Cordless landline telephones

Cordless telephones for house and garden transmit conversations between a base station connected to the telephone landline and a portable talk/listen device (handset) via radio. In Germany, now only devices that use the DECT standard (Digital Enhanced Cordless Telecommunications) devices may be operated. DECT base stations can also be integral parts of routers or cable modems. The latter are not connected to the telephone landline network but to a broadband cable network.

Cordless DECT landline telephone

The DECT standard

The DECT standard has originally been an industry standard allowing the setting up of wireless telephone systems in cellular structures that are similar to those used in mobile communications.

In order to ensure the smooth operation of the handsets, the base station transmits a continuous control signal. Devices in which the control signal can be deactivated during periods without an active call connection (standby mode) are becoming increasingly available. This contributes to reducing the exposure of the user.

DECT telephones transmit voice and data between the base station and the handset by means of high-frequency electromagnetic waves. The frequency range employed by DECT is between 1800 and 1900 megahertz (MHz). Similarly to GSM mobile communications, the DECT standard uses a time slot technique. This enables several users to make phone calls over one base station at the same time. A time frame of ten milliseconds (ms) is split up into 24 time slots with a duration of 0.42 ms each. A particular handset transmits data to the base station in only one of the 24 time slots during a phone call. In the remaining time slots, the other handsets can exchange data with the base station. This results in a pulsed transmission signal with a repetition frequency of 100 hertz (Hz) for the individual handset.

Transmitter power and SAR value

The maximum transmitter power of base station and handset is 250 milliwatts (mW) each. The outdoor range is thus limited to about 300 metres. On account of the time slot technique, the time-averaged radiated power during a phone call results in a maximum of 10 mW. If several handsets are connected to a base station, the average radiated power of the base station may increase accordingly.

Investigations to date show that the specific absorption rates (SAR) for conventional DECT handsets are under 0.1 watt per kilogram (W/kg). This value clearly falls below the limit value of 2 W/kg for partial body exposure recommended by the German Commission on Radiological Protection (SSK) and the International Commission on Non-Ionizing Radiation Protection (ICNIRP).

Power control in DECT telephones

Unlike in the technical standard for mobile phones, an automatic transmitter power control is not a mandatory specification of the original DECT standard.

In devices without automatic transmitter power control, the base station and the handset always transmit at maximum power during a phone call, irrespective of how far away the user is from the base station. Modern devices can progressively adapt their transmitter power as required. The transmitter power of base station and handset can additionally be reduced manually for some devices.

Precautionary measures for minimising radiation exposure

According to current knowledge, cordless phones using DECT technology do not present a hazard to human health. If possible, however, preventable exposures should be reduced for general precautionary considerations. Anyone who would like to avoid exposure from cordless telephones can fall back on corded landline phones. When using cordless devices, the exposure can be minimised by simple measures:

• Devices

  • which do not emit any control signal in standby mode,
  • where the user can limit the range to the necessary level and can thus reduce the radiated power,
  • which automatically adapt the current radiated power to the requirements

  are recommended.

• Place the base station in a location suitable for radio transmission where individuals are not permanently present, for example in the corridor.
• Use the speakerphone function.
• Anyone who would like to avoid continuous head exposure to electromagnetic fields should keep phone calls short while using the handset.

Blue Angel (German ecolabel) for digital cordless telephones

For some time now, manufacturers have been able to have their products labelled with the ecolabel "Blue Angel" if they have specific technical and other characteristics. The ecolabel for digital cordless telephones mainly focuses on the minimisation of energy consumption and the precautionary reduction of exposure to radiofrequency electromagnetic radiation. Further information is available from the website of the Blue Angel.