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The 802.11 service
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Despite the utilization of acknowledgments, the 802.11 layer only provides an unreliable connectionless service like Ethernet networks that do not use acknowledgments. The 802.11 acknowledgments are used to minimize the probability of frame duplication. They do not guarantee that all frames will be correctly received by their recipients. Like Ethernet, 802.11 networks provide a high probability of successful delivery of the frames, not a guarantee. Furthermore, it should be noted that 802.11 networks do not use acknowledgments for multicast and broadcast frames. This implies that in practice such frames are more likely to suffer from transmission errors than unicast frames.
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In addition to the data and control frames that we have briefly described above, 802.11 networks use several types of management frames. These management frames are used for various purposes. We briefly describe some of these frames below. A detailed discussion may be found in [IEEE802.11]_ and [Gast2002]_.
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A first type of management frames are the `beacon` frames. These frames are broadcasted regularly by access points. Each `beacon frame` contains information about the capabilities of the access point (e.g. the supported 802.11 transmission rates) and a `Service Set Identity` (SSID). The SSID is a null-terminated ASCII string that can contain up to 32 characters. An access point may support several SSIDs and announce them in beacon frames. An access point may also choose to remain silent and not advertise beacon frames. In this case, WiFi stations may send `Probe request` frames to force the available access points to return a `Probe response` frame.
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IP over 802.11
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Two types of encapsulation schemes were defined to support IP in Ethernet networks : the original encapsulation scheme, built above the Ethernet DIX format is defined in :rfc:`894` and a second encapsulation :rfc:`1042` scheme, built above the LLC/SNAP protocol [IEEE802.2]_. In 802.11 networks, the situation is simpler and only the :rfc:`1042` encapsulation is used. In practice, this encapsulation adds 6 bytes to the 802.11 header. The first four bytes correspond to the LLC/SNAP header. They are followed by the two bytes Ethernet Type field (`0x800` for IP and `0x806` for ARP). The figure below shows an IP packet encapsulated in an 802.11 frame.
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IP over IEEE 802.11
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The second important utilization of the management frames is to allow a WiFi station to be associated with an access point. When a WiFi station starts, it listens to beacon frames to find the available SSIDs. To be allowed to send and receive frames via an access point, a WiFi station must be associated to this access point. If the access point does not use any security mechanism to secure the wireless transmission, the WiFi station simply sends an `Association request` frame to its preferred access point (usually the access point that it receives with the strongest radio signal). This frame contains some parameters chosen by the WiFi station and the SSID that it requests to join. The access point replies with an `Association response frame` if it accepts the WiFI station.
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The 802.11 working group defined the `basic service set (BSS)` as a group of devices that communicate with each other. We continue to use `network` when referring to a set of devices that communicate.
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In fact, the [IEEE802.11]_ frame format contains a fourth optional address field. This fourth address is only used when an 802.11 wireless network is used to interconnect bridges attached to two classical LAN networks.
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