k summary of the whole 802.11 family. The 802.11 specifications were initially introduced in 1997 for operation in the unlicensed 2.4GHz band, and they included two spread spectrum methods for transmission: 1Mbps Frequency Hopping Spread Spectrum (FHSS) and 1Mbps and 2Mbps Direct Sequence Spread Spectrum (DSSS). An infrared method was also specified. Ultimately, the Wi-Fi Alliance dropped both FHSS and infrared, but the 1Mbps DSSS method is still used by WAPs to advertise themselves (in a process known as beaconing). Now 802.11 is sometimes called 802.11 legacy. WLAN Speeds Despite the maximum speeds quoted in the specifications, the actual data throughput is generally quite a bit lower. This is because 802.11 uses a collision avoidance access tech- nique (Carrier Sense Multiple Access/Collision Avoidance [CSMA/CA]) rather than the collision detection method (Carrier Sense Multiple Access/Collision Detection [CSMA/ CD]) used in wired Ethernet LANs. Unlike wired LANS, wireless systems cannot detect col- lisions. The CSMA/CA method waits for an acknowledgment from the other end to determine whether the packet was transmitted properly, thus reducing the throughput. In addition, the speed of a WLAN depends on the distance. The farther away the remote device is from the WAP, the lower the speed, or bit rate. Depending on the dis- tance involved, different modulation techniques are used, including BPSK, QPSK, 4-QAM, 16-QAM, and 64-QAM. The bottom line is that you should assume that the actual transmission speed of a WLAN is going to be 50% or less of the rated speed. WLANs Table 15.5 The IEEE 802.11x Standards Standard Description 802.11 The original 1Mbps and 2Mbps, 2.4GHz radio frequency and infrared standard (1999) 802.11a A 54Mbps, 5GHz standard (1999; products began shipping in 2001) 802.11b Enhancements to 802.11 to support 5.5Mbps and 11Mbps (1999) 802.11c Bridge operation procedures; included in the 802.1D standard (2001) 802.11d International (country-to-country) roaming extensions (2001) 802.11e Enhancements 