it with the received. Depending on the result of the comparison occurs or the continuation of the communication, or separation.
If someone wants to eavesdrop transmitted over terrestrial interface information, he has to steal authentication key, you must know the algorithm for determine the key of "request" and "response". The definition of "reverse" algorithm - time-consuming and costly procedure.
Ensuring privacy of information transmitted may when encrypting data in the transmission process. This can be applied the same encryption key, and when authenticating ARB. The receiving party uses it to decrypt the same information. This process is part of the DECT standard (but optional).
For systems DECT standard selection 1880...1900 MHz. Used ten its frequencies, each of which includes 12 duplex channels division time. The spacing between adjacent frequencies is 1,728 MHz. The transmission broken down into 10-millisecond frames, consisting of 24 timeslots. Each the subscriber is allocated one frame for transmission and one for reception (duplex, but not only the frequency, as in analog systems, and temporal). Average radiated power is 10 mW (peak 250 mW). Unlike cellular systems, where the decision on the allocation of the channel receives the base station and the channel is allocated to the duration of the connection, DECT channel selection is performed by the tube, and in the process conversation this channel is changing almost continuously. This mode of operation called a continuous dynamic channel selection - CDSC.
BRB continuously transmits at least one channel signal containing technical information needed to establish the connection, speaking so way as a beacon to the ARB. The transmission can be part of an active connection, and can be blank. ARB connected to the transfer of the lighthouse, analyze the transmitted information and determine whether they have access rights to the system, whether the capabilities of the system services required by the ARB, and does BRB the free capacity to establish radio communication with ARB.
All equipment DECT regularly scans all available radio channels at least one every 30 C. When scanning is measured local RF signal. This the process is carried out as a background and creates a list of free and busy channels (list RSSI: Received Signal Strength Indication - Indication power the received signal), one for each combination of time slot/carrier", which will be required when you select the channel. Free same time slot until it is used to transmit or receive. In the list of low RSSI values of power signal mean free channels without interference, and high values are busy channels or channels with interference. Using the RSSI information or ARB may choose BRB the optimal (least interference) channel to establish new lines of communication.
The channels with the highest RSSI values are continually evaluated in ARB to check that the transmission is coming from the base station to which it has rights access. The work of the ARB is synchronized with BRB having the strongest signal, as defined by the DECT standard. The channel with the lowest RSSI values are used to establish radio communication with BRB, if the user ARB decides to establish a connection, or in the case where the mobile DECT handset transmit the signal of an incoming call through the reception of paging messages (not for speech).
In DECT base station when selecting a channel for establishing transmission of the beacon channels are used with low RSSI values. Dynamic choice and highlight a channel ensures that the connection is always established on the net from the available channels.
Thanks to the use of CDCS in DECT systems do not require frequency planning. The solution to this problem is shifted to the ARB, which makes the installation of such systems a simple procedure, but also allows to increase the total the number of channels by simply adding new base stations.
Now consider how the call (outgoing call). Already it was said that the initiative selection in basic telecommunication applications DECT always belongs to the ARB. Using dynamic channel selection, it selects the best of available and binds it with BRB. To BRB found attempts to establish communication from the ARB, their work is synchronized with constantly the transmitted base station service information. On the basis of this information ARB can determine the exact moment when a successful access on BRB the selected channel. To ARB could use all ten radio frequency carrier DECT, BRB consistently constantly scans your unoccupied channels in search of attempts ARB to establish a connection.
When an incoming call on the ARB access network sends an appropriate identifier for the paging channel. ARB, taking paging message with its ID, sets the radio channel for maintenance an incoming call, using the same procedure that is applied by establishing uplink.
When establishing a connection in DECT systems ensures the passage of cells in the honeycomb without any disconnection. The process by which this occurs is the name "handover" and is a mechanism of avoiding channels that are prone to interference, or channels with weak signals. ARB can get away from compounds containing interference, parallel to establishing the second connection to the newly selected channel. First, it supports both the wireless on them is transmitted is identical to the speech information and analyzes the quality of the connection. Then after some time, the base station determines which the connection has the best quality, while freeing up another channel.
Finally, a few words about the prospects for the development of the standard. DECT strengthens its position. Today the market share of DECT phones among all cordless phones for the home in Europe is 53%. Among all the standards used for networking wireless access Wireless local loop - WLL), he is a leader, occupying 32% of the the total number of installed lines. It is expected that the total volume of sales in 2000. approximately 30 million terminals.
Promising for the development of DECT is the possibility of this interaction standard GSM (since they are both based on digital transmission technology TDMA). Integration capabilities of DECT systems, manufacturing busy traffic, but imposing a limit on the speed of movement of the subscriber, and GSM with less capacity but providing subscribers with the ability to move in the car or train and provide roaming allowed to create two-mode user terminal. He works in the DECT standard, while the subscriber is in the area of cellular DECT system in which he is registered. Once the subscriber leaves the area, the phone automatically switches to work in the GSM standard. The integration of the two standards have a role to play in personalization of communication.
One of the possible applications of DECT - organization of subscriber radio access - allows operators to minimize initial costs and gradually increase the network capacity due to the revenues derived from its operation, which will also be contribute to the popularity of the standard.
When introducing systems of the DECT standard we recommend you to read the documents, regulating this activity. It is a decision of the SCR from Russia 26.08.96 G. (Protocol No. 39/7 without any response from) "On the use of the frequency band 1880-1900 MHz for equipment wireless telephone DECT technology"; decree of the Ministry of communications of Russia No. 128 of 13.11.96 G. "On the procedure for introduction of DECT equipment on the Russian networks telecommunication"; decision of the SCR from Russia 27.04.98 G. (Protocol № 6/2) "On the use of the frequency band 1880-1900 MHz for DECT equipment"; order The state Committee of Russia No. 134 of 11.08.98 G. "On the procedure for introduction of DECT equipment on the Russian telecommunication networks".