A cordless telephone is a telephone that features a portable handset which communicates with a base unit using radio waves. The base unit (or base station) is connected to a Plain Old Telephone System (POTS) line, just like an ordinary telephone. The handset is powered by a rechargeable battery and can be used within a range of a few hundred feet of the base unit.
Cordless phones were first introduced around 1980. The initial models had a frequency of only 27 megahertz and had many drawbacks including:
- Too much interference.
- Bad quality.
- The ability to use a neighbour's base unit without his knowledge.
- Easy eavesdropping using radio sensors.
Due to these concerns, the FCC (Federal Communications Commission, a United States government agency) set out rules in the year 1986 that all cordless phones should have a frequency range between 47 to 49 megahertz, because higher frequencies mean less noise. However, many other appliances that came into use began using similar frequencies, necessitating an increase in the frequency range to 900 megahertz in 1990. At this time, the cost was still high, at about 500 USD.
Further advancements came about five years later with the introduction of digital cordless phones (1994) and a technology called Digital Spread Spectrum (DSS). DSS uses "frequency hopping" through which the audio signal can be spread out over a wide range of frequencies in such a way that only its own base unit will be able to receive the signal, hence making eavesdropping extremely difficult.
The frequency range was increased to 2.4 gigahertz in 1998 by the FCC, thus further increasing quality and security. The 5.8 gigahertz band is commonly used by cordless phones today. The cost of today’s cordless phones has gone down to below 100 USD.
In Europe standards appeared in the early 1990s when the European Telecommunications Standards Institute (ETSI) introduced the first Digital Enhanced Cordless Telecommunications (DECT) standards in 1992. Recently, the 1.9 gigahertz bandwidth was allocated for cordless phones under the DECT standard.
Cordless telephones can operate over a wide range of frequencies ranging from just 1.7 megahertz to more than 5 gigahertz. However, most of today’s cordless phones use one of the following frequencies:
- 900 MHz
- 1.9 GHz
- 2.4 GHz
- 5.8 GHz
Analogue cordless phones usually do not beyond the 900 MHz frequency band, as most phones beyond that band are digital.