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There are several WiFi protocols known by their letter designated IEEE 802.11 specifications, such as 802.11a, 802.11b, 802.11g and 802.11n. Generally their differences are related to frequency (aka band or spectrum), data rate (aka throughput or transfer speed), bandwidth, modulation and range.
Bandwidth is often confused with data rate. While there is often a correlation between greater bandwidth and greater data rate, in terms of radio the bandwidth refers to the difference between the upper and lower frequencies of a given channel as measured in hertz. For example, with the 802.11g protocol the first channel will have a lower frequency of 2.400 GHz and an upper frequency of 2.422 GHz for a total of 22 MHz bandwidth. An 802.11n based network using 40 MHz bandwidth will occupy nearly twice the spectrum as the 22 MHz wide 802.11g channel and similarly achieve a much faster data rate.
Modulation also affects data rate, with the most common modulation type being OFDM or Orthogonal frequency-division multiplexing. In addition to being a mouthful, it’s a digital encoding technique used to cram a lot of data on a small amount of spectrum. It’s the same technology used in DSL modems and 4G mobile broadband. The important takeaway is that OFDM supersedes the older DSSS modulation technique used in 802.11b.
802.11a and 802.11b were the first mainstream WiFi protocols, introduced in 1999. 802.11a operates in the 5 GHz band with speeds up to 54 Mbps while 802.11b operates in the 2.4 GHz band with speeds only up to 11 Mbps. These networks are more rare to find, though when they are it’s typically indicative of aging infrastructure.
Nowadays 802.11g and 802.11n are more commonly found with data rates up to 54 Mbps and 150 Mbps respectively. Both operate in the 2.4 GHz band with the latter capable of operating in the 5 GHz band as well.
An important thing to consider about protocols is that WiFi radios operating on newer protocols almost always contain backwards compatibility, so an access point using the 802.11g standard may be just as enticing to a client device capable of using the newer 802.11n standard.