 extended service set (ESS) (Figure 5.1-4). A BSS has a unique identifier, the BSSID. An ESS also Despite its name, Ethernet was conceived as a cable- has an identifier, the ESSID, that is unique to that ESS based technology. Putting all the stations on a cable and but shared by all the component BSSs (is that enough limiting the cable length allowed meant that all stations acronyms in one sentence for you?). Unfortunately, in- could always hear one another, and any collision would be terfaces for many commercial 802.11 implementations detected by all the stations on the cable. However, in use the nomenclature SSID, which in practice usually a wireless network it is unlikely that all stations will be Station 1 Station 2 Preamble Start Destination Source Length Error Start Destination Source Length Error Payload Preamble Payload Frame address address type check Frame address address type check Logical Link Control [802.2] Logical Link Control [802.2] 802.11 802.11 Wireless Media Access Control [MAC] Wireless Media Access Control [MAC] Direct Sequence physical layer Direct Sequence physical layer 2.4 GHz radio 2.4 GHz radio Figure 5.1-3 802.11 as a means of moving Ethernet frames. CHAPTER 5.1 Wireless local area networks access point distribution system to network access stations points basic service set extended service set Figure 5.1-4 Definition of the BSS and ESS. able to receive all transmissions all the time: this is detected, the medium is considered to be busy and the known as the hidden station problem. An example is station defers its transmission. A virtual carrier sense depicted in Figure 5.1-5: the access point can commu- mechanism-the network allocation vector (NAV)-is nicate with stations A and B, but A and B cannot receive provided to further reduce the likelihood of collisions. each other's transmissions directly. Because of this fact, Each packet header involved in an exchange that lasts collision detection would fail if A and B both attempted longer than a single frame, such as a data frame that 