Wi-Fi Spectrum: ISM, U-NII, FCC Rules
Wi-Fi operates within defined bands of frequencies as dictated by the Federal Communications Commission, specifically Part 15 of FCC rules. Wi-Fi operates in unlicensed frequency spectra, which means that anyone, within specified transmission power restrictions, can operate a transmitter within these bands.
There are 12 industrial, scientific, and medical (ISM) bands. Of these, two are in use: the 26 MHz centered around 915 MHz, and the 100 MHz centered around 2.450 GHz. Currently, only the latter band is used for Wi-Fi. The former is used for devices such as home automation (Zwave) and baby monitors. The 2.4 GHz band has a total of 14 channels, numbered 1-14. Channels 1-11 are allowed for use in the United States; 1-13 are used in Europe and Israel; 1-14 are used in Japan. Each channel is 22 MHz wide, with center frequencies of each channel separated by 5 MHz. As shown in Figure 1, only three channels do not overlap in the 11-channel model; therefore, only these channels can be used in a multi-channel architecture, or adjacent channel interference will render the network unusable.
As of this writing, there are 4 Unlicensed National Information Infrastructure bands (U-NII). There are a total of 23 U-NII channels available for use.
4 bands within 5 GHz
The four bands in the 5GHz spectrum are:
- U-NII-1 (lower) comprises four 20 MHz channels, 36-48 in the 100 MHz 5.150-5.250 GHz band.
- U-NII-2A (lower middle) comprises four 20 MHz channels, 52-64 in the 100 MHz 5.250-5.350 GHz band.
- U-NII-2C (upper middle) comprises 11 20 MHz channels, 100-140 in the 250 MHz 5.470-5.725 GHz band.
- U-NII-3 (upper) comprises four 20 MHz channels, 149-161 in the 100 MHz 5.725-5.825 GHz band.
All four bands operate under power restrictions, with U-NII-1 being relegated to indoor use only. A recent rule change effectively opened up this band for general use, removing the indoor-only stricture, and allowing transmit power to rise to 1 W from 50 mW. For comparison, U-NII-2A and 2C can operate up to 250 mW, and U-NII-3 can operate up to 1 W. Another change will be to add ISM channel 165, at the center frequency of 5.825 GHz, to the U-NII-3 band, bringing another 25 MHz into play.
More Wi-Fi bands proposed
Proposed new bands are under consideration by the FCC to add even more bandwidth for Wi-Fi use. U-NII-2B will add 120 MHz bandwidth in the 5.350-5.470 band. U-NII-4 will add another 75 MHz in the 5.850-5.925 GHz band.
The 20 MHz wide channels are actually very narrow in terms of data throughput. Newer Wi-Fi standards allow for the bonding of multiple channels to allow up to 160 MHz wide channel combinations. This will have the effect, along with faster data rates and multiple transceivers, to allow a theoretical throughput of up to 7 GBPS when the latest ratified Wi-Fi amendment, IEEE 802.11ac, is fully implemented.
– Daniel E. Capano, owner and president, Diversified Technical Services Inc. of Stamford, Conn., is a certified wireless network administrator (CWNA). Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering, firstname.lastname@example.org.
www.controleng.com/blogs has other wireless tutorials from Capano on the following topics:
– What is wireless?
– Radio frequency basics
– Comparative modulation: Spread spectrum modulation terms and definitions for wireless networking
www.controleng.com/webcasts has wireless webcasts, some for PDH credit.
Control Engineering has a wireless page.