CWAP Reference Guide so far… #CWAP12

I’ve tried to condense my notes from the study for CWAP-403 exam. The exam consists of lot of details which need to be learnt if you have not done enough capturing and analyzing 802.11 wireless frames before.

  1. CWAP 403 – Start > Introductory blog
  2. 802.11 Mac Header explained
  3. Key 802.11 Frames
  4. Troubleshooting WLAN issues #mindmap
  5. PHY Layer
  6. WLAN medium contention
  7. 802.11 Frame Exchanges (Security)
  8. How to capture WLAN Frames?
  9. Troubleshooting WLAN issues with 802.11 Frames
  10. 802.11n | HT Operations
  11. 802.11ac | VHT Operations
  12. Spectrum Analysis

Spectrum Analyzer – CWAP#13

Spectrum Analyzer – comprises of 15% syllabus for CWAP-403 exam. You can roughly expect around 9 questions from this section. Spectrum Analyzers range in price from a few hundred $ to thousands. They are not free to use like in the case of Wireshark etc..

I was unable to source spectrum analyzers easily and ended up not reading thoroughly on this topic My first attempt for CWAP-403 exam did not achieve the required passing score. I scored 44% on this section, hence I am writing this blog post to strengthen my understanding and also for those who are on the lookout for studying and understanding the concepts. The knowledge that is tested around this section comprises mainly around the terminologies related to Spectrum Analyzers and the understanding the patterns in tables/layouts displayed in the application.

The terminologies include Duty Cycle, Sweep Cycle, FFT, Resolution Bandwidth, Utilization, Domains and a few more. An understanding is required to demonstrate that one is able to locate the interfering devices and recognizing patterns using various Spectrum Analyzer applications.

A few known concepts before diving into Spectrum Analysis.

Cycle / Wavelength : A wave form which starts at the center, climbs in energy to the highest point, called the peak; returns to the center; then drops to the weakest point, called the trough and then continues till it finally attenuates and looses the energy. Wavelength is often measured from peak to the next peak.

Amplitude : Determined by the height, force or power of the wave.

One cycle

Frequency: Number of cycles within one second. E.g. 2.4GHz generates 2.4 billion times of cycles every second.

λ = Wavelength
C = Speed of light (186000 miles per second)
2.4 GHz = 12.5cm
5 GHz = 6cm

Free Space Path Loss (FPSL) – Loss of signal as it travels through free space. This is a theoretical value, as in the real world, there may be many obstacles, reflection, scatter which need to be accounted when estimating the signal at a location. FPSL is based on inverse square law, originally developed by Isaac Newton. You don’t need to know below formula for the exam.

FSPL = 36.6 + (20log10(f)) + (20log10(d))
FSPL = path loss in dB
f = frequency in MHz, d = distance in miles between antenna

RF Mathematics

dB is logarithmic ratio of values

  • We add gains +3dB = x2
  • We subtract losses -3dB = /2
  • We add gains +10dB = x10
  • We subtract losses -10dB = /10
  • dBm is power measurement relative to 1mW
  • dBi is the forward gain of an antenna compared to istropic antenna.


  • RSSI is a metric that is specified by measuring the amount of energy associated with the bits received via wireless NIC.

Noise Floor

  • Background level of radio energy that exist in a medium on a specific channel which is analyzed.


  • SNR can be presented as a dB value or as the difference between the RSSI(signal) and the noise floor(noise). High SNR is better performance.

Receive Sensitivity

Receive Sensitivity refers to the power level of an RF signal required to be successfully received by the receiver radio.

Wireless NICs

  • Use Antenna and coding filter to keep out of unwanted RF and bits.
  • Will use some of the specific information gleaned from the RF to bit transition process to actually add info to the wireless frame.
  • The additional information is added to the receiving station and known as Radiotap Header.
  • All the info shown on the Radiotap Header is in reference to “receiving station” and “not transferring station”

The Hardware

  • Mobile / Integrated
  • Mobile spectrum analyzers like protocol analyzers use the adapters present in laptops.
  • Integrated spectrum analyzers use APs to monitor the RF.
  • Popular mobile analyzers include AirMagnet, Spectrum XT, Metageek Wi-Spy DBx etc..
  • Integrated spectrum example may include Cisco Clean Air software to pull spectrum data from the AP.

The Software

  • Three popular applications available for mobile analysis, they are AirMagnet Spectrum XT, Metageek Chanalyzer, Cisco Spectrum Expert.

Spectrum Analysis Terminology

Duty Cycle

  • Duty Cycle indicates the fraction of time a resource is busy.
  • FFT (Fast Fourier Transform) Duty Cycle measurements are important way to determine potential interference/impact of an RF transmitter on WLAN operations. Duty cycle measures the amount of time in which the amplitude is above some arbitrary threshold. Threshold can vary from each software. (such as -95dBm, or 15dB above noise floor or -75dBm).

Sweep Cycles

  • In higher end spectrum analysis tools, a sweep is measured as a single scan of the bandwidth span. So if one is measuring 100 MHz of spectrum, a sweep is how long it takes to scan that 100 MHz band a single time.
  • For e.g. a real-time FFT plot shows amplitude (Y axis) plotted over frequency (X axis). Within the real-time FFT chart, there may be a trace for the maximum amplitude over the last sweep or possibly a “max hold” over all previous sweeps. When the plot updates after the next sweep, the data will be refreshed with new information and will be relative to the previous sweep.

Resolution Bandwidth

  • RBW is a reference to the smallest frequency that can be resolved by the receiver.
  • RBW should be low enough to resolve spectral components of the transmission being measured.

Waterfall Plots

  • Same data from FFT plot but adding the time dimension.
  • A waterfall plot is a three-dimensional plot in which multiple curves of data, typically spectra, are displayed simultaneously.
Waterfall plot using Chanalyzer

Wi-Fi Integration

As spectrum analysis tools have improved and developed, more emphasize is given on data reporting and analysis have moved tighter integration and correlation with Wi-Fi information.

RF Signature Characteristics

  1. Shape
  2. Patterns
  3. Uniqueness
  4. Frequency
  5. Pulse vs. Constant
  6. Duty Cycle
  7. Frequency Hopping

I have not documented individual interference examples from various sources. This is nicely documented on the metageek website and you can click here for references.

Other useful references

FFT Spectrum Analysis

EXAM MOMENT: When locating devices, use a directional antenna to aid in the location of the signal source. Study about Duty Cycles, FFT, Swept Spectrogram and other interference patterns.

802.11ac and a bit of 802.11ax #CWAP11

June 1997: 
September 1 999: 
802.11m 802.11b 
March 2007: 
March 2012: 
February 2014? 
June 2003: 
802.1 lg 
September 2005: 
June 2004: 
802.1 li 
September 2009: 
802.11 n 
September 2009: 
May 2008: 
802.11k, 802.1 Ir 
2007 2008 2009 
September 2011: 
802.11 v,802.11u 
2010 2011 2012 2013 2014 
Ratified Late 2020 
1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 
Figure 1-1. 802.11 timeline

This blog post will be focusing on 802.11ac in particular. We visited the aspects of 802.11n in the last blog post.

802.11ac introduced the VHT (Very High Throughput) along with some core technological advancement like MU-MIMO, 256 QAM addition & support for 80MHz/160MHz channels. One of the key differences also lie in the support of only 5GHz band. So there is still a dependency on 802.11n for 2.4Ghz support, however the upcoming 802.11ax will support both 2.4GHz and 5GHz.

