The 802.11a standard was released in 1999, around the same time as 802.11b. While 802.11b only supported a data transfer rate of 11 Mbps, most routers and wireless cards at that time were manufactured using the 802.11b standard. Therefore, 802.11b remained more popular than 802.11a for several years. In 2003, 802.11g replaced both of the previous standards. 802.11g supports transfer rates of up to 54 Mbps (like 802.11a), but uses the same 2.4 GHz band as 802.11b.
NOTE: In order for an 802.11a connection to work, each device on the wireless network must support the 802.11a standard. For example, if a base station broadcasts an 802.11a signal, only computers with Wi-Fi cards that support 802.11a will be able to communicate with it. While many routers are backward-compatible with older standards, some must be manually configured to work with older 802.11a and 802.11b devices.