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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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802.11a An extension to 802.11 that applies to wireless LANs <strong>and</strong> provides<br />

up to 54 Mbps in the 5 GHz b<strong>and</strong>. 802.11a uses an orthogonal<br />

frequency division multiplexing (OFDM) encoding scheme rather<br />

than FHSS or DSSS.<br />

802.11b Also known as 802.11 High Rate or WiFi. An extension to 802.11 that<br />

applies to wireless LANs <strong>and</strong> provides 11 Mbps transmission with<br />

fallbacks to 5.5, 2, <strong>and</strong> 1 Mbps in the 2.4 GHz b<strong>and</strong>. 802.11b uses<br />

only DSS. 802.11b was a 1999 ratification to the original 802.11<br />

st<strong>and</strong>ard, allowing wireless functionality comparable to Ethernet.<br />

802.11g Applies to wireless LANs <strong>and</strong> provides 20+ Mbps in the 2.4 GHz<br />

b<strong>and</strong>.<br />

For additional information about the 802.11 family of st<strong>and</strong>ards, see:<br />

http://www.ieee802.org/11/<br />

Operation<br />

WiFi operates as a non-switched Ethernet network. Every 100 ms, Wireless<br />

Application Protocols (WAPs) broadcast service set identifiers (SSIDs) using<br />

beacon packets. Clients who receive these beacons can opt to wirelessly<br />

connect to the WAP. This determination is usually established by some<br />

combination of the following factors:<br />

► Whether or not the client has been configured to connect to the broadcasted<br />

SSID.<br />

► The signal strength of the WAP. In particular, a client might receive two<br />

beacons from two different WAPs, each one broadcasting the same SSID. In<br />

this instance, the client should opt to connect to the WAP demonstrating the<br />

stronger signal.<br />

► The level of encryption offered by a WAP.<br />

Each beacon is broadcast at 1 Mbps, ensuring that any client who receives the<br />

beacon at a minimum supports communication at this speed. All of the area to<br />

which a WAP beacon can be received is referred to as a hotspot. Though WiFi<br />

hotspots can be several miles long, such an implementation requires multiple<br />

WAPs to overlap their individual hotspots using the same SSID.<br />

WiFi can also be used in peer-to-peer mode, allowing mobile devices to<br />

communicate with one another in the absence of a wireless network. Although<br />

this method of operation does not provide any sort of connectivity to the Internet,<br />

it does lend itself to other applications such as backing up data or gaming.<br />

398 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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