Table 1. 
Calculating the speed of 802.1 lac and 802.1 lax 
802.1 lac 
802.1 lax 
(as number of 
data subcarriers) 
234 (80 MHz) 
2 234 (160 MHz) 
980 (80 MHz) 
2 x 980 (1 60 MHz) 
Data bits per 
5/6 log2(256) 
= 6.67 
5/6 x log2(1 024) 
= 8.33 
Time per 
OFDM symbol 
(800ns G') 
4 ps 
13.6 vs 
1 .56 
G bps 
G bps 
G bps 
G bps 
Table 1. 
Calculating the speed of 802.1 lac and 802.1 lax 
802.1 lac 
802.1 lax 
(as number of 
data subcarriers) 
234 (80 MHz) 
2 234 (160 MHz) 
980 (80 MHz) 
2 x 980 (1 60 MHz) 
Data bits per 
5/6 log2(256) 
= 6.67 
5/6 x log2(1 024) 
= 8.33 
Time per 
OFDM symbol 
(800ns G') 
4 ps 
13.6 vs 
1 .56 
G bps 
G bps 
G bps 
G bps 

Multi-user MIMO

  • One of the greatest potential of 802.11ac
  • Prior to this all the 802.11 standards used single user.
  • If there are two receivers located in sufficiently different directions, a beamformed transmission may be sent to each of them at the same time.
  • Enables better spatial reuse. As per the below example, the MU-MIMO builds on small-cell approach by enabling even more tightly packed networks. As a result AP can send independent transmissions within its own coverage area. Just as 802.3(Ethernet) reduces collision domains, MU-MIMO intends to reduce spatial contention of transmissions.
Downlink Multi-User MIMO

802.11ac Wave 1 and 2 – The first wave of 802.11ac products will be driven by the enthusiasm for higher speeds. APs will typically have three stream capabilities, but with 802.11ac providing 80 MHz channels and 256-QAM modulation, the speed will go from 450 Mbps to 1.3 Gbps. The second wave of 802.11ac products will add even wider channels and possibly even multi-user MIMO support, as outlined in the figure below.

Maximum number of spatial streams 
Channel width 
Maximum modulation 
Typical maximum speed 
Beamforming support 
MU-MIMO support 
First wave 
80 MHz 
1.3 Gbps 
Varies (depending on vendor) 
Second wave 
3 or 4 
160 MHz 
26 Gbps 



  • OFDM based transmission, 802.11ac divides the channel into OFDM sub carriers each 312.5kHz
  • To increase throughput, 802.11ac introduces two new channel widths. Supports 80MHz and further added 160MHz channel option for even higher speeds.
  • 802.11ac channels have exactly the same shape as previous OFDM channels (802.11a,g,n)


  • MCS Index tends to be much simpler than 802.11n. First 7 are mandatory and others are supported.
MCS index value 
Code rate (R) 
  • 802.11ac retains the ability to select a shortened OFDM guard interval if both Tx and Rx are capable of processing it. The GI shrinks from 800ns to 400ns, providing a 10% boost in the throughput.

VHT Signal Fields

The purpose of the Signal Field is to help the receiver decode the data payload, which is done by describing the parameters used for transmission. 802.11ac separates into Signal A and Signal B fields. For CWAP purposes this has not been dealt in depth. There are 2 parts in VHT Signal A field are referred as VHT-SIG-A1 & VHT-SIG-A2.


  • Bandwidth
    • 0 – 20MHz, 1 – 40MHz, 2- 80MHz & 3 – 160MHz
  • STBC
    • If the payload is encoded with STBC (Space-time block coding may be used when the number of radio chains exceed the number of spatial streams, it tx a single data stream across 2 spatial streams.) for extra robust-ness, this field is set to 1, otherwise will be 0.
  • Group ID
    • Frames to AP > group ID =0
    • Frames sent to STA Client > group ID = 63
  • Number of space-time streams
    • Starts from 0, e.g. if field is set to 3, then there are 4 space time streams.
  • Partial AID
    • Last 9 bits of the BSSID.
  • Transmit power save forbidden
    • Field will be 0, if AP in network allows client to power off radios when they have opportunity to transmit frames. Otherwise will be 1.
  • Short GI – Field set to 1 for 400ns, 0 for otherwise.
  • Short GI disambiguation – Extra symbol may be required denoting 1 or 0 for not required.
  • Coding – Field is 0 when convolutional coding is used to protect the data field, 1 when LDPC is used.
  • LDPC Extra Symbol – Field is set to 1 if extra symbol is required.
  • MCS – MCS Index value of the payload.
  • Beamformed – If matrix is applied to the transmission, the bit is set to 1 otherwise set to 0.
  • CRC – Error correction
  • Tail – 6 zeros are included to terminate the convolutional coder that protects the Signal A field.


  • Used to setup the data rate, as well as tune in the MIMO reception.
  • VHT Signal B Length (17, 19 or 21 Bits)
  • Reserved bits – Set to 1.
  • Tail bits
< IEEE 802.11ac Figure 22-19—VHT-SIG-A2 structure > 
SU VHT-MCS/MU[1-3] coding 
Rese rved 
Composite Name 
SU Name 
MU Name 
8 us 
Composite Name 
SU Name 
MU Name 
OFDM PHY Modulation 
Rese rved 
VHT Modulation 
84 ag 
8 us 
NSTS/Partial AID 
Partial AID 
< IEEE 802.1 lac Figure 22-18— 
-Al structure >

Air Magnet Pro can help you scan through the PHY frames


Frame aggregation was introduced in 802.11n, 802.11ac however adds an interesting new take on the aggregation. All frames transmitted use the aggregated MPDU (A-MPDU) format. Even the single frame transmitted in one shot is transmitted as aggregate frame.

Table 3-1. Size comparisons of transmissions for different 802.11 PHYs 
MSDU (MAC payload) size 
MPDU (MAC frame) size 
A-MSDU (aggregate MAC payload) 
PSDU (PLO payload) size 
PPDU frame) size 
Implied by maximum MSDU 
Not used with 802.1 la 
4,095 bytes 
Implied by maximum PSDU 
Implied by A-MSDU size 
65,535 bytes 
5.484 ms (mixed mode) or 10 
ms (greenfield mode) 
802.1 lac 
Implied by maximum 
MPDU size 
5.484 ms

Management Frames

  • VHT Capabilities Information element.
v VHT Capabilities Info: 
. . .01 
= Maximum MPDU Length: 7 991 
00.. = Supported Channel Width Set: Neither 160MHz nor 80+80 supported (OXO) 
. 1 = Rx LDPC: Supported 
1. = Short Gl for Supported 
= Short GI for 160MHz and 80+80MHz: Not supported 
1... = Tx STBC: Supported 
. = Rx ST BC: 1 Spatial Stream Supported (Oxl) 
. . .01 
. 1... = SU Beamformer Capable: Supported 
. = SU Beamformee Capable: Supported 
. = Beamformee STS Capability: 4 (Ox3) 
. 011. . 
= Number of Sounding Dimensions: 2 (Oxl) 
= MU Beamformer Capable: Not supported 
. = MU Beamformee Capable: Supported 
. ..Ø. = TXOP PS: Not supported 
. = +HTC—VHT Capable: Not supported 
.. 11 1... . .. 
. . = Max A-MPDU Length Exponent: 1 048 575 
. = VHT Link Adaptation: No Feedback (OXO) 
= Rx Antenna Pattern Consistency: Not supported 
= Tx Antenna Pattern Consistency: Not supported 
00.. = Extended NSS BW Support:
  • VHT Operations Information element
v Tag: VHT Operation 
Tag Number: VHT Operation (192) 
00 = Basic 
ll.. = Basic 
11 = Basic 
= Basic 
= Basic 
= Basic 
= Basic 
= Basic 
Tag length: 5 
v VHT Operation Info 
Channel Width: 20 MHz or 40 
Channel Center Segment 0: 
Channel Center Segment 1: 
Basic MCS Map: Oxfffc 
. ll.. 
.. 11 
. ll.. 
.. 11 
0-7 (OXO) 
Suppo r ted 
Suppo r ted 
Suppo r ted 
Suppo r ted 
Suppo r ted 
Suppo r ted 
Suppo r ted

NOTE: Greenfield mode was offered with 802.11n. The efficiency gains from greenfield mode were often lost because airtime-devouring CTS-to self

messages were required before transmitting in the greenfield mode. As a result, greenfield mode was removed from 802.11ac.

