avel farther than those in higher bands because these waves are longer (refer to Figure 7-1). The 802.11b and 802.11g standards cover a range of about 100 to 150 feet; 802.11a covers only about 75 feet. Because they cover smaller areas, networks that utilize 802.11a antennas require more antennas. Equipment supports both standards is referred to as dual-band equipment. As its name implies, dual-band equipment supports two frequency bands. The higher speeds achieved by 802.11a and 802.11g are possible because both are based on Orthogonal Frequency-Division Multiplexing (OFDM). This is because OFDM sends multiple streams of bits simultaneously. See the section "The LTE and WiMAX Orthogonal Frequency-Division Multiplexing Air Interface," earlier in this chapter, to read more about OFDM in LTE and WiMAX networks. Wi-Fi Standards, Architecture, and Use in Cellular Networks The following factors impact achievable speeds. Network congestion. The distance from antennas. Overhead, which is the number of bits required in packet headers for infor- mation such as addressing and error correction. Interference from thick walls or other material. These factors can also decrease range. Worldwide, the number of channels available in each standard varies. When governments set requirements for the number of channels that can be used for each standard, they do not always do it uniformly. The standards themselves often define flexible requirements. 802.11a: Higher Speeds, Smaller Coverage Area, and More Channels For the most part, 802.11a equipment is sold to enterprises that want greater capac- ity because it supports more simultaneous users per access point (antenna plus trans- ceiver). This is the result of 802.11a having 24 channels rather than only three. Another advantage beyond capacity is that the 5GHz frequency is less crowded with devices that also operate on 2.4GHz, such as microwave ovens and mobile devices equipped with Bluetooth. These devices can cause interference with Wi-Fi equipment on the same 2.4GHz band. Enterpri