So we’ve looked at what a wave is but now I want to talk about what frequencies these waves use. In the 802.11 space we generally use the Industrial, Scientific and Medical (ISM) 2.4Ghz and 5Ghz bands but sometimes the 4.9Ghz band is also used for public safety applications. Each 802.11 standard can be deployed within certain bands that are determined by the IEEE standard and country’s regulatory requirements. In Australia there are 13 5Mhz channels available within the 2.4Ghz band starting from 2400Mhz to 2483.5Mhz.
The 5Ghz space is slightly more complicated in that there are three sub-bands that are consumed depending on indoor or outdoor deployments, the IEEE also dictated that each channel must have a width of 20Mhz. UNII-1 contains 4 channels namely channels 36, 40, 44 and 48 with a frequency range of 5.150Mhz to 5.250Mhz. UNII-2 contains another 4 channels which include channels 52,56, 60 and 64 between the ranges of 5.250Mhz to 5.350Mhz. 11 channels are available within the UNII-2 extended band which reside between the frequencies between 5.470Mhz through to 5.725Mhz and include channels 100,104,108,112,116,120,124,128,132,136 and 140. Finally channels 149, 153,157 with frequencies ranging from 5745Mhz to 5805Mhz are available in the UNII-3 band. Each 5Ghz channel is 5Mhz wide but why are only every fourth channel usable? Well, each channel frequency is the centre of frequency of the corresponding channel and since the IEEE requires a 20Mhz non-overlapping width this extends the channel by 10Mhz in each direction.
UNII-1, UNII-2 and UNII-2 Extended can be used for indoor deployments while UNII-2, UNII-2 Extended and UNII-3 can be used for outdoor deployments. The FCC and Australia communication regulatory agencies insisted that devices in the UNII-2, UNII-2 Extended and UNII 3 bands deploy Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) and was ratified by the IEEE under the standard 802.11h. These requirements were enforced in order to avoid interference with military applications and radar. In recent years the FCC has been investigating expanding the 5Ghz range to include additional channels between UNII-2 and UNII-2 Extended as well as channels above the UNII-3 band. Each country also apply power restriction for each band and it is very important that these power and band requirements are carefully adhered too.