Beamforming Basics

  • As 802.11ac beamforming is based on explicit channel measurements, both the transmitter and receiver must support it.
  • Any device that shapes its transmitted frames is called beamformer, receiver of such frames is called beamformee.
  • The AP initiates frame exchange with the STA, which helps it to measure the channel. The result of the channel measurement is a derivation of the steering matrix.
  • Steering Matrix describes how to setup each element of transmitter’s antenna system to precisely overlap transmissions to reach farther.
  • To steer transmissions in a particular direction, a beamformer will subtly alter what is transmitted by each array. A simple phase shift can alter/steer the transmission.

Null Data Packet (NDP) – Standardizes beamforming methods. 802.11ac method of beamforming is termed as null data packet sounding. Sounding is the term used to denote the process  performed by the transmitter to acquire channel state information (CSI) from each of the different users by sending training symbols and waiting for the receivers to provide explicit feedback containing a measure of the channel.

VHT beamformer shall initiate a sounding feedback sequence by transmitting VHT NDP announcement frame followed by a VHT NDP after a SIFS.

for mec 
Beamfcgm— I 
•ing Relx»rt r 
Fr ame S 
IS Frmne 
. 1. AVHT

SU Beamforming

  • Begins with the beamformer sending a NDP announcement packet followed by NDP. The NDP has fixed known format. The beamformee receives the NDP, analyzes it and computes back in form of feedback matrix. The feedback matrix is sent in reply to the NDP in the form of compressed beamforming frame (CBF).
Beamfor mee 

MU Beamforming

  • As opposed to Tx to one device, MU-MIMO Aps are capable of simultaneously transmitting data to multiple device groups.
  • The key distinction between them is that with MU-MIMO beamforming and beamformer requires a response from all beamformees in order to conclude channel sounding.
  • The CBF packet is 802.11 action frame which contains a channel matrix that specifies the CSI for each client. The CBF is the largest contributor to the overhead caused by MU-MIMO transmission and is size is determined by
    • Channel Width
    • Number of radio chain pairs
    • Bit count of each CSI unit
Beamformee 1 
Be amforming 
Report poll 
Beamformee 2 
Beamformee 3 
Beam f 

Recommended Reading

Cisco 802.11ax White Paper
Wifi Certified 6 Highlights
802.11 Framing in Detail
802.11ac Channel Planning
802.11ac VHT PHY
Research Paper on VHT MU-MIMO
802.11ac – A Survival Guide

802.11n | HT Operations #CWAP10

The blog post will cover the topics related to High Throughput Throughput technologies in conjunction with the exam objectives laid down for CWAP-403 exam. 802.11 Frame Exchanges cover 25% of the knowledge domain required for the exam. Analysing HT & VHT transmission methods are one of the sub topics under this section. I will be focusing on the HT/802.11n type in this blog, apparently it has gone a little longer than i thought. There are certain section which might take further reading from 802.11n Survival Guide if you are keen.

802.11n ~ High Throughput

  •  Ratified Sep 2009
  • Clause 20 technology, backward compatible with HR-DSSS (Clause 18), OFDM (Clause 17).
  • Can be used for both 2.4GHz & 5GHz bands.

MIMO Enhancements

  • Transmit Beamforming (TxBF) – Tx(Transmitter) Radios multiple antenna can transmit in the best direction of the Rx (Receiver).
  • Spatial Multiplexing (SM) – Tx multiple radios at the same time with each unique stream containing different data.
  • Space-Time Block Coding(STBC) – Transmitting redundant copies of data stream from different antenna thereby increasing the signal quality.
  • Antenna Selection (ASEL) – Increase signal diversity by dynamic selection of antennas.

Spatial Multiplexing

  • Takes advantage of multipath (when signal tends to reflect, scatter, diffract or refract).
  • Multiple streams follow different paths to the receiver because of the space between the transmitting antenna is known as spatial diversity and is also called as spatial multiplexing.
  • When using SM, both Tx and Rx should participate and be MIMO systems.
FIGURE 10.2 
MuItiple spatial streams 

HT Channels

  • Use 20 MHz OFDM channels.(NON-HT)
    • Each 20MHz OFDM channel contains 64 subcarriers which are each 312.5 KHz wide and can be separately modulated.
    • First 6 & Last 5 sub carriers are null as they act like guard band for the channel + center subcarrier is also null. This leaves 52 subcarriers.
    • Out of 52, 48 transmit data while 4 used in dynamic calibration between Tx and Rx.
  • 20MHz OFDM channels (HT)
    • Each 20MHz OFDM channel has 56 subcarriers, 52 transmit data, 4 are used for calibration between Tx and Rx.
  • 40 MHz Channels
    • Use 114 OFDM subcarriers, 108 transmit data, 6  are used for calibration.
    • A 40MHz channel doubles the frequency bandwidth available for transmission of the data.
    • A 40MHz channel used by HT radios essentially 2x 20MHz OFDM channels bonded together.
Table 3-1. Channel description attributes 
+25, +53 
PHY standard 
Subcarrier range Pilot subcarriers 
-57 to +57 
+7, +21 
+7, +21 
Subcarriers (total/data) 
52 total, 48 usable 
56 total, 52 usable 
114 total, 108 usable

Modulation and Coding Scheme (MCS Index)

  • Value that describes the number of spatial streams, modulations (BPSK, QPSK, 16-QAM or 64-QAM and further) and error correction code used in Tx.
  • 802.11n supports equal modulation, in which all SS are transmitted in same manner, and unequal modulation, in which the spatial streams may be modulated differently.
  • 802.11n defines 77 different combinations of modulation and coding.
  • There are 8 mandatory MCS for 20 MHz HT channels.
TABLE 10.1 
MCS index 
Mandatory modulation and coding schemes—20 MHz channel 
Data rates 
800 ns Gl 
6.5 Mbps 
13.0 Mbps 
19.5 Mbps 
26.0 Mbps 
39.0 Mbps 
52.0 Mbps 
58.5 Mbps 
65.0 Mbps 
400 ns Gl 
7.2 Mbps 
14.4 Mbps 
21.7 Mbps 
28.9 Mbps 
43.3 Mbps 
57.8 Mbps 
65.0 Mbps 
72.2 Mbps

Guard Interval (GI)

  • The GI is the space between the symbols being transmitted.
  • May be confused with IFS, the GI is there to eliminated inter-symbol interference where is referred to as ISI.
  • ISI happens when echoes from one symbol interfere with another.
  • A good rule of thumb specifies that GI should be 4x the highest multipath delay spread. When 802.11a was designed, designers used conservative value of 200ns for the delay speed, and choose to make the GI 800ns.


I’ve discussed this topic in details under this blog post.

Wi-Fi Alliance

  • Before the 802.11n amendment was ratified, the HT technology was already being certified and sold. The Wi-Fi alliance had developed a vendor certification program called Wi-Fi CERTIFIED 802.11n draft 2.0. The Cert Program as name suggested, certified products against the amendment. Draft 2.0 supports a max data rate of 300Mbps which is half max data rate specified in ratified document.
  • Details about the Wi-Fi certified “n” features can be found here

HT Control Field

  • The 802.11n amendment adds a new field in 802.11 MAC header, called the HT Control Field. It is 4 octets long and follows QoS control field in 802.11 MAC header.
  • Any MPDU that contains an HT control field is referred to as +HTC MPDU.

The Order Bit – The 802.11n amendment uses the existing but relatively unused order bit in the Frame Control field of the MAC header to indicate the presence of an HT Control Field in QoS data & management frames. Original purpose of this bit was to indicate that data muse be sent using a strict ordered class of service.

