frames

1. Frames
One of the maintopicsthat we shouldlearnisthe conceptof 802.11 framestotake successfully
the CWAP exam.Inthischapter,we will introduce the evolutionof generalframe since creation
until current802.11ac frame and explainbrieflyeach fieldrelatedtogeneral frame.
802.11 General Frame Format from 802.11-1997 and 1999
In 802.11-2007 were incorporatedsome changes.The Frame Bodyfieldisnow from0-2304 and
because of 802.11e QoS, wasaggregatedQoSControl fieldtopassinformation QoSanddefine
queue operationsinthe STAs.
802.11 General Frame Format from 802.11-2007
With802.11n ratification,in2009 was incorporated802.11-2012. ThispermitsalongerFrame
BodywithA-MSDU, alsowe can see that was introducedthe HTControl fieldthatcontains
informationrelatedtoTransmitBeamformingandAntennaSelection(ASEL) andanothermore.
802.11 General Frame Format from 802.11-2012
The 802.11ac formatshows the Frame Body fieldas variable forlengthwithaminimumof 0octets
and maximumlengthbasedinMMPDU,MSDU, A-MSDU and MPDU that are receivedforthe
PPDU format.Finally,fieldslikeQoSControl,Address4,HT Control,TKIP,CCMPGCMP andMIC
can impact the lengthof Frame Body.
802.11 General Frame Format from 802.11ac-2013

2. Now we will do a brief description of each field related to a general frame 802.11:
Frame Control
Frame Control Field 802.11-2012
1. The Protocol Version bits are always set a 00.
2. The Type and Subtype fields (6-bits) define the frame type (management, control o data) and
the subtype. Next table lists the important valid values for these bits.
3. The subfields ToDSand From DS bits determine whetheraframe is transmitted from a STA to
the AP, from one STA to another in a IBSSS or using the four-address MAC header format

3. (mesh BSS). Next table shows the To DS and From DS values appropriate as defined in the
802.11 standard.
4. The More Fragments subfield (1-bit) determineswhether current frame is part of fragmented
frame or not.
5. The Retry Field (1-bit) isusedwhenaframe isretransmittedthatinclude the Retry field set to
1.
6. The PowerManagementfield(1-bit) isabitindicatingwhetherpowermanagement is used by
the STA.
7. The ProtectedFrame field (1-bit)indicateswhetherthe MSDU isencryptedinthe frame if it is
set to 1.
8. The Order field issetto1in a non-QoSdataframe to indicate thatit containsan MSDU. It is set
to 1 in a QoS data or management frame to indicate that the frame contains an HT control
field.
Duration ID
The Duration/ID field is used for two purposes. First, it may contain the duration of the frame.
Second, it may contain the association identifier (AID) of the STA that transmitted the frame.
Address1, 2, 3 and 4
The 802.11 general frame formatspecifiesfouraddressfields.The nexttable providesanoverview
of the use of these fields.

4. Sequence Control
It is used with fragmentation and for the removal of duplicate frames should they occur. It is
divided intoa4-bitfragmentnumberanda 12-bit sequence number(16-bittotal).Whenan MSDU
is fragmented, all fragments have the same sequence number and the fragment number is
incrementedby1for eachframe until all fragmentsare delivered. The sequencenumber start at 0
until it reaches 4095, then simply resets to 0 and begins again.
QoS Control
It is a 16-bit field that identifies the category to which the frame belongs for queuing purposes.
The most important factor in this field is the user priority (UP). EDCA is implemented in QoS
WLANsbasedon WMM certification the bits0-3in the QoS Control fieldare mappedwithpossible
values from 0 to 7. The next table lists the mapping of WMM access categories (ACs) to 802.1d
tags.
HT Control
It is used to specify parameters related to the HT operations and VHT operations.
HT Control Field in 802.11-2012

5. HT Control Field in 802.11ac