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802.11b
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When developing its family of standards, the `IEEE 802.11 working group <http://www.ieee802.org/11/>`_ took a similar approach as the `IEEE 802.3 working group <http://www.ieee802.org/3/>`_ that developed various types of physical layers for Ethernet networks. 802.11 networks use the CSMA/CA Medium Access Control technique described earlier and they all assume the same architecture and use the same frame format.
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../../protocols/wifi.rst:35
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The architecture of WiFi networks is slightly different from the Local Area Networks that we have discussed until now. There are, in practice, two main types of WiFi networks : `independent` or `adhoc` networks and `infrastructure` networks [#fBSS]_. An `independent` or `adhoc` network is composed of a set of devices that communicate with each other. These devices play the same role and the `adhoc` network is usually not connected to the global Internet. `Adhoc` networks are used when for example a few laptops need to exchange information or to connect a computer with a WiFi printer.
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An 802.11 independent or adhoc network
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Most WiFi networks are `infrastructure` networks. An `infrastructure` network contains one or more `access points` that are attached to a fixed Local Area Network (usually an Ethernet network) that is connected to other networks such as the Internet. The figure below shows such a network with two access points and four WiFi devices. Each WiFi device is associated to one access point and uses this access point as a relay to exchange frames with the devices that are associated to another access point or reachable through the LAN.
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An 802.11 infrastructure network
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An 802.11 access point is a relay that operates in the datalink layer like switches. The figure below represents the layers of the reference model that are involved when a WiFi host communicates with a host attached to an Ethernet network through an access point.
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802.11 devices exchange variable length frames, which have a slightly different structure than the simple frame format used in Ethernet LANs. We review the key parts of the 802.11 frames. Additional details may be found in [IEEE802.11]_ and [Gast2002]_ . An 802.11 frame contains a fixed length header, a variable length payload that may contain up 2324 bytes of user data and a 32 bits CRC. Although the payload can contain up to 2324 bytes, most 802.11 deployments use a maximum payload size of 1500 bytes as they are used in `infrastructure` networks attached to Ethernet LANs. An 802.11 data frame is shown below.
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802.11 data frame format
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