Control Wrapper Frame – is/are described using the carried frame name + HTC, for example RTS+HTC or CTS+HTC

HT Control Field Format – figure below shows the format of HT Control field. (Honestly some of the stuff went way over my head but might have to figure this out by looking at a few pcaps & studying them :|)

Link Adaptation Control (16 bits)

  • TRQ – Training Request > Set to 1 to request the responder to transmit a sounding PPDU. Set to 0 to indicate that the responder is not requested to transmit a sounding PPDU.
  • MAI (MCS Request (MRQ) or ASEL Indicator) – When set to “14”, it is an ASEL indicater which indicates that you would interpret the MFB/ASELC subfield as an ASEL command.
  • MFSI – MCS Feedback Sequence Identifier- A MCS Feedback (MFB) frame is sent in response to a MCS Request.
  • MFB/ASELC – MCS feedback and Antenna Selection Command -When ASEL indicator is present, the MFB/ASELC subfield interpreted as ASELC subfield. Otherwise it is interpreted as MFB subfield. A value of 127 indicates that no feedback is present

Calibration Position (2 bits)

  • Set to 0 indicates this is not a calibration frame.
  • Set to 1 indicates calibration start.
  • Set to 2 indicates sounding response.
  • Set to 3 indicates sounding complete.

Calibration Sequence – Each of the four packets within the calibration exchange will have the same sequence number.

CSI/Steering – When using sounding frames to transmit feedback about the channel, the Channel State
Information (CSI)/Steering subfield identifies the type of feedback being used.

NDP Announcement – indicates that an NDP will be transmitted after the frame. It is set to 1 to indicate that an NDP will follow; otherwise, it is set to 0. NDP are used to send sounding PPDU when no other data needs to be transmitted. If a frame transmitted that require an immediate response and also has the TRQ=1 (request for sounding PPDU) then receiver can either transmit the MPDU response withing a sounding PPDU or send the response MPDU with the NDP Announcement bit set to 1, indicating that NDP will be transmitted following the current PPDU.

Reverse Direction Protocol – 802.11n amendment which improves the efficiency of data transfer between STAs.

HT Action Frames & Information Elements

Information Elements

HT Capabilities, HT Operations, 20/40 BSS Coexistence & Overlapping BSS Scan Parameters,

HT Capabilities Element

  • Can be seen in Beacon, Probe Req/Response, Association Req/Response & Reassoc Req/Response.
  • You can figure out the MCS values supported by the 802.11n AP from this section in the pcap.
  • Determine A-MPDU parameters

HT Operation Element

  • STA operation within an HT BSS environment.
  • Found in Beacon, Reassociation Response, and Probe Response frames transmitted by an AP.

HT information elements

  • Primary Channel, Secondary Channel Offset and STA channel width.
    – When the Supported Channel Width Set subfield is equal to 1(as in above), indicating a 20/40 MHz BSS, then the Primary Channel field indicates the primary channel number.
    – Secondary Channel – Directly above or below the primary channel.
  • Protection Mechanisms – To ensure backward compatibility with older 802.11 a/b/g radios, an HT access point will operate in one of four protection modes. 00 in above pcap example.
  • RIFS mode – The 802.11e QoS amendment introduced the capability for a transmitting radio to send a burst of frames during a transmit opportunity (TXOP). (prohibited in above pcap case).
  • Basic MCS Set – Last in Operations element, similar to MCS set field in HT Capabilities Element.

For Further Reading

Troubleshooting WLAN issues with 802.11 Frames – #CWAP9

I have pen down a some troubleshooting scenarios which I’ve come across while studying for CWAP exam.

To begin with,

Management Frames > Foundation of how wireless radios detect, join and operate on WLAN.
Control Frames > Frames which control the delivery of Data frames.
Data Frames > Carry actual data payload from/to layers 3-7.

Some scenarios which frames can provide an insight for.

  • Client Roaming Observations – In some cases, there might be some issues with clients not able to perform seamless roaming or the roaming might be delayed when client moves from one AP to another. In some cases we may need to find which type of roaming method are supported by the AP to diagnose other issues. Let’s see how the frames can help.
    • To find the roaming handoff time from one AP to another we have to examine the frames from type > Reassociation Type to the completion of 4-way handshake. E.g. frame below
s 0.029712 
8 0.053240 
: 2. : 98 
: db:Sc 
- - -nnrr_h- 'h 
o ch&ge E 
7 cipher Spec, Fessaqe 
24 3 
  • Total roaming time can be calculated by subtracting the EAPOL M4 time (0.105180) with Reassociation Request Frame(.003857)= .101323 ~ approx. 101ms
  • Type of roaming method can be deduced from the Tagged Parameters set in 802.11 Wireless LAN section. Below example uses Over-the-air Fast BSS, value of 1 will denote it using Over-the-DS BSS.
Tag: Mobility Domain 
Tag Number: Mobility Domain (54) 
Tag length: 3 
Mobility Domain Identifier: øxcd64 
FT Capability and Policy: Oxøø 
Fast BSS Transition over DS: Oxø 
Resource Request Protocol Capability: 
  • Management Retries – Generally anything under 20% of Management retries in the network is considered OK or acceptable. There is no set vendor recommended management retry. In a prod environment it is bound to have certain % of retries even if the AP or Client placement/AP Tx Power/Interference and Channel settings are set to optimal. In any case anything over constant 20% retries could indicate some concerns in the WLAN environment which need investigation.
Total Retransmissions Across All Clients 
Mgmt Retre o.øax (408) 
Mgmt 439% (1212) —l 
Data Retre 083% 
vgmt Retries: 094% (408) 
Data Fran-— 15g3% (5039) — 
control 7702% (33,946)
IEEE 8ø2.11 Authentication, Flags: .R... 
Type/Subtype: Authentication (øxoøøb) 
Frame Control Field: exbøø8 
. .øø = Version: 0 
= Type: management frame (e) 
— Subtype: 11 
: øxe8 
. .øø = DS status: Not leaving DS or network is operating in AD—HOC mode (To DS 
More Fragments: This is the last fragment 
Retry: Frame is being retransmitted 
PWR MGT: STA wilt stay up 
. = More Data: No data buffered 
. = Protected flag: Data is not protected 
. = Order flag: Not strictly ordered 
: e From DS: e) 
.øøø eøøø eølø lløø = Duration: 44 microseconds 
Receiver address: Cisco_bf:a4:2e (øø: 
Destination address: Cisco_bf:a4:2e (eø:a7:42:bf:a4:2e) 
Transmitter address: 5e:a7:ec:a8:33:ab (5e:a7:ec:a8:33:ab) 
Source address: 5e:a7:ec:a8:33:ab (5e:a7:ec:a8:33:ab) 
BSS Id: 
= Fragment number: ø 
— Sequence number: 1 
eøøø eøøø eøøl -
  • We can also check this on the Wireshark IO graphs as below to highlight the management retries. Below network has lot of management retries and needs further investigation
Wireshark • 10 Graphs • airtool_2019-11-28_02.47.29.PM .pcap 
Wireshark 10 Graphs: .pcap 
HO Ver over the graph for details. 
Display Filter 
Time (s) 
Interval 10 sec 
Y Axis 
Y Field 
Graph Name 
Al packets 
TCP errors 
SMA Period 
Mouse O drags 
O zooms 
Copy tram v 
Time of day 
Log scale 
Save As...
  • Duration/ID field
    • 16 bits in length, used for virtual carrier-sense, legacy power management & contention-free period.

In the below RTS frame, the duration value is 2048ms. The radio is asking for permission to reserve airtime to pending transmission. The receive radio can allow or deny this request. But higher duration value can indicate the delays it is causing in allowing/denying the request. This can cause some weird behavior in client operation, may also cause disruption in network services. We have to closely check the change log on the WLAN environment. If this is a result of some WLAN controller/AP software update or other updates which may cause the issues. Also NOTE: Please check the device and not always high duration value can be a problem.

