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- Figure A6-20 shows the frame format of the IEEE 802.11 WLAN specification. Below describes
the subfields and flags in the Frame Control field:
Protocol Version Indicates the version of the 802.11 protocol. A receiving station uses this
value to determine whether it supports the version of the protocol of the
received frame.
Type and Subtype Determine the function of the frame – management, control, or data.
The type and subtype fields for each frame type determine the specific
function to perform.
To DS and
From DS
Indicates whether the frame is destined to or exiting from the distributed
system (DS). All frames of wireless stations that are associated with an
access point (infrastructure mode) will have one of the DS bits set. The
interpretation of the Address fields depends on the setting of these bits.
More Fragments Indicates whether there are more subsequent fragments for a particular
management or data frame are to follow. Control frames are not
fragmented, hence this bit is always set to 0 for control frames.
Retry Indicates whether the management or data frame is being retransmitted.
Power
Management
Indicates whether the sending wireless station is in active or powersaving
mode.
More Data Used to inform a wireless station which is in power-saving mode that the
access point has more frames to send to it. Also used by an access points
to indicate that additional broadcast or multicast frames are to follow.
This bit is only being used in management and data frames; hence this bit
is always set to 0 for control frames.
Protected Indicates whether encryption and authentication are used for the frame.
Control frames may not be encrypted; hence this bit is always set to 0 for
control frames.
Order Indicates that all received data frames must be processed in sequence.
- Below shows how to interpret the To DS and From DS bits:
From DS = 0 From DS = 1
To DS = 0
All management, control, and data
frames within an IBSS (ad-hoc).
Data frames arrived at a wireless station
(from AP) in an infra. WLAN.
To DS = 1
Data frames transmitted from a wireless
station (to AP) in an infra. WLAN.
288
Data frames on a wireless bridge
(WDS, Wireless Distribution System).
- The Duration/ID field is used in all control frames (except with the subtype of PS-Poll) to
indicate the remaining duration needed to receive the next frame transmission.
- Wireless stations may want to save battery power by turning off antennas. When the subtype is
PS-Poll, it contains the association identity (AID) of the waking transmitting station, which
indicates which BSS the station belongs to. Note: PS is referred to as Power Save.
Copyright © 2008 Yap Chin Hoong
yapchinhoong@hotmail.com