Digital Enhanced Cordless Telecommunications

Digital Enhanced Cordless Telecommunications (DECT, Digital European Cordless Telephony originally ) is an international standard for Telecommunications by means of wireless technology, especially for cordless phones. DECT is a trademark of European Telecommunications Standards Institute (ETSI ) with extensive specifications.

4.1 electrosmog
4.2 ECO DECT

6.1 Safety risk with and without encryption

History

DECT was introduced in 1993, a year after the alternative CT2. The initiative came in 1985 from the Conférence Européenne des Administrations des Postes et des Télécommunications (CEPT ), while developed in the UK with CT2 and CT3 in Sweden with first digital technology for cordless phones. In the fall of 1987, the CEPT had a demonstration prototype, decided in January 1988 but for its own approach and handed the project to the newly founded ETSI, where the specification was in 1991 drafted by mid-year and adopted in March 1992 after public consultation. In the winter of 1989/1990, the CEPT, the frequency band 1880-1900 MHz for DECT decided to provide Europe. In June 1991, the Council of the European Union pointed to the Member States to protect the latest from New Year's Day 1992 this frequency band for DECT, to ensure the implementation of DECT by the end of 1992. In summer 1994, a license for the operation of DECT in this frequency band followed the recommendation of the CEPT, Europe demand. In other regions, different frequency bands for DECT were elected, some of which require a license. In Germany, for 2009 went to the type of other cordless phones, because their frequency bands were assigned elsewhere.

Areas of application

DECT operates basically connection-oriented and is primarily designed for so-called picozellulare telephony within buildings, where a range or a cell radius can be reached from 30 to 50 meters; outdoors are transmission lines of 300 meters are possible. The maximum allowed output power is 250 mW.

In contrast to mobile radio systems, DECT is a pure access technology, each with small number of participants ( 1 to 6) on a common base station to a suitable base network. DECT does not describe the network itself, the connection is connected to a gateway, which is commonly referred to as a base station. In most cases, the conversion takes place in the public telephone network. Newer technologies such as IP telephony are also available on the market. However, there are devices in which no gateway exists in a network, such as baby monitors.

DECT used below 2.45 GHz frequency ranges other than Wi-Fi, Bluetooth, etc., and therefore does not interfere with these networks. For transmitting voice of the G.726 codec is used, which has a bit rate of 32 kbit / s uses here. It takes into account the low data rates and the limited computational power of mobile devices that were available in the standardization of DECT.

Mobility characteristics

DECT supports short-term mobility with specified quality and good intelligibility. The change of the base station within a multi-cellular radio network is done by forwarding to another cell ( automatic handover ) and long-term mobility is achieved by logging into a foreign network ( automatic roaming ). Alternatively, done manual log-in to another base station or other base network. This mobility properties are not fully standardized and require type conditionally appropriate precautions in the terminals or manual settings.

In the absence of mobility can be bridged in conjunction with a directional antenna or with reduced mobility at close range with repeaters a distance of several kilometers.

Data transfer properties

The features for data transfer are different for DECT devices depending on the device type and device family based on standards of the ITU or IETF.

Radio transmission and frequencies

The transmission is based on a Time Division Duplex and Time Division Multiple Access and Frequency Division Multiple Access method and works in Europe in the frequency range of 1880 MHz to 1900 MHz, in which 10 channels are defined, each with 1728 kHz bandwidth. But also ETSI specified extension bands in the areas of 1900-1980 MHz, 2010-2025 MHz and 2400-2480 MHz.

DECT uses a frame of 10 ms duration is divided into 24 time slots. Each time slot can be used both in the uplink and in the downlink. By coupling of time slots and asymmetric transfer rates up to a ratio 23:1 are possible.

In that time slot of 416.7 microseconds duration, a burst is sent, which typically takes 368 microseconds and contains 424 bits. This results in a bit duration of 868 ns and a bit rate of 1.152 MHz.

Modulation Gaussian Frequency Shift Keying ( GFSK ) is used. A binary one is transmitted by a frequency increase of 288 kHz, a binary zero by a frequency reduction of 288 kHz. Stable in radio communications and a 4 -level or 8 -level modulation can be used, which are transferred in each step 2 or 3 bits.

The 424 bits of a burst can be divided into the following fields:

32 bits synchronization (S- box )
388 bits of data (D - box ), which 64 bits header field (A- field)
320 bits of user data (B- field)
4 bits for determining the channel quality (X field)

The resulting standard data rate of the payload is 32 kbit / s, which is in both directions.