IEEE 8ø2.11 Request-to-send, Flags: ..... ...C 
Type/Subtype: Request—to—send (Oxøølb) 
v Frame Control Field: exb40e 
. .øø = Version: 0 
= Type: Control frame (1) 
= Subtype: 11 
. .øø = DS status: Not leaving DS or network is operating in AD—HOC mode (To DS: 
ø . — More Fragments: This is the last fragment 
— Retry: Frame is not being retransmitted 
- PWR MGT: STA wilt stay up 
. = More Data: No data buffered 
. — Protected flag: Data is not protected 
Order flag: Not strictly ordered 
.øøø løøø oøøø ooøø - 
Duration: 2ß48 microseconds 
Receiver address: App 92:ga) 
Transmitter address: 7a:8a:2ø:øf:bg:6f 
Frame check sequence: øx4d4e67bf (unverified] 
[FCS Status: Unverified] 
e From DS: e) 
  • Null Data Frames / Power Management

The null data frames are in fact not null as per their description. They can help in troubleshooting few WLAN issues. Null data is categorised under control frame. It is only transmitted from a STA/Client. The sole purpose is to carry power management frames controlled field. The power management bit will either be set to 0 or 1. Below are the examples.

STA = 0, it is informing AP that it(STA) is In active power state (awake) and transmission of frames from AP to STA should be normal.

IEEE 8ø2.11 Nun function (No 
Type/Subtype: Nutt function 
Frame Control Field: ex48e1 
. .øø = Version: e 
data), Flags: ...TC 
(No data) (øx0024) 
= Type: Data frame (2) 
= Subtype: 4 
. ..øl = DS status: Frame from STA to DS via an AP (To DS: 
= More Fragments: This is the last fragment 
Retry: Frame is not being retransmitted 
PWR MGT: STA will stay up 
More Data: No data buffered 
. = Protected flag: Data is not protected 
= Order flag: Not strictly ordered 
1 From DS: e) 
.øøø eøøø eølø lløø = Duration: 44 microseconds 
Receiver address: RuckusWi_cf:d2:7c (2c:5d:93:cf:d2:7c) 
Transmitter address: Apple_51:44:de (94:f6:d6:51:44:de) 
Destination address: (2c:5d: 93: cf :d2:7c) 
Source address: Apple_51:44:de (94: f6:d6:51:44:de) 
BSS Id: Ruckuswi_cf

STA =1, is informing AP that it is going offline and any frames that come into the AP from this STA should be buffered at the AP till the STA returns and sends a NULL frame of 0, active state.

IEEE 8ø2.11 Null function (No 
data), Flags: 
Type/Subtype: Nutt function 
(No data) (øx0024) 
Frame Control Field: ex4811 
. .øø = Version: e 
= Type: Data frame (2) 
= Subtype: 4 
. ..øl = DS status: Frame from STA to DS via an AP (To DS: 
= More Fragments: This is the last fragment 
Retry: Frame is not being retransmitted 
PWR MGT: STA will go to sleep 
More Data: No data buffered 
. = Protected flag: Data is not protected 
. = Order flag: Not strictly ordered 
1 From DS: e) 

PSM > Power Save Mode allows the client STA to go into sleep mode. It can essentially turn of the NIC functions including the radio thereby consuming less battery and conserving it. Some devices can benefit from this but there are some which may have aggressive power save mode options. So one needs to check the client driver details to troubleshoot any issues relating to client.

Some known issues with Power Management are described in below links

Another reason why client STA may inform AP about changing the bit to 1 is when it is roaming. Suppose client has reached the roaming limits of the AP it was connected to and wants to switch to the nearby one, in order to to this it may go off the channel sending the buffer frames signal to AP and resume its connection.

How to capture WLAN Frames? #CWAP8

This blog post will focus on tools I’ve used for performing Wireless Frame Captures. I’ve been largely dependent on Macbook for capturing the wireless frames. I would highly suggest you for sourcing a Macbook for frame capture as Windows PC option involves getting a third party WLAN pcap which is not cheap. Thank you Apple for making it possible to capture frames natively on Mac.

The Hardware

  • Macbook Pro

Other Utilities Required/Recommended.

  • Wireshark is available as free tool to download. It is highly recommended to optimize it using the wireless configuration profiles available at Metageek. This is our primary tool for capturing and analyzing the frames.

It is recommended to add (Absolute Time, Relative Time & Delta Time) values on the Wireshark as it is important when analyzing the wireless frame analysis. In roaming scenarios, one may need to acquire the time it took for a client to move between one AP to another.

  • Airtool is also available for free. This tool is not mandatory but good to have. Since it is free, then why not? It helps capture frames on few mouse clicks and helping you easily move them analyze them on wireshark or via online (Packets)
  • Packets (Arista) – Phenomenal tool for analyzing the frames. Birds eye view of various frame types in the wireless environment, management retries, problem clients etc. Free account available up to 100MB of pcap (more than sufficient for your CWAP studies).
  • WiFi Explorer – Highly Recommended if you can purchase, the professional version costs around $20 USD. Can really help with identifying the WLAN discovery and metrics of the environment.
  • If you own an iPhone or iPad, one can configure Wi-FI Diagnostics on the phone. Thanks for George Stefanick for explaining it so nicely.

802.11 Frame Exchanges – Security #CWAP7

802.11 Frame Exchanges section account for 25% of syllabus for CWAP-403 exam. Potentially around 15 questions out of 60 in the exam can be expected from this section. This blog post focuses on the “security” component of 802.11 Frame Exchange. I will be focusing on other sections in the subsequent posts in the next week or two. Let’s begin!


1st step required to connect to 802.11 BSS. Both authentication and association must occur in order to successfully pass wireless traffic over to the AP and further. IEEE 802.11i-2004 defines RSNA. Open System & Shared Key Authentication are Prior to RSNA (Pre-RSNA) methods. The 802.11 authentication merely establishes an initial connection between the client and the access point, basically validating or authenticating that the STA is a valid 802.11 device.

  • Open System Authentication > Allows any device to authenticate and then attempt to communicate with the AP. The STA can communicate only its Wired Equivalent Privacy(WEP) keys match the AP
  • Shared Key Authentication > Not used anymore. Requires static WEP key configured on STA and AP.

Open System authentication and association between client STA and AP occurs prior to 802.1x/EAP authentication exchange between client STA and Radius server.

WLAN Encryption Methods

  • WEP
    • Pre-RSNA
    • Weak / Vulnerable / No Protection against replay attacks
    • Open/Shared Authentication
  • TKIP (Temporal Key Integrity Protocol) (RSN)
    • Uses dynamically created encryption keys as opposed to static keys.
    • 128-bit temporal key can either be a pairwise transient key (PTK) or group temporal key (GTK) used to encrypt
    • WPA-PSK & WPA-Enterprise
    • Can be vulnerable against certain attacks.
  • CTR with CBC-MAC Protocol (CCMP) (RSN)
    • CTR – Counter mode is used for data confidentiality
    • CBC MAC(Cipher-block chaining message authentication code) is used for integrity.
    • Used with AES block cipher suite with 128 bit key
  • SAE (Simultaneous Authentication of Equals)
    • Uses SAE known as Dragonfly Key Exchange, with forward secrecy feature
    • WPA3 Personal – 128 Bit SAE, Enterprise – 192 bit SAE
    • Not Vulnerable to KRACK attacks and offline dictionary attacks.

The info that is protected by these L2 encryption methods is data found in layers of 3-7. L2 encryption methods are used to provide data privacy for 802.11 data frames. These methods encrypt MSDU payload of an 802.11 data frame.

Security Protocols WEP TKIP (WPA) CCMP (WPA2) OWE (Opportunistic Wireless Encryption)
Cipher RC4 RC4 AES AES-GCM & Elliptical Curve Cryptography
Key 40/104 bits 128 bits 128 bits 192 bits
Authentication N/A IEEE 802.1X/EAP/PSK IEEE 802.1X/EAP/PSK WPA3 Personal / Enterprise
Data integrity CRC32 MIC CCMP Secure Hash Algorithm-2 for each input
IV Length 24 bits 48 bits 48 bits 24 bits

RSNA (Robust Security Network Association)
First published & ratified as IEEE 802.11i-2004, defined stronger encryption and better authentication methods. Now part of 802.11-2007 standard. Association between two stations is referred to as RSNA which means the two radios should share dynamic encryption keys that are unique between those two radios. CCMP/AES s mandatory, TKIP RC4 is optional. All client stations have to undergo a unique RSNA process called the 4-way handshake.

