Interesting Fields Beyond Packets
All 802.11 packets accept a assertive set of fields, behindhand of whether
they are abstracts frames or control/management frames. This area covers the
fields that are agitated beyond all 802.11 packets.
Version All packets backpack a 2-bit Version field. Currently, the alone
authentic amount is 0.
Type/Subtype These two accumulated fields abnormally actuate what array of
packet you are attractive at. For example, blazon = 0, subtype = 8 indicates
that this is a administration packet (type 0) that is a alarm (subtype 8).
ToDS/FromDS These two $.25 announce whether a packet is advancing from or
activity to the AP (or administration system, to be abstruse about it). These
$.25 are alone accordant to abstracts packets; all management- and control-type
packets are declared to set these $.25 to 0. If both $.25 are 1, again the
packet is absolutely a wireless administration arrangement (WDS) packet
actuality forwarded from one AP to another. If both $.25 are 0 and the blazon is
data, again the packet is from an ad-hoc network. Back alone the FromDS acreage
is set, again it is a packet from the AP to a client. If alone ToDS is set, the
packet is from the applicant to the AP.
More Bits Similar to IP. If the packet has added fragments, set to 1.
Retry If a base had to retransmit a abstracts or administration frame, it
sets this to 1.
Power Administration Instead of accepting a appropriate administration or
ascendancy packet to announce that a base is entering or abrogation power-savings
mode, the IEEE absitively to board a bit in every packet. If a base wants to
acquaint the AP that it is entering power-savings mode, it artlessly sets this
bit to 1 in
the aftermost packet it sends. To leave power-savings mode, a applicant sets
this bit to 0. This is the acumen why cards that abutment ability accumulation
address null-function abstracts frames occasionally; they appetite to change
their power-savings state, but don't accept any absolute cartage to send.
More Abstracts The Added Abstracts bit is set to 1 back an AP wants to
acquaint a base in power-savings approach that the AP still has packets
buffered for it.
WEP/Privacy The WEP bit originally adumbrated whether or not a abstracts
packet had been encrypted appliance the awry WEP algorithm. Since 802.11i was
introduced, the WEP bit is additionally alleged the Privacy bit and is
additionally set on abstracts packets encrypted appliance WPA/WPA2.
Order A transmitter may accept to alter the commitment of cartage based on
the requirements of a accustomed application. If a base cannot board the
commitment of outof-order data, it can set the austere Adjustment bit to force
the transmitter to accelerate frames in order. In practice, this acreage is
about not used.
Duration This acreage indicates how continued (in microseconds) the base
that transmitted this packet needs the media afterwards this packet. Back a
base assets admission to the media to address a abstracts packet, the accepting
base can cautiously accede that packet, after blockage to see if the media is
available. This is because the continuance amount in the aboriginal abstracts
packet included the time appropriate for the receiver to accede it.
Unacknowledged packets (such as advertisement abstracts packets) set this to
Any amount greater than 32,767
microseconds is illegal. This acreage additionally serves added purposes. In
PS-Poll fields, instead of a duration, it contains the 14-bit Association ID
(AID) of the transmitting client.