In addition to the normal Basic burst to 424 bits in 368.1 microseconds there are three more:

Short burst with 96 bit in 83.3 microseconds at the beginning of a time slot. This burst can be used, for example, when the base station transmits no conversation, but still needs to broadcast their identifier.
Low Capacity burst with 184 bit in 159.7 microseconds. This burst is only half of a time slot, so that two bursts can be sent within a time slot. The B-field for the payload decreases while but disproportionately from 320 to 80 bits, so that the data rate is reduced to a quarter.
High Capacity burst with 904 bit in 784.7 microseconds. This burst occupies two time slots and always starts on an even time slot. The B-field increases to 800 bits, so that the net data rate is increased by a factor of 2.5.

The choice of transmission frequency and time slot in DECT is done always by the mobile device.

DECT provides dynamic channel selection and allocation. For this purpose all DECT devices cause an RSSI list ( Received Signal Strength Indication ). At regular intervals ( at least every 30 seconds) to scan all the idle channels and added to the list. If a new channel is required, the mobile device or the base station selects the channel with the least interference based on the RSSI list.

Health aspects

Electrosmog

The maximum transmission power of base station and the handset is in each case 250 mW. For smooth operation sends a conventional DECT base station outside the talk time permanently pulses to allow the mobile device synchronization. For short bursts can be used having only one quarter the length of the normal burst so that the average transmission power is reduced accordingly. The maximum permissible power remains unchanged, the actual output power is kept constant to the handset or other reception conditions for simple devices regardless of the distance from the base station. With modern instruments, the transmission power is controlled to a minimum required value down. DECT devices without such a scheme are due to high electromagnetic stress in the criticism.

The average radiated power of a DECT device is max. about 9 mW. This results in radiation exposure at an average of less than 0.1 W / kg, which is far below the recommended limit of 2 W / kg for medium -specific absorption rates (SAR ). The radiation exposure during pulsed bursts, however, are higher by a factor of 27 and thus exceed (albeit for only short periods of time ) the allowable limit. Since orient the electromagnetic limits in Europe exclusively on the mean absorption rate for the underlying stress model of heating of human tissue, even this favorable value is considered.

Possible effects that are not caused by global warming as may occur, for example due to resonance effects in cell structures are not taken into account in the exposure model. So far, other effects besides the low heating ( thermal effect ) of non-ionizing radiation, as it also represent the frequencies of DECT, not been investigated in numerous studies either or not detected. Nevertheless, the Federal Office for Radiation Protection ( BfS) was published in a press release dated 31 January 2006, the following recommendation:

" In order to avoid potential health risks, the BfS recommends to minimize personal exposure to radiation through their own initiative."

Long calls are often perceived by users as unpleasant, because the local heating close to the handheld device can be felt. To avoid this and to take the guess by BfS own initiative, the following measures are recommended:

Avoid long phone calls with the handset on the ear (for example, by using the speaker function, headphones or a wired headset)
Setting up the base at a place where you do not permanently resides (for example, in the hallway instead of directly on the bedroom or workplace)
Selection and use of DECT equipment that are particularly low radiation
Selection and use of base stations that send pulses when the handset in the base station plugged

ECO DECT

Under the aspect of a precautionary reduction of radiation exposure, the Federal Office for Radiation Protection demanded in January 2006 to the manufacturer, that the base stations automatically switched off in stand- by mode and the phones are equipped with a targeted control of the transmission power. Low-radiation DECT telephones ("Low Radiation" ) reduce the transmission power of the handset when it is sufficiently close to the base station, and particularly criticized duration ends of the base station on-hook to the base station handset is finished. For this it is necessary, however, that a maximum of a handset is registered on that base station. For these functions, the term ECO DECT always find further spread. Meanwhile, devices with a "Full Eco Mode " on the market that only send when phoning or the phone rings. Some manufacturers refer to this mode as " Eco Mode Plus". The handset is in the normal state in a receive-only mode, only when a call is received, the base station establishes a wireless connection. This means that a pass or two ringing signals without the handset mitklingelt. Another disadvantage: The user can carry the handset from the range of the base station without realizing that no wireless connection is possible. They also reduce considerably with many devices in "Eco " mode, the standby battery life.

DECT ULE (Ultra Low Energy)

(DECT ULE - Ultra Low Energy ) In January 2011, an energy-saving variant of DECT radio was on the DECT World in Barcelona for the first time in a White Paper Sitel Semiconductors (now Dialog ) is presented. In the same year were announced first interoperability tests and founded in February 2013 its own umbrella organization, the ULE Alliance, the DECT Forum in Bern.

