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Early systems used these frequencies for the base units: 1.665, 1.690, 1.695, 1.710, 1.725, 1.730, 1.750, 1.755, 1.770 MHz.
And for the handsets: 46.610, 46.630, 46.670, 46.710, 46.730, 46.770, 46.830, 46.870, 46.930, 46.970, 49.670, 49.770, 49.830, 49.845, 49.860, 49.875, 49.890, 49.930, 49.970, 49.990 MHz
Both base and handset (portable unit) used frequency modulation. The 1665 kHz, 1690 kHz and 1710 kHz frequencies eventually became part of the expanded AM broadcast band.
The second-generation of analog cordless phones were 10-channel FM units operating in the 46.61 MHz to 46.97 MHz (base stations) and 49.67 MHz to 49.99 MHz under Part 15 47 CFR 15.233 using the following frequency pairs.
- Base Frequency 46.610 MHz - Handset Frequency 49.670 MHz
- Base Frequency 46.630 MHz - Handset Frequency 49.845 MHz
- Base Frequency 46.670 MHz - Handset Frequency 49.860 MHz
- Base Frequency 46.710 MHz - Handset Frequency 49.770 MHz
- Base Frequency 46.730 MHz - Handset Frequency 49.875 MHz
- Base Frequency 46.770 MHz - Handset Frequency 49.830 MHz
- Base Frequency 46.830 MHz - Handset Frequency 49.890 MHz
- Base Frequency 46.870 MHz - Handset Frequency 49.930 MHz
- Base Frequency 46.930 MHz - Handset Frequency 49.990 MHz
- Base Frequency 46.970 MHz - Handset Frequency 49.970 MHz
The "49 MHz" cordless phones were rapidly adopted by consumers, resulting in heavy frequency congestion. The 49.830-49.890 MHz frequencies were and still are heavily used by other devices including baby monitors in continuous transmit mode. 15 frequencies were added bringing the total number of channels to 25.
The FCC quickly moved to authorize additional frequency space for cordless phones. With the 900 MHz ISM band 902-928 MHz becoming available next. 900 MHz cordless phones are still used today. Analog FM, digital voice and various frequency hopping spread-spectrum or FHSS cordless phones became available for the 900MHz band. If FHSS technology is used, base stations may transmit up to 1 watt ERP. This increase in transmitter power combined with the interference rejection capabilities of spread spectrum gave the higher-end 900 MHz cordless phones significant range. Unlike the older-generation models, 900 MHz cordless telephones never had a standardized channel plan or base and handset frequency pair or channel plan. This meant that manufacturers simply created their own, with widely differing channel steps, number of channels and other options. Even cheap analog FM 900 MHz cordless phones usually featured automatic channel selection. In other words, if the first channel pair was busy the system simply switched to the next clear channel. This same technology was (and is) included in digital voice cordless telephones operating in the 900 MHz band, the 2.4 GHz band and the 5.8 GHz band as well as those using modern DECT technology.
The 2.4 GHz band became available shortly after 900 MHz and originally provided excellent results. However, the proliferation of devices operating in the 2.4 GHz band now means that 2.4GHz cordless phones often suffer from (or cause) interference and/or have reduced range compared to the other bands. The 5.8 GHz allocation was opened up to alleviate the congestion on 2.4 GHz. Many 5.8 GHz band cordless phones are sold today. Some "5.8 GHz" phones are actually cross-band and use 5.8 GHz for handset to base but use 900 MHz or 2.4 GHz for base-to-handset.
DECT (branded as "DECT 6.0" in the United States and Canada) actually operates in the 1.8 GHz and 1.9 GHz band but provides interference free cordless phone service and is extremely adaptable for business, retail, industrial, office or home use. DECT phone systems may be expanded with numerous base stations, full-duplex intercoms and other systems in a cellular-like network. This includes PBX telephone systems in offices, retail stores, etc. DECT is effectively the convergence of cellular technologies and single-base/single-handset cordless phones.
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