FIGURE 9.12 
Key 1 
RSNA within a BSS 
Group temporal key (GTK) 
Key 2 
Key 4 
Key 3 Access Point 
4-Way Handshake 
Keyl Key 4 
Key2 Key4 
Keys 1, 2, and 3 encrypt 
unicast traffic 
Key 4 encrypts 
broadcast/multicast traffic 
Key3 Key4

The RSN information element field is found in 4 management frames: beacon, probe, association request and reassociation request frames. Client STA use the association request frame & reassociation request (in case of roaming to/from) to inform the AP about their security capabilities.

FIGURE 9.18 
Client STA 
Client station RSN security capabilities 
Association request frame 
RSN information element: 
Client STA security capabilities 
1) CCMP/AES encryption 
2) 802.1X authentication 
Association response frame 
Success! - Welcome to the BSS! 
Access Point 
Original AP 
Roaming Client STA 
Reassociation request frame 
RSN information element: 
New AP 
Client STA security capabilities 
1) CCMP/AES encryption 
2) 802.1X authentication 
Reassociation response frame 
Success! - Welcome to the new BSS!

RSN information element – AES(CCMP) used in the below frame example.

v Tag: RSN Information 
Tag Number: RSN Information (48) 
Tag length: 20 
RSN Version: 1 
Group Cipher Suite: (leee 802.11) AES (CCM) 
Pairwise Cipher Suite Count: 1 
Pairwise Cipher Suite List (leee 802.11) AES (CCM) 
Auth Key Management (AOI) Suite Count: 1 
Auth Key Management (AKM) List (leee 802.11) WPA 
RSN Capabilities: 
— RSN Pre—Auth capabilities: Transmitter does not support pre—authentication 
0. — RSN No Pairwise capabilities: Transmitter can support WEP default key 0 simultaneously with Pairwise key 
10.. = RSN PTKSA Replay 
10 = RSN GTKSA Replay 
. 0.. = Management Frame 
= Management Frame 
= Joint Multi—band 
= PeerKey Enabled: 
Counter capabilities: 4 replay counters per PTKSA/GTKSA/STAKeySA (Ox2) 
Counter capabilities: 4 replay counters per PTKSA/GTKSA/STAKeySA (Ox2) 
Protection Required: False 
Protection Capable: False 
RSNA: False 


The 802.1X standard is port-based access control standard which provides an authorization framework that allows or disallows traffic to pass through port thereby granting access to the network resources.  802.1X can be implemented in either wireless/wired environments. The L2 protocol called EAP (Extensible Authentication Protocol) is used and consists of 3 major components of this framework.

  • Supplicant > Client STA
  • Authenticator > AP or WLAN Controller.
  • Authentication Server > Usually Radius(NPS), ISE (Cisco)
FIGURE 9.23 
comparison-autonomous access point and WLAN controller 
Autonomous AP 
Client station 
RADIUS server 
(authentication server) 
Network resources 
802. IX—Autonomous AP 
Client station 
WLAN controller 
RADIUS server 
(authentication server) 
Network resources 
802. IX—WLAN Controller


Defined in IETF RFC 2284 and ratified in the IETF RFC 3748, provides support to many authentication methods.

  • L2 Protocol
  • Two way authentication also called as mutual authentication.
  • EAP messages are encapsulated in EAP over LAN (EAPOL)
  • Five major types of EAPOL messages as shown below
TABLE 9.1 
Packet type 
0000 0000 
0000 0001 
0000 0010 
0000 0011 
0000 0100 
EAPOL messages 
EAPOL- Encapsulated - 
This is an encapsulated EAP frame. The majority 
of EAP frames are EAP-Packet frames. 
This is an optional frame that the supplicant 
can use to start the EAP process. 
This frame terminates an EAP session and 
shuts down the virtual ports. Hackers some- 
times use this frame for DOS attacks. 
This frame is used to exchange dynamic keying 
information. For example, it is used during the 
4-Way Handshake. 
This frame is used to send alerts, such as 
SNMP traps to the virtual ports.
FIGURE 9.27 
Generic EAP exchange 
1 802.11 association 
2 EAPoL-start 
4 EAP-response/identity (username) 
8 EAP-challenge-response 
Controlled and 
ports blocked 
Uncontrolled port opens 
Dynamic encryption keys created 
4-Way Handshake 
13 Controlled port opens

EAP Protocols

The stronger and more commonly deployed methods of EAP use TLS (Transport Layer Security) or TLS-tunneled authentication. EAP-MD5 and EAP-LEAP have only 1 supplicant identity making them weaker EAP types. EAP-TLS uses 2 supplicant identities – outer and inner identity. The outer identity is effectively a bogus username and can be seen clear text, and then inner identity is the true identity protected with TLS tunnel.  Table describes all the protocols with their characteristics.

802.1X EAP Types Feature / Benefit MD5 Message Digest 5 TLS Transport Level Security TTLS Tunneled Transport Level Security PEAP (WIDELY USED) Protected Transport Level Security FAST Flexible Authentication via Secure Tunneling LEAP Lightweight Extensible Authentication Protocol
Client-side certificate required no yes no no no (PAC) no
Server-side certificate required no yes yes yes no (PAC) no
WEP key management no yes yes yes yes yes
Rogue AP detection no no no no yes yes
Provider MS MS Funk Software MS Cisco Cisco
Authentication Attributes One way Mutual Mutual Mutual Mutual Mutual
Deployment Difficulty Easy Difficult (because of client certificate deployment) Moderate Moderate Moderate Moderate
Wi-Fi Security Poor Very High High High High High when strong passwords are used.

4-Way Handshake

802.11-2007 standard requires EAPOL-Key frames be used to exchange cryptographic information between STA supplicants and the authenticator, which is usually an AP. EAPOL key frames are used for the implementation of three different frames exchanges: 4-way handshake, group key exchange & peerkey handshake. 4 way handshake is the final process used to generate pairwise transient keys (PMK / GTK) for the encryption of unicast transmissions and the group temporal key for encryption of broadcast/multicast transmissions.

The 4-way handshake uses pseudorandom functions, it hashes various inputs to derive a value (PRF). The PMK is one of the inputs combined with other inputs to create the pairwise transient key (PMK). Some of the other inputs used by the PRF are called nonces. A nonce is a random numerical value that is generated one time only. In the case of 4-way handshake, a nonce is associated with the  PMK. Two nonces are created in 4-way handshake – authenticator nonce (anonce), supplicant nonce (snonce).

PTK = PRF (PMK + anonce + snonce + aa(Authenticator Mac)+ spa (Supplicant Mac).

M1 – Message 1

  • Authenticator sends EAPOL-Key frame containing “anonce” to supplicant
  • With this info,  supplicant have all the necessary input to generate PTK using PRF

M2 – Message 2

  • Supplicant sends an EAPOL-Key frame containing “snonce” to the authenticator
  • Authenticator has all the inputs to create PTK
  • Supplicant also sends RSN IE capabilities to Authenticator & MIC (message integrity code)

M3 – Message 3

  • If necessary, Authenticator will derive GTK from GMK
  • Authenticator sends EAPOL-key frame containing “anonce”, RSN-IE and a MIC.
  • GTP (encrypted with PTK) delivered to the supplicant.
  • Message to supplicant to install temporal keys.

M4 – Message 4

  • Supplicant sends final EAPOL-key frame to authenticator to confirm temporal keys have been installed.

