connected by landline or to mobile phones (usually via Bluetooth), now use DECT 6.0. 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. Outside the United States In Europe, the 1.9 GHz (1880–1900 MHz) band was set aside for the DECT phone standard from its inception. DECT, on its European band & related frequencies, has largely displaced all other cordless phone standards worldwide except for North America. Performance Many cordless phones in the early 21st century are digital. Digital technology has helped provide clear sound and limit casual eavesdropping. Many cordless phones have one main base station and can add up to three or four additional bases. This allows for multiple voice channels that allow three-way conference calls between the bases. This technology also allows multiple handsets to be used at the same time, and up to two handsets can have separate conversations with outside parties. Manufacturers usually advertise that their higher frequency systems improve audio quality and range. In the ideal case, higher frequencies actually have worse signal propagation as shown by the basic Friis transmission equation, and path loss tends to increase at higher frequencies as well. Practical influences on quality and range are signal strength, antenna quality, the method of modulation used, and interference, which varies locally. "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. Typical bandwidth is 3.6 kHz; only a fraction of the frequencies that humans can hear, but enough to make the voice intelligible. No phone handset can improve on this quality, as it is a limitation of the phone system itself. Higher-quality phones can transfer this signal to the handset with less interference over a greater range, however. Most cordless telephones, no matter what frequency band or transmission method is used, will hardly ever exactly match the sound quality of a high-quality wired telephone attached to a good telephone line. This limitation is caused by a number of issues, including the following: Sidetone: hearing one's own voice echoed in the receiver speaker A noticeable amount of constant background noise (this