The power requirement of the DECT ULE standard is despite full transmit power at 250 mW in the microampere range and is therefore particularly suitable for battery-powered products. For the use of DECT ULE usually no new DECT base is necessary. Meanwhile, there are various commercial products in the field of security and home automation system that will use the new DECT ULE wireless. These include products such as remote-controlled ( "smart" ) sockets, motion and smoke detectors or door stations. In December 2013 it was announced that the ULE Alliance and ETSI have to joint development and marketing of the DECT ULE concluded an agreement standards.

Security

Unauthorized use and unauthorized monitoring are in DECT, as with other mobile radio systems also by three methods difficult, two of which are compulsory:

In addition, there is a third, optional method of the DECT specification, the encryption. It is not implemented in many devices and are often turned off by default in deployed devices. The user data (speech or data) to be coded during the radio link and decoded at the other end, wherein a key is used, the two remote stations is known, but is not itself transferred by radio. The encryption standard used is called DECT Standard Cipher. Eavesdropping is not guaranteed even when correctly implemented encryption.

Security risk with and without encryption

According to press reports, conversations held with little effort can be monitored via DECT. For example, it is a actually provided for VoIP PC Card, the Com- On-Air - map, it is possible to listen to a run over DECT conversation. This requires a special software on Linux is necessary, which was developed by members of the Chaos Computer Club and scientists at the TU Darmstadt. The wiretapped conversations are stored on the computer and can be digitally processed by the attacker. A primary criticism also that on the packaging of the device is not clear whether the phone has implemented an encryption or not.

The Federal Office for Security in Information Technology criticized the strength of the DECT encryption algorithm, it suffices It had been shown that the standard-compliant and properly implemented encryption with only 64 bit " far from today's requirements for secure encryption. " - At the performance of modern computer - could be broken in less time. In addition, critics bemoan the design weakness of DECT. How could an attacker the base unit, even without knowing the key, to send data cause, and then to analyze these for decryption, intercepted phone calls so do not need to. The BSI recommends that you use DECT phones only "where phone calls with sensitive content can be excluded and the use of a corded landline phone would require a disproportionate effort. " This applies both to encrypt and for nichtverschlüsselnde devices.

Technical problems

DECT and DECT base stations can interfere with the reception of certain channels of satellite TV, if the wiring from the LNB to the satellite TV receiver is not properly shielded, since DECT uses the same frequency range that is used for the signal transport of certain transponder between the LNB and the receiver. For analog satellite reception, for example via Astra 19.2 ° East, the intermediate frequency was 1891 MHz on the DECT frequency, and the 10 ms long DECT frame could cause noise bars in the TV picture. ( The " frequency " of DECT complies with 100 Hz exactly twice the 50 Hz field frequency of PAL. ) Transponder ( 11,641 MHz horizontal) has been empty for analogue switch-off. With digital satellite reception on Astra beta digital transponder ( 12.480 MHz vertical) is disturbed with the intermediate frequency 1880 MHz, which is why some of the programs of the ProSiebenSat.1 Group, which took advantage of the transponder, since April 2007 on a different transponder are emitted. Furthermore, on this transponder send but for example, Sport 1 and Tele 5

DECT GAP Profile

A subset of DECT, DECT -GAP (Generic Access Profile ) allows communication of DECT equipment from different manufacturers with each other. CAP is only one of the defined by ETSI profiles that help, DECT, which in itself virtually replaced the data cable to integrate into larger networks. While there used to be compatibility issues between devices from different manufacturers, DECT phones WITHOUT CAP few out.

Although CAP guarantees that a handset from the manufacturer to the base of another manufacturer works, but extends only to pure telephony, not on convenience functions such as listening to voicemail or browsing through the phone book. In addition, also for the mobile unit must first be registered to the base station. Because different logon procedures of the various manufacturers often can in this case difficulties arise.

Other profiles

Public Access Profile ( PAP), predecessor of the CAP
Radio in the Local Loop Access Profile ( RAP)
DECT Packet Radio System ( DPRS )
DECT multimedia profiles (DMAP)
Multimedia in the Local Loop Access Profile ( MRAP )
Data Service Profile ( DSP)
ISDN interworking profile ( IIPs )
CTM Access Profile ( CAP)
DECT / GSM Interworking Profile ( GIP)
DECT / UMTS Interworking Profile ( UIP)

Technical standard Specification (technical standard) International Telecommunication Union Internet Engineering Task Force Burst mode (computing) Universal Mobile Telecommunications System ISM-Band CAT-iq Institute of Electrical and Electronics Engineers Stiftung Warentest

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