Why can't the same 2.4 GHZ be used for a WLAN access point and telephone?
802.11b radios operate in the unlicensed 2/4 GHz ISM frequency band, along with 2.4 GHz cordless phones, Bluetooth devices, and HomeRF products. Not all of these devices use the same modulation technique - for example, Bluetooth uses Frequency Hopping Spread Spectrum (FHSS), while 802.11b uses Direct Sequence Spread Spectrum (DSSS).
Two 802.11b access points based DSSS can use non-overlapping 22 MHz-wide channels to peacefully share the ISM band. However, devices that use different modulation techniques interfere with each other by competing for the same spectrum. Intersil and Silicon Wave recently demonstrated Blue802, a technology that uses time-slicing to let both protocols coexist by switching rapidly between them.
This said, cordless phones that are based on 2.4 GHz DSSS *should* coexist with 802.11b access points. Some channel tuning may be required to keep the AP and phone on non-overlapping channels. Many have conducted informal tests that substantiate this claim and prove there are exceptions. Not all phones are created equal. For example, read "When Wireless Networks Collide", an article by Toni Kistner.
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