ses that use 802.11a need to equip employee laptops with 802.11a network interface cards or chips to take advantage of the extra capacity. The 802.11a standard is available in access points sold to residential consumers, but often at higher prices than those with only 802.11b and 802.11g. The Longer Range and Higher Capacity of 802.11n The 802.11n standard is a Wi-Fi standard that is backward compatible with 802.11a, b, and g networks. It enables WLANs to cover longer distances by overcoming a cer- tain amount of interference. It also increases achievable speeds, increases throughput (user data minus packet headers), and supports more users per access point. Improve- ments in spectral efficiency-the number of users supported in a given amount of air- space-are critical as the number of users and applications on WLANs increases. This is particularly true in networks in which more employees use tablet computers and in homes that stream video from the Internet for viewing on High-Definition Televisions (HDTVs). Wi-Fi equipment for consumers and enterprises generally includes 802.11n. Mobile and Wi-Fi Networks The Spectral Efficiency in 802.11n Antennas Improvement in antennas is the reason 802.11n uses spectrum more efficiently and overcomes many "dead spots." A dead spot is an area that access points don't cover because of interference from building materials or the distance from an antenna. The 802.11n access points use Multiple-Input Multiple-Output (MIMO) technology. Like MIMO antennas used in advanced cellular services, MIMO antennas in Wi-Fi net- works simultaneously transmit multiple streams at different frequencies within a sin- gle channel. The four types of MIMO antennas are classified by the number of transmit and receive antennas installed within access points and user devices. 2x2 antennas have two transmit and two receiver antennas. 2x3 antennas have two transmit and three receiver antennas. 3x3 antennas have three transmit and three receiver antennas. 4x4 antennas have four transmit and four receiver 