Group Key Handshake

The 802.11-2007 standard also defines a two-frame handshake that is used to distribute a new group temporal key to client STA that have already obtained a PTK and GTK in a pervious 4-way handshake. The GKH is used only to issue a new group temporal key to client STA that have previously formed security associations. Effectively GKH is identical to M3/M4 in 4 way handshake.

Fast BSS Transition (FT)

Published in 2008, 802.11r – technical name for standardized fast secure roaming. An Amendment to improve handoff from one AP to another. The handoff is the same with or without 11r, the device is what ultimately decides when and where to roam.  802.11r are often discussed in context with WLAN controller architecture. Mobility domain is a group of AP that belong to the same ESS where the client STA can roam in a fast and secure manner. FT BSS transitions can happen over-the-air or over-the-DS (Distribution System).

FIGURE 9.33 
Fast BSS transition information element 
Element ID 

FT over-the-air (AP to AP, Same Controller)

  1. Client associates with AP1 and requests to roam to AP2
  2. Client sends a FT authentication request to AP2 and receive FT authentication response from AP2.
  3. Client sends FT reassociation request to AP2 and receives FT re-association response from AP2.
  4. Client completes the roaming from AP1 > AP2

FT over-the-air (AP-CONTROLLER|CONTROLLER-AP)(Inter-Controller)

  1. Step 1 & 2 similar to above steps.
  2. WLC1 ends PMK and mobility message to WLC-2 about the roaming client that uses mobility infrastructure.
  3. Client completes the roaming from AP1 > AP2
Tag: Mobility Domain 
Tag Number: Mobility Domain (54) 
Tag length: 3 
Mobility Domain Identifier: øxcd64 
FT Capability and Policy: exøø 
ø = Fast BSS Transition over DS: exø 
= Resource Request Protocol Capability: 
v Tag: Fast BSS Transition 
Tag Number: Fast BSS Transition (55) 
Tag length: 88 
MIC control: øxeeøø 
eøøø eøøø 
= Element Count: e 
MIC: eøøøooøøøøeeøøøeoøøøooøøøøooøøøe 
ANonce: øeoøøøøooøøøooøøøooøøøøooøøøooøøøooøøøøooøøøooøøm 
SNonce: cc1f8ga18b7615afa146b8249a7311283587dd66ca57eeeg„. 
Subelement ID: PMK—Rø key holder identifier (ROKH—ID) (3) 
Length: 4 
PMK-RO key holder identifier (ROKH-ID): cea814øa

FT over-the-DS (AP to AP, Same Controller)

  1. Client Associates to AP1 and requests to roam to AP2
  2. Client sends a FT authentication request to AP1 and receives a FT authentication response from AP1
  3. The controller sends the pre-authentication info to AP2 as the AP are member of same controller.
  4. Client sends a FT re-association request to AP2 and receives a FT re-association response from AP2.
  5. Client completes its roaming

FT over-the-DS (AP to AP, Different Controller)

  1. Step 1 and 2 are similar to above steps.
  2. WLC-1 sends PMK and mobility message to WLC-2 about the roaming client
  3. Client completes its roam from AP1 to AP2.
Tag: Mobility Domain 
Tag Number: Mobility Domain (54) 
Tag length: 3 
Mobility Domain Identifier: øxcd64 
FT Capability and Policy: Oxø1 
Fast BSS Transition over DS: Oxl 
Resource Request Protocol Capability: 
Tag: Fast BSS Transition 
Tag Number: Fast BSS Transition (55) 
Tag length: 88 
MIC control: øxeeøø 
eøøø eøøø 
= Element Count: e 
MIC: eøøøooøøøøooøøøooøøøooøøøøooøøøo 
ANonce: øeoøøøøooøøøooøøøooøøøøooøøøooøøøooøøøøooøøøooøø„. 
SNonce: f71ebøf1ef1b8725392f92f2979186ed912676cb6cb5cb53-. 
Length: 4 
PMK-RØ key 
ID: PM-Re key holder identifier (ROKH-ID) (3) 
holder identifier (ROKH—ID): cea814øa

Recommended Readings

How to Run and for little longer!

I’ve always wanted to write about “Running” as a focus on health/fitness regimen. Running is one of the best forms of cardio exercises which can improve overall health. I’ve been running for the past 10+ years. The maximum i’ve run is 22kms~13.1 miles for a half marathon. I normally don’t run any less than 3-5kms in a given run. Generally make sure that I run 2x 10kms and around 5x 5kms in a month. As they say quite right about ‘Health being the best form of Wealth’ anyone can possess. Running also provides much needed stress relief and provides clarity in thinking helping you with effective decision making.

I’d like to provide some tips which can benefit you with running. This article is aimed at people who starting on the course of their journey to be more fit and improve their overall training which can help with achieving the health/fitness goals.

  1. Always Hold the Vision > If you aim to run tomorrow have the vision of completing it in your mind the night before. Envision the track or the trail and see yourself completing it. I’ve seen this help a lot as its important for your brain assimilate your goals.
  2. Invest in good Running Shoes > I’ve learnt that running shoes play a major role when it comes to running and sustainability. Incorrect shoe size or uncomfortable shoes can lead to short/long term foot injury and issues. Good shoes may cost you at the time of purchase but the benefits outweigh as they provide the much needed comfort while running enabling you to run for longer without any injury and pain.
  3. What to eat and not> Don’t eat a heavy dinner/meal the night before you plan for the run. Green leafy veggies salad with beetroot can help with sustainability and pace for the running. Believe it or not Beetroot can do wonders for running as it is packed with nitrates which can increase the blood flow capacity and lowering the amount of oxygen muscles need. Even if you end up eating other than salad make sure you have it 3 hours before the bedtime. Don’t eat or drink anything new before or during the workout.
  4. Aim to get 6-8hours of Sleep > Sleep is important, especially a longer sleep cycle ensures that your body goes through the REM state enabling your digestion process to complete and making your muscles rest so that they are prepared for the run.
  5. Bowel Clearance > This is an important element, the gut has to be happy so that you can focus on running. Ensure you drink a warm/hot glass of water upon getting up in the morning helping you with clearing the bowel.
  6. What gets measured gets improved > I am sure you have a smart phone or perhaps a smart watch as well. I am not saying this required all the time but it is good to have your run/activity tracked. Tracking helps you know where you stand and helps in improving the performance. I’ve been using Fitbit and Strava for recording the activity.
  7. Utilities – Carry a bottle of water if you are new to running. Hydration is very important for any physical activity. Make sure you put on some sunscreen to block harmful UV if you are running between 9am to 5pm. If you are running for 10+ KMs, might pay to carry a small sack containing a couple of bananas and some peanuts. That’s the best natural energy bar you can have without spending much money. Wear according to the forecast/temperature in your area. Don’t wear too much warm attire/clothing because we tend to warm up soon.
  8. Don’t forget the Warm-up & Stretch > Before commencing the run perform some warm up exercises, this can include some push ups or pull ups. After completing the run/activity make sure you perform some stretching exercises to relieve the muscular tension. I’ve seen that missing the stretching routine can cause leg sprains.

WLAN Medium Contention – #CWAP6

Accounts for 10% of the CWAP knowledge domain areas, approx. 6/60 questions

Medium Contention :Protocols that allow large number of devices to effectively share the wireless channel. All AP & STAs will contend with each other on a common transmission medium.

CSMA / CA – The AP/STAs (802.11) use carrier sense multiple access with collision avoidance as opposed to collision detection used by the Ethernet (802.3) realm.

802.11 devices must avoid multiple devices transmitting simultaneously over a shared medium which can cause failed transmissions. Wireless mediums cannot detect collision but find ways to avoid them. Collision handling is not straight forward and may be time consuming at times. Hence one of the reasons that 802.11(WLANs) have much lower throughput-to-data rate ratio than 802.3(Wired LANs).

CSMA/CA uses DCF (Distributed Coordination Function)  for non-QoS WLANs & HCF (Hybrid Coordination Function) for QoS WLANs using EDCA (Enhanced Distributed Channel Access).

