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Beacon frame is one of the management frames in IEEE 802.11 based WLANs. It contains all the information about the network. Beacon frames are transmitted periodically to announce the presence of a wireless LAN. Beacon frames are transmitted by the Access Point (AP) in an infrastructure Basic service set (BSS). In IBSS network beacon generation is distributed among the stations. == Components of a Beacon frame == Beacon frames consist of MAC header, Frame body and FCS. Some of the fields are listed below. * Timestamp :: After receiving the beacon frame all the stations change their local clocks to this time. This helps with synchronization. * Beacon interval :: This is the time interval between beacon transmissions. The time at which a node (AP, station when in ''ad hoc'' or P2P GO mode) must send a beacon is known as Target Beacon Transmission Time (TBTT). Beacon interval expressed in Time Unit (TU). It is a configurable parameter in the AP and typically configured as 100 TU. * Capability information :: Capability information field spans to 16 bits and contain information about capability of the device/network. Type of network such as AdHoc or Infrastructure network is signaled in this field. Apart from this information, it announces the support for polling, as well as the encryption details. * SSID * Supported rates * Frequency-hopping (FH) Parameter Set * Direct-Sequence (DS) Parameter Set * Contention-Free (CF) Parameter Set * IBSS Parameter Set * Traffic indication map (TIM) Infrastructure network access points send beacons at a defined interval, which is often set to a default 100ms. In the case of an ad hoc network where there are no access points, a peer station is responsible for sending the beacon. After an ad hoc station receives a beacon frame from a peer, it waits a random amount of time. After that random timeout has elapsed, it will send a beacon frame unless another station has already sent one. In this way, the responsibility of sending beacon frames is rotated amongst all the peers in the ad hoc network, while ensuring that beacons will always be sent. Most access points allow the changing of the beacon interval. Increasing the beacon interval will cause the beacons to be sent less frequently. This reduces load on the network and increases throughput for clients on the network; however, it has the undesirable effect of delaying association and roaming processes as stations scanning for access points could potentially miss a beacon while scanning other channels. Alternatively, decreasing the beacon interval causes beacons to be sent more frequently. This increases load on the network and decreases throughput for users, but it does result in a quicker association and roaming process. An additional downside of decreasing the beacon interval is that stations in power save mode will consume more power as they must more frequently awake to receive beacons. Inspecting an idle network with packet-monitoring tools such as tcpdump or Wireshark would show that most traffic on the network consists of beacon frames, with a few non-802-11 packets mixed in, such as DHCP packets. If users joined the network, responses to each beacon would begin to appear, along with regular traffic generated by the users. Beacon frames must be sent with the CSMA/CA algorithm. This means that if a station is currently sending a frame when the beacon needs to be sent, it must wait. This means that beacons may not be sent as frequently as the beacon interval would indicate. However, stations are able to compensate for this difference by inspecting the timestamp in the beacon frame when it is finally sent. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Beacon frame」の詳細全文を読む スポンサード リンク
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