Wi-Fi is based on the IEEE 802.11 specifications. There are currently four
deployed 802.11 variations: 802.11a, 802.11b, 802.11g, and 802.11n.
The b standard permits up to 11Megabits/second, while both a and
g allow up to 54Mbs. The new n specification will allow even
higher speeds (up to 100Mbs and beyond). The 802.11a standard works in the 5GHz
frequency band, and the others work in the 2.4GHz band. In fact, n is a
superset of g , and g is a superset of b , so there
is some interoperability among them. In most of the world, these frequencies do
not require user licenses from local regulators (eg, the Federal Communications
Commission in the US).
The most widespread version of Wi-Fi in the US market today (based in IEEE
802.11b/g) operates in the 2,400 MHz to 2,483.50 MHz. It allows to operate in 11
channels (5 MHz each), as follows
Channel 1 - 2,412 MHz; Channel 2 - 2,417 MHz; Channel 3 - 2,422 MHz; Channel
4 - 2,427 MHz; Channel 5 - 2,432 MHz; Channel 6 - 2,437 MHz; Channel 7 - 2,442
MHz; Channel 8 - 2,447 MHz; Channel 9 - 2,452 MHz; Channel 10 - 2,457 MHz;
Channel 11 - 2,462 MHz
In Europe, France and Spain have adopted their own allowed channels set, and
Japan has also done so. In all areas, the maximum radio transmitter power and
the maximum effective radiated power (essentially the power output at the
antenna) are strictly limited. In the US, maximum transmitter power is 1 watt,
and maximum effective radiated power is 4 watts; in Europe these limits are
somewhat lower. An antenna which concentrates 1 watt of transmitter energy into
1/4 of an 'omnidirectional' sphere will achieve 4 watts of effective power. Most
WiFi equipment (eg, PCMCIA or Cardbus cards for laptops, PCI cards for desktop
equivalent computers, or standalone units often with other functions included)
has transmitter power levels of between 15mw and perhaps 200mw, so antennas with
some gain are permissible.
New standards beyond the 802.11 specifications, such as 802.16 (WiMAX, with
ranges of several miles and data rates of 70Mbs or so), are currently in the
works and offer many enhancements, anywhere from longer range to greater
transfer speeds. 802.16a permits operation between 2 and 11 GHz, so there may
eventually be some interoperability between 802.11 units and some 802.16a units.
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