 WLAN and a wired LAN. It connects the MAC sublayer of the data layer and the physical layer, as medium over which the APs communicate to the medium well as the electrical specifications of the physical layer. of the wired LAN-coaxial cable or twisted pair lines, for Figure 5.2-5 illustrates the relationship between the example. physical and MAC layers of several types of networks In addition to the functions Wi-Fi provides for dis- with upper-layer application software interfaced through tributing data throughout the network, two other im- a commonly defined logical link control (LLC) layer. The portant services, although optionally used, are provided. LLC is common to all IEEE local area networks and is independent of the transmission medium or medium access method. Thus, its protocol is the same for wired local area networks and the various types of wireless STA 1 networks. It is described in specification ANSI/IEEE STA 3 standard 802.2. STA 2 The Medium Access Control function is the brain of the WLAN. Its implementation may be as high-level STA 4 digital logic circuits or a combination of logic and a microcontroller or a digital signal processor. EEE 802.11 and its supplements, (which may be generally designated Figure 5.2-1 Basic service set. 802.1 1x), prescribe various data rates, media (radio CHAPTER 5.2 Short-range wireless applications and technologies STA 4 STA 2 STA 1 STA 3 STA 5 PORTAL WIRED LAN Figure 5.2-3 Extended service set. waves or infrared), and modulation techniques (FHSS, The fundamental access method in IEEE 802.11 is the DSSS, CCK, ODFM). These are the principle functions DCF, more widely known as CSMA/CA (carrier sense of the MAC: multiple access with collision avoidance). It is based on a procedure during which a station wanting to transmit Frame delimiting and recognition may do SO only after listening to the channel and de- Addressing of destination stations termining that it is not busy. If the channel is busy, the Transparent transfer of data, including fragmentation station must w