In order to understand how cordless telephones work, it's important to understand the technology they replace. From its inception in the 1870s, a standard telephone has actually contained two cords. One leads directly to a phone company's closest relay station through a grid of wires supported on poles. This cord also provides a constant source of direct current from batteries located in the phone company's facilities. Whenever an incoming call is received, the direct current powers the ringer until the phone's owner breaks the connection by picking up the handset.
This handset contains the second cord in a standard phone. Once the caller picks up the handset, the cord leading to the base completes a circuit and the handset begins to send and receive low-powered electrical impulses. The mouthpiece contains sensitive components which convert the vibrations of a human voice to electrical impulses. An earpiece takes these electric vibrations and turns them back into recognizable speech through a small amplifier.
The problem is that a corded phone must remain physically connected to the phoneline in order to function. Longer cords could be purchased in order to give the phone's owner a little more flexibilty, but this isn't always a convenient solution. Wires can become damaged by the exposure, and they could also become a tangled mess or a tripping hazard for children.
A cordless phone solves both the flexibility problem and the dangers of cords. A typical cordless phone system consists of a base unit, which still connects to the phoneline through hardwiring, and a battery-powered handset, which may contain the dialer and volume controls. Additional features such as voice mail or answering machine are usually built into the base unit itself. These extra features can require additional electrical power, so most base units are also connected to an outlet through a power converter. This is an important element to consider, because many cordless phones will not function at all without electricity. Standard phones are powered remotely by batteries located at the phone company.
Once a cordless phone is properly connected to the jack and an external outlet, the handset's battery must be fully charged. This battery powers what is essentially a minature radio station in the handset itself. The handset also contains a strong antenna which is tuned to a specific radio frequency. The base unit receives these radio waves and converts them to an electronic medium suitable for the phone company's needs. Because the handset's power is limited to a specific area of broadcast, the user can be several hundred feet away from the base unit's receiver without a noticeable drop in reception. A cordless phone is often advertised as possessing a certain amount of power, typically 900 Mhz or greater. This does not mean that the phone's range is dramatically larger, but the vocal clarity may be better with more megahertz available.
A cordless phone is not the same as a wireless cellular system, although they may share a few technologies. Cordless phones must still be hardwired into a standard landline in order to work. The radio signal from the handset is sent no further than the base unit's receiver. Once the base unit receives the dialing information and the call is connected, a cordless works much like a standard model. Electrical impulses are carried along a wire or glass fiber, connected to the other party through a switcher, and received as electronic signals at the other end.
A cordless phone is most vulnerable to illicit eavesdropping within the range of the handset's transmitter. Unscrupulous people could conceivably match the frequency of the phone's transmitter and hear the conversations, provided they and their equipment were located within a few hundred yards of the phone's location. Once the phone's base unit transmits the call to the landline, the system becomes closed. Only a physical connection to the landline itself would provide any information for would-be wiretappers. Early cordless phones did transmit in a frequency range just above the AM radio band, as did wireless baby monitors. Some commercial radios could be 'detuned' to pick up conversations held on cordless phones. Since then, however, the technology has been changed to discourage illicit activities. Modern cordless phones often transmit at frequencies which are difficult to pick up without specialized equipment. Users can also select other channels to improve vocal clarity or discourage illegal listening. Some cordless phone systems will also electronically scramble the signal between the handset and the base unit. The base unit will then unscramble the signals and send them into the closed phone system.
