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How Cordless Telephones Work


Cordless phones have many of the same features as standard telephones, and there are many models, offering lots of different features.

Major Features

Remember that a cordless telephone is a combination of a telephone and a radio transmitter/receiver. Because it is a radio transmitter/receiver, you have the following issues that you do not have on a standard cord phone:

  • range
  • sound quality
  • security

The range is the distance that the handset can be from the base. The sound quality can be affected by the distance, the way the information in the radio signal is transmitted, and interfering structures such as walls and appliances. Security is an issue because the radio signals from both handset and receiver go over the open airways, where they can be picked up by other devices (other cordless phones, baby monitors, radio scanners).

The above issues relate to the following features of your cordless phone:

Frequency Because your cordless phone is a radio transmitter/receiver, it operates on various radio frequencies, which are set by the Federal Communications Commission (FCC) as with any other radio. Cordless phones operate over three major frequency bands (base and receiver use two closely related but separate frequencies within the band so that you can talk and listen at the same time):

  • 43-50 MHz
  • 900 MHz
  • 2.4 GHz
  • 5.8 GHz

The 43-50 MHz band was common to early cordless telephones and is still available in low-cost models. Because of the low frequency, these phones have short ranges (about 1,000 ft / 330 m) and poorer sound quality (due to interference from structures and appliances). The 43-50 MHz phone signals can also be picked up easily on radio scanners and nearby baby monitors.

The 900 MHz band (actually 900-928 MHz) is the most common frequency for cordless phones today. The higher frequency gives it a greater range (5,000 to 7,000 ft / 1,500 to 2,100 m) and better sound quality. However, 900 MHz signals can be picked up easily by most commercially available radio scanners.

In 1998, the FCC opened up the 2.4 GHz range for cordless phone use. A 2.4 GHz or 5.8 GHz cordless phone can operate over a greater distance and is above the frequencies that can be picked up by most commercially available radio scanners; therefore, it is more secure than lower frequency models.

Analog vs. Digital Analog technology is common in cordless telephones, especially in inexpensive models. Analog signals tend to be more noisy, or prone to interference with respect to sound quality. In addition, analog signals are easily picked up and interpreted by radio scanners.

In contrast, digital technology, like that found in a CD, allows the phone signals to sound clearer. Furthermore, digital signals are more secure. In 1995, digital spread spectrum (DSS) was introduced for cordless phones. DSS spread the digital information in pieces over several frequencies between the receiver and the base, thereby making it almost impossible to eavesdrop on cordless phone conversations.

Channels Each frequency band (43-50 MHz, 900 MHz, 2.4 GHz or 5.8 GHz) can be subdivided into different increments or channels. For example, on some models, when you're talking on your 900 MHz phone, the base searches for a pair of frequencies (channels) within that range, that is not already in use, in order to talk to the handset. So, if the base is capable of searching more increments, it can more easily find a frequency pair that is clear from interference, providing better sound quality. The number of cordless phone channels can vary as follows:

  • 10 to 25 channels - 43-50 MHz phones, some inexpensive 900 MHz phones
  • 20 to 60 channels - most 900 MHz phones
  • 50 to 100 channels - high-end 900 MHz and 2.4/5.8 GHz phones