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How Wi-Fi Works

 

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Wi-Fi: How it works

The typical Wi-Fi setup contains one or more Access Points (APs) and one or more clients. An AP broadcasts its SSID (Service Set Identifier, Network name) via packets that are called beacons, which are broadcasted every 100 ms. The beacons are transmitted at 1 Mbit/s, and are relatively short and therefore are not of influence on performance. Since 1 Mbit/s is the lowest rate of Wi-Fi it assures that the client who receives the beacon can communicate at at least 1 Mbit/s. Based on the settings (i.e. the SSID), the client may decide whether to connect to an AP. Also the firmware running on the client Wi-Fi card is of influence. Say two AP's of the same SSID are in range of the client, the firmware may decide based on signal strength (Signal-to-noise ratio) to which of the two AP's it will connect. The Wi-Fi standard leaves connection criteria and roaming totally open to the client. This is a relative strength of Wi-Fi, but also means that one wireless adapter may perform substantially better than the other. Since Windows XP there is a feature called zero configuration which makes the user show any network available and let the end user connect to it on the fly. In the future wireless cards will be more and more controlled by the operating system. Microsoft's newest feature called SoftMAC will take over from on-board firmware. Having said this, roaming criteria will be totally controlled by the operating system. Wi-Fi transmits in the air, it has the same properties as non-switched Ethernet network. Even collisions can therefore appear like in non-switched Ethernet LAN's.

Wireless Protocols

802.11 refers to a family of specifications developed by the IEEE (Institute of Electrical and Electronics Engineers) for wireless LAN technology. 802.11 specifies an over-the-air interface between a wireless client and a base station or between two wireless clients. The IEEE accepted the specification in 1997.

There are several specifications in the 802.11 family:

  • 802.11 - applies to wireless LANs and provides 1 or 2 Mbps transmission in the 2.4 GHz band using either frequency hopping spread spectrum (FHSS) or direct sequence spread spectrum (DSSS).

  • 802.11a - an extension to 802.11 that applies to wireless LANs and provides up to 54 Mbps in the 5GHz band. 802.11a uses an orthogonal frequency division multiplexing encoding scheme rather than FHSS or DSSS.

  • 802.11b (also referred to as 802.11 High Rate or Wi-Fi) - an extension to 802.11 that applies to wireless LANS and provides 11 Mbps transmission (with a fallback to 5.5, 2 and 1 Mbps) in the 2.4 GHz band. 802.11b uses only DSSS. 802.11b was a 1999 ratification to the original 802.11 standard, allowing wireless functionality comparable to Ethernet.

  • 802.11g - applies to wireless LANs and provides 20+ Mbps in the 2.4 GHz band.

  • 802.16 - Commonly referred to as WiMAX or the Air Interface Standard, IEEE 802.16 is a specification for fixed broadband wireless metropolitan access networks (MANs).  This protocol is still in development, but holds great promise for delivering economical broadband internet in lightly populated areas not served by wireline technologies.
     
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