Cordless telephone
A cordless telephone differs functionally from a mobile phone in its limited range and by depending on the base station on the subscriber premises.
In specialized models, a commercial mobile network operator may maintain base stations and users subscribe to the service.
Because the range was intended to cover the widest possible service area, capacity was extremely low, and the early tube technology made equipment rather large and heavy.
Beginning in 1963, a small team of Bell Laboratories engineers were tasked with developing a practical and fully functional duplex wireless telephone.
The system operated under an experimental license on crystal controlled channels in the 35 and 43 MHz bands using FM, a low power transmitter and a sensitive superhet receiver.
[3] Sweigert, a radio operator in World War II stationed at the South Pacific Islands of Guadalcanal and Bougainville, developed the full duplex concept for untrained personnel, to improve battlefield communications for senior commanders.
Sweigert was an active proponent for directly coupling consumer electronics to the AT&T-owned telephone lines in the late 1960s.
They quickly became popular because of their convenience and portability, despite fears that their reliance on radio signals would make them vulnerable to eavesdropping or other malfeasance.
This technology enabled the spreading of the digital voice transmission over multiple frequencies, improving privacy and reducing interference between different subscribers.
1.7 MHz cordless phones were the earliest models available at retailers, and are generally identifiable by their large metal telescoping antennas.
43–50 MHz cordless phones had a large installed base by the early 1990s, and featured shorter flexible antennas and automatic channel selection.
Although less susceptible to interference than previous AM units, these models are no longer in production and are considered obsolete because their 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.
Digital transmission is immune to static interference but can experience signal fade (brief silence) as the phone goes out of range of the base.
This technology enabled the digital information to spread in pieces among several frequencies between the receiver and the base, thereby making it almost impossible to eavesdrop on the cordless conversation.
There is no specific requirement for any particular transmission mode on the older bands, but in practice many legacy phones also have digital features such as DSSS and FHSS.
However, DECT 6.0's late start compared to DECT elsewhere has led to a large installed base of legacy cordless phones using other frequencies, many of which remain in use today despite increasingly common interference with the ever growing use of Wi-Fi, Bluetooth and other unlicensed digital radio standards, especially at 2.4 GHz and 5.8 GHz.
DECT, on its European band & related frequencies, has largely displaced all other cordless phone standards worldwide except for North America.
"Plain old telephone service" (POTS) landlines are designed to transfer audio with a quality that is just adequate for the parties to understand each other.
Many analog phone signals are easily picked up by radio scanners, allowing anyone within range to listen in on conversations (though this is illegal in many countries).
Spreading the signal out over a wider bandwidth is a form of redundancy, and increases the signal-to-noise ratio, yielding longer range and less susceptibility to interference.
Only the base unit using a matching pseudorandom number can decode the signal, and it chooses from one of thousands of such unique codes each time the handset is returned to the cradle.
