The intial revision of the 802.11 specification in 1997 had a second physical layer based on direct sequence spread spectrum (DSSS) technology. The DSSS PHY in 802.11 had data rates of 1 Mbps and 2 Mbps. Although it operated at the same speed as the frequency hopping PHY, it quickly became clear that direct sequence technolgies had the potential for higher speeds than frequency hopping technologies. As a result, even though the two had equivalent speeds, direct sequence became the PHY of choice. In 1999, a PHY with data rates of 5.5 Mbps and 11 Mbps was specified in 802.11b. The older 1 and 2 Mbps PHYs and the newer 5.5 and 11 Mbps PHYs are often combined into a single interface, even though they are described by different specifications. (It is usually referred to as "802.11b" support, even though the two lower rates are not part of 802.11b.) This chapter describes the basic concepts and modulation techniques used by the direct sequence physical layers. It also shows how the PLCP prepares frames for transmission on the radio link and touches briefly on a few details of the physical medium itself.