IEEE 80211 Wireless LANs

The most popular type of wireless LAN today is based on the IEEE 802.11 standard, which is known informally as Wi-Fi. The 802.11 specification defines what happens on the wireless network to let two or more devices send and receive data.

Wireless LAN communication is really a shared LAN because only one station effectively can transmit at one time, at least in a particular constrained geography. 802.11 signals effectively might reach another device as far as 300 feet away. So, you can have lots of people around the planet using the airwaves for 802.11 at any one point in time, but only one device can send at a time when within range of the wireless signals.

Wireless LANs typically include one or more computers that have a wireless 802.11 LAN card, plus one or more wireless access points (APs). Access points bridge or route traffic from the wireless LAN to the "wired" LAN and vice versa. Figure 11-14 depicts the general idea.

Figure 11-14 Wireless Access Point and 802.11 Framing

PCs with 802.11 Wi-Fi Cards

802.11

802.2

Data

802.3

802.3

802.2

Data

802.3

The AP shows two antennae protruding from the corners—indeed, a component of wireless communications is the antennae used to receive and transmit wireless radio signals. The two PCs at the top of the figure also have antennae, typically connected to the end of the 802.11 LAN card and protruding out from the PC.

Note that 802.11 calls for the use of IEEE 802.2 LLC, as well as the same format of addresses defined in 802.3. 802.11 does use a different MAC header than 802.3, however. So, to bridge the traffic, the access point simply swaps an 802.11 header for an 802.3 header, and vice versa, using the same MAC addresses. Some wireless APs route traffic from the wireless network to the wired network.

802.11 includes several standards. 802.11b transmits at 11 Mbps using the 2.4 GHz frequency band, but it is shared, with the maximum throughput capped at about 7 Mbps. 802.11a, which runs in the 5 Ghz frequency band, now can run at speeds up to 54 Mbps, as can 802.11g, which uses the 2.4 Ghz band, like 802.11b. When this book was published, there was still debate in the industry as to which of the higher-speed wireless standards would emerge as the more popular technology in the marketplace.

0 0

Post a comment