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 in an
infrastructure Basic service set. 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 must send a beacon is known as Target
Beacon Transmission Time. Beacon interval expressed in Time Unit. 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 Parameter Set
Direct-Sequence Parameter Set Contention-Free Parameter Set
IBSS Parameter Set Traffic indication map
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 be 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. Beacons Functions
While beacon frames do cause some nontrivial overhead for a network, they
are vital for the proper operation of a network. Radio NICs generally scan all
RF channels searching for beacons announcing the presence of a nearby
access point. When a radio receives a beacon frame, it receives information
about the capabilities and configuration of that network, and is also then able
to provide a list of available eligible networks, sorted by signal strength.
This allows the device to choose to connect to the optimal network.
Even after associating with a network, the radio NIC will continue to scan for
beacons. This has several benefits. Firstly, by continuing to scan for other
networks, the station has options for alternative networks if the current
access point’s signal becomes too weak to continue communication. Secondly, as
it still receives beacon frames from the currently associated access point, the
device is able to use the timestamps in those beacons to update its internal
clock. Beacons from the currently associated access point also inform
stations of imminent configuration changes, such as data rate changes.
Finally, beacons enable devices to have power saving modes. Access points will
hold on to packets destined for stations that are currently sleeping. In the
traffic indication map of a beacon frame, the access point is able to
inform stations that they have frames waiting for delivery.