Cordless Telephone - Frequencies

Frequencies

In the United States, seven frequency bands have been allocated by the Federal Communications Commission for uses that include cordless phones. These are:

• 1.7 MHz (1.64 MHz to 1.78 MHz & up to 5 Channels, AM System)
• 43–50 MHz (Base: 43.72-46.97 MHz, Handset: 48.76-49.99 MHz, allocated in November 1984 for 10 channels, and later 25 Channels, FM System)
• 900 MHz (902–928 MHz) (allocated in 1993)
• 1.9 GHz (1880–1900 MHz) (used for DECT communications outside the U.S.)
• 1.9 GHz (1920-1930 MHz) (developed in 1993 and allocated U.S. in October 2005, esp. re DECT 6.0)
• 2.4 GHz (allocated in 1998)
• 5.8 GHz (allocated in 2003 due to crowding on the 2.4 GHz band).

1.7 MHz cordless phones were the earliest models available at retailers, generally identifiable by their large metal telescoping antennas. Channels just above the AM broadcast band were selected manually by the user. Some of the frequencies used are now part of the expanded AM radio band can be clearly heard by anyone with an AM radio. There are reports of people still using these phones and even using them as make-shift AM radio stations that can be heard for a couple of city blocks. These models are no longer in production. They are considered obsolete because they are susceptible to eavesdropping and interference, especially from fluorescent lighting and automobile ignition systems. However, before the cell phone age, a person under the right conditions could get 1/2 mile or more range out of these AM systems. These phones are sharing the 49.8 Mhz band and are directly shared with baby monitors. (49.830 - 49.890) EST.1979

43-50 MHz cordless phones had a large installed base by the early 1990s, and featured shorter flexible antennas and automatic channel selection. Due to their popularity, an over crowding of the band led to an allocation of additional frequencies, thus manufacturers were able to sell models with 25 available channels instead of just 10 channels. Despite being less susceptible to interference, these models are no longer in production and are considered obsolete because these frequencies are easily heard on practically any radio scanner. Advanced models began to use voice inversion as a basic form of scrambling to help limit unauthorized eavesdropping.

900 MHz cordless phones are still sold today and have a huge installed base. Features include even shorter antennas, up to 30 auto selecting channels, and higher resistance to interference. Available in three varieties; analog, analog spread spectrum (100Khz bandwidth), digital, and digital spread spectrum, with most being sold today as budget analog models. Analog models are still susceptible to eavesdropping, However, older used models can still be found that are fully capable of receiving this spectrum. Digital variants can still be scanned, but are received as a digital hiss and therefore are difficult to eavesdrop upon. Digital transmission is immune to static interference but can experience signal fade (brief silence) as the phone goes out of range of the base. Digital Spread Spectrum (DSS) variants spread their signal over a range of frequencies providing more resistance to signal fade.This technology enabled the digital information to spread in pieces over several frequencies between the receiver and the base, thereby making it almost impossible to eavesdrop on the cordless conversation. The FCC only allows DSS model phones to transmit at the full power of 1 watt, which allows increased range over analog and digital models.

Virtually all telephones sold in the US use the 900 MHz, 1.9 GHz, 2.4-GHz, or 5.8 GHz bands, though legacy phones can remain in use on the older bands. There is no specific requirement for any particular transmission mode on 900, 1.9, 2.4, and 5.8, but in practice, virtually all newer 900 MHz phones are inexpensive analog models with digital features such as DSSS and FHSS generally available only on the higher frequencies.

Some cordless phones advertised as 5.8 GHz actually transmit from base to phone on 5.8 GHz and transmit from phone to base on 2.4 GHz or 900 MHz, to conserve battery life inside the phone.

The recently allocated 1.9 GHz band is used by the popular DECT phone standard and is considered more secure than the other shared frequencies.