There are two carrier sense protocols used by the stations to indicate whether a channel is busy or idle.

  • Physical Carrier Sense, also known as CCA (Clear Channel Assessment)
  • Virtual Carrier Sense, also known as NAV (Network Allocation Vector)

Both QoS & non-QoS use either of the above protocols for transmitting data.

CCA (Layer 1) > Identify whether the channel is unused and available prior to the packet transmission.

  • Channel Occupied = State of Busy ~ Energy Detection Levels.
  • Channel Clear = State of Idle

Apply to 802.11 modulation, if the AP or STA is too far away to detect any transmission at requisite energy level, the CCA may go into the idle state even though the channel is still occupied.

NAV (Layer 2) > is a timer that counts down toward zero(0). When a device has a NAV value greater than zero, the device says quiet. Once the NAV = 0, the medium is considered clear.

As discussed earlier, CCA may fail to keep other devices on the channel quiet (Too far transmitting device, obstruction, interference), the design of the NAV keeps APs and stations quiet.

Duration value in the 802.11 header set the NAV values for AP and STAs.

It is vital for the AP and STA to stay with the RSSI data range in order to successfully demodulate a transmitted frame so that the Duration/ID field in the header can be accurately set.

Interframe Spaces

When 2 or more STAs begin frame transmission at the same time in the idle environment, collisions are bound to happen. Hence we have additional medium contention protocols beyond CCA & NAV. These protocols must keeps AP and STAs quiet like CCA/NAV & also allow differentiated medium access.

IFS is the quiet period that AP & STA must wait before any 802.11 frame transmission.

TIPS to Remember!

  • If the contention has been completed, then a reduced IFS (RIFS) or short IFS (SIFS) will be used. Most cases it is SIFS but RIFS is only used between consecutive frames transmitted by 802.11n device.
  • If the contention/arbitration is not determined, then arbitration IFS (AIFS) or DCF IFS (DIFS) will be used. The AIFS is used for WLANs that support 802.11e QoS, and the DIFS is used for WLANs that do not support 802.11e QoS.
  • If an AP or STA has received a corrupted frame as defined by having an incorrect FCS, then extended IFS will be used.
  • PCF IFS (PIFS) is part of PCF and therefore not used in real world. (May be ignored for CWAP prep!)
  • 802.11 FHSS network use 50ms slot time.
  • Steps involved for a STA to go through before starting the frame transmission in the wireless medium (Source : 802.11 Arbitration CWNP White Paper)
— Arbitration Flowchart 
is clear 
idle F 
"S inte rval 
it one is 
timer one slot. 
Is NAV O• 
Transmit Frame


  • Foundation of all IFSs.
  • 10ms for 802.11b/g/n (2.4GHz)
  • 16ms for 802.11a/n (5GHz).
  • It is used after contention/arbitration is completed. Exception being 802.11n device using MIMO to transmit frames then RIFS is used.


  • Simplest IFS to understand.
  • Length is always the same 2ms.
  • Only for devices which use 802.11n/MIMO.
  • It precedes for only “data” frame.


  • Designed to force AP and STA with ordinary data in the queue to stay quiet for enough time to allow QoS frames to have access to the channel.
  • It is used when arbitration process has not yet completed.
  • DIFS is equal to length of SIFS + 2 slot times. Slot times are quiet periods, similar to IFS.
  • They are equal to 9ms for 802.11a/n/ac operating in 5GHz and 802.11g/n with 2.4GHz.
  • The 20ms slot is used if the HT or ERP is used with long preamble and 802.11b/g/n 2.4 GHz DSSS.
  • The short preamble is default setting when HT or ERP is used.


  • Designed to give AP and STA a chance to retransmit after a failed frame.
  • This happens when AP/STA failed to receive ACK after transmission.
  •  EIFS = SIFS + DIFS plus the time taken acknowledge the frame to transmit.
  • 802.11b/g/n(2.4GHz) using DSSS= 364ms, 802.11a/n(5GHz) & 802.11g/n (2.4GHz) = 160ms. EIFS is the longest of the IFS.

Near/Far Problem : STA closer to AP may cause problem to STA at far. When data is transmitted between AP and nearby STAs they can use higher data rate than far stations. (This is why STA dynamically switch their data rates downward when moving away from the AP). The frame therefore will appear to be corrupt even though it was successfully transmitted. The far STA have to stay quiet for an EIFS at the beginning of the arbitration process, while the near STA will be allowed to use the shorter DIFS.

PIFS > Equal to one slot time + 1 SIFS and it is designed to give AP the chance to send the beacon in order to begin the CFP (Contention Free Period). In real-world the PIFS is only used with Channel Switch Announcement frame, which is one of the Action frames from 802.11h.


The mechanism which prevents collision by differentiating 802.11 channel access is the Random Backoff. Unlike the IFS, the random backoff is not static. It is the period of time that changes based on a random number chosen by AP or STA.

AP and STA stay quiet during the random backoff by randomly choosing a number of slot times and then counting down until the number of slot times equal to zero. Transmission resumes after slot time equals zero.

  • For the random backoff to work, there must be an upper and lower limit to the number of slot times that ca be chosen.
  • The lower limit is always 0. The upper limit for the random backoff is equal to the contention window (CW). 
  • The CW is derived from the equation 2x – 1, where x is a value that increments with each failed frame. For DSSS-based networks, x starts at 5, which results in a CW of 31. For OFDM-based networks, x starts at 4, which results in a CW value of 15. For both DSSS and OFDM-based networks, the x value stops incrementing at 10, which results in a CW value of 1023.
  • Failed frames cause the contention window to grow exponentially. More quiet time means a less efficient channel thus causing latency and throughput issues.
Immediate access when 
Medium is idle DIFS or AlFSti1 
Busy Medium 
Defer Access 
Contention Window 
Backoff Slots 
Slot time 
Next Frame 
Select Slot and Decrement Backoff as long 
as medium is idle 
Figure 2.4: The DCF Operation Overview



  • Used by QoS enabled STA to transmit all data, management, PS-Poll, RTS, CTS (when not transmitted as response to RTS), Block Ack Req and Block Ack (when not transmitted as a response to Block Ack Req).
  • Slot times in AIFS is called as AIFSN (slot number).
  • 802.11e specifies Voice (AV_VO), Video (AV_VI), Background (AV_BK) & Best Effort (AV_BE).
  • Video and Voice = 2 Slot times
  • Best Effort = 3 Slot times
  • Background = 7 Slot times
  • Calculate AIFS for a given Access Category = AIFSN[AC] x Slot Time x SIFSTime


  • Transmit Opportunity or TXOP is the amount of time a STA can send frames when it has won contention for the wireless medium. This is in relation to EDCA (Enhanced Distributed Channel Access).
  • When a STA sends QoS data, it must first contend for the access to the wireless medium.
  • STAs perform CCA and determine if the channel is idle. It must have its NAV set to 0. Then it must wait for the appropriate InterFrame Spacing.
  • Then it would wait for the contention window to complete. CW has 4 categories as discussed in the previous section. Each category has different TXOP.
IEEE Std 802.11-2016 
IEEE Standard for Information Technolog—ocal and Metropolitan Area Networks—Specific Requirements 
part 1 1: Wreless LAN MAC and PHY Specifications 
Table 9-137—Default EDCA parameter Set element parameter values if dot110CBActivated is false 
T.xop limit 
aCW min 
ac Wmin 
(aCWmin + I 
(a CWmin+ 
CW max 
a CWmax 
a CWmax 
ac Wmin 
(a C Wmin+ I 
in Clauw IS 
3.264 ms 
6.016 ms 
Clause 17, 
Clauw IS, 
4.096 ms 
2.080 ms 
For denned 
in 22 
22.56 ms 
(BCU. 60r7 
16.92 ms 
(BCU. S MHz) 
1128 ms 
(BCU 60r7 
S MHz)