ystem operates in the 5-GHz band employing orthogonal frequency division multiplex modulation (OFDM) offering data rates from 6 to 54 Mbps. IEEE 802.11b specifies operation in the 2.4-GHz band providing data rates of 1 or 2 Mbps of the initial 802.11b specification and an added capability for 6.6 Mbps and 11 Mbps with CCK (complementary code keying) modulation which makes more efficient use of the radio-frequency spectrum. The 802.11 series are commonly referred to as wireless LANs (WLANs). These WLANs can be used either to replace wired LANs or as an extension of the wired LAN infrastructure. In the WLAN arena we can have a basic service set (BSS) consisting of two or more wireless nodes or stations (STAs) which have recognized each other and have established communications. In the most basic form, stations communicate directly with each other on a peer-to-peer level sharing a given call coverage area. This type of network is often formed on a temporary basis and is commonly referred to as an ad hoc network, or independent basic service set (IBSS). In most instances, the BSS contains an access point (AP). The main function of an AP is to form a bridge between wireless and wired LANs. The AP is analogous to a base station used in cellular phone networks. When an AP is present, stations do not communicate on a peer-to-peer basis. All communication between stations or between a station and the wired network client go through the AP. APs are not mobile, and they form part of the wired network infrastructure. A BSS in this configuration is said to be operating in the infrastructure mode. The extended service set (ESS) illustrated in Figure 13.1 consists of a series of over- lapping BSSs, each containing an AP and are connected together by means of a DS (distributed system). Although the DS could be any kind of network, it is almost invari- ably an Ethernet LAN. Mobile nodes can roam between APs, and seamless campus-wide coverage is possible. The MAC layer in the IEEE 802.11 standard is a set of protocols that is resp