WiFi: How it Works
WiFi networks use radio technologies called IEEE 802.11 to provide secure,
reliable, fast wireless connectivity. A typical WiFi 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 usually broadcast every 100 ms.
The beacons are transmitted at 1 Mbit/s, and are of relatively short duration
and therefore do not have a significant effect on performance. Since 1 Mbit/s is
the lowest rate of WiFi it assures that the client that receives the beacon can
communicate at at least 1 Mbit/s. Based on the settings (e.g. the SSID), the
client may decide whether to connect to an AP. If two APs of the same SSID are
in range of the client, the client firmware might use signal strength to decide with which of the two
APs to make a connection.
The WiFi standard leaves connection criteria and roaming totally open to the
client. This is a strength of WiFi, but also means that one wireless adapter
may perform substantially better than another. Since WiFi transmits in the air,
it has the same properties as a non-switched wired Ethernet network, and
therefore collisions can occur. Unlike a wired Ethernet, and like most packet
radios, WiFi cannot do collision detection, and instead uses an acknowledgment
packet for every data packet sent. If no acknowledgement is received within a
certain time a retransmission occurs. Also, a medium reservation protocol can be
used when excessive collisions are experienced or expected
(RequestToSend/ClearToSend used for Collision
Avoidance or CA) in an attempt to try to avoid collisions.
A WiFi network can be used to connect computers to each other to the
internet and to wired networks (which use IEEE 802.3 or Ethernet). WiFi
networks operate in the unlicensed 2.4 (802.11b/g) and 5 GHz (802.11a/h) radio
bands, with an 11 Mbit/s (802.11b) or 54 Mbit/s (802.11a or g) data rate or with
products that contain both bands (dual band). They can provide real world
performance similar to the basic 10BaseT wired Ethernet networks.
WiFi's Wireless Sub-Channels
Except for 802.11a/h, which operates at 5 GHz, WiFi devices historically
primarily use the spectrum in 2.4 GHz, which is standardized and unlicensed by
international agreement, although the exact frequency allocations and maximum
permitted power vary slightly in different parts of the world. Channel numbers,
however, are standardized by frequency throughout the world, so authorized
frequencies can be identified by channel numbers. The 2.4 GHz band is also used
by microwave ovens, cordless phones, baby monitors and Bluetooth devices.
The maximum number of available channels for WiFi enabled devices are:
- 13 for Europe. A typical channel layout for 802.11b would be 1/7/13 (or
1/6/11 for compatibility to devices bought in North America). For traffic that
is predominantly 802.11g, 1/5/9/13 provides a fourth frequency enabling a much
better frequency plan.
- 11 for North America. Only channels 1, 6, and 11 are recommended for
802.11b/g to minimize interference from adjacent channels.[1]
- 14 for Japan [2]
