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<article id="post-5246" class="post-5246 post type-post status-publish format-standard hentry category-cwap category-wireless-qos tag-cwap tag-difs tag-edca tag-eifs tag-interframe-spaces tag-rifs tag-sifs">
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<h1>CWAP – 802.11 Medium Contention</h1> <a href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/" class="post-permalink"> <p class="post-date">
<strong>12</strong>
<em>Sunday</em>
<span>Oct 2014</span>
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Posted <span class="by-author"> by <span class="author vcard"><a class="url fn n" href="https://mrncciew.com/author/nayarasi/" title="View all posts by nayarasi" rel="author">nayarasi</a></span></span> in <a href="https://mrncciew.com/category/cwap/" rel="category tag">CWAP</a>, <a href="https://mrncciew.com/category/wireless-qos/" rel="category tag">Wireless QoS</a> </p>
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<strong>≈ <a href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/#comments">10 Comments</a></strong>
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<p><strong>Tags</strong></p><p><a href="https://mrncciew.com/tag/cwap/" rel="tag">CWAP</a>, <a href="https://mrncciew.com/tag/difs/" rel="tag">DIFS</a>, <a href="https://mrncciew.com/tag/edca/" rel="tag">EDCA</a>, <a href="https://mrncciew.com/tag/eifs/" rel="tag">EIFS</a>, <a href="https://mrncciew.com/tag/interframe-spaces/" rel="tag">Interframe Spaces</a>, <a href="https://mrncciew.com/tag/rifs/" rel="tag">RIFS</a>, <a href="https://mrncciew.com/tag/sifs/" rel="tag">SIFS</a></p> </div><!-- end .post-extras -->
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<p><span style="color:#00ffff;">DCF</span>– <span style="color:#ff00ff;">Distributed Coordination Function</span> : <span style="color:#00ff00;">Non-QoS WLAN</span><br>
<span style="color:#00ffff;">HCF with EDCA</span> – <span style="color:#ff00ff;">Hybrid Coordination Function</span> : <span style="color:#00ff00;">QoS WLAN</span><br>
<span style="color:#ff00ff;">EDCA</span>– <span style="color:#ff00ff;">Enhanced Distributed Channel Access<br>
</span><span style="color:#00ffff;">PCF</span> – <span style="color:#ff00ff;">Point Coordination Function</span> (<em><span style="color:#00ff00;">not implemented practically</span></em>)</p>
<p><span style="color:#00ffff;">Physical Carrier Sense</span>: <span style="color:#ff00ff;">Layer 1 – Clear Channel Assessment (CCA)</span><br>
– ED (Energy Detection)<br>
– CS (Carrier Sense or Preamble detection)<br>
<span style="color:#00ffff;">Virtual Carrier Sense</span> : <span style="color:#ff00ff;">Layer 2 – Network Allocation Vector (NAV</span>) – Duration value set in each frame’s MAC header where other stations set their NAV to this if the sense medium is busy.</p>
<p>These are the steps a station go through prior to transmit a frame to the wireless medium<br>
1. STAs use a physical carrier sense (Clear Channel Assessment—CCA) to determine if the wireless medium is busy.<br>
2. STAs use virtual carrier sense (Network Allocation Vector—NAV) to detect if the medium is busy. When the virtual timer (NAV) reaches zero, STAs may proceed.<br>
3. If conditions 1 and 2 are met, STAs wait the necessary IFS interval, as prescribed by the protocol.<br>
4. If conditions 1 and 2 are met through the duration of condition 3, STAs generate a random backoff number in accordance with the range of allowed values.<br>
5. STAs begin decrementing the backoff timer by one for every slot time duration that the wireless medium is idle.<br>
6. After decrementing the backoff value to zero, with an idle medium, a STA may transmit the allotted frame exchange, in accordance with the parameters of the obtained transmission opportunity (TXOP).<br>
7. If another STA transmits before Step 6 is completed, STAs observe steps 1, 2, 3, and 5 until the backoff timer is equal to zero.<br>
8. After a successful transmission, repeat as needed. Below diagram show the flow of the above steps (source : 802.11 Arbitration CWNP white paper)</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-01.png"><img data-attachment-id="5250" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-01/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-01.png?w=529&h=398" data-orig-size="699,526" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-01" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-01.png?w=529&h=398?w=300" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-01.png?w=529&h=398?w=529" class="aligncenter size-full wp-image-5250" src="./values_files/cwap-contention-01.png" alt="CWAP - Contention-01" width="529" height="398" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-01.png?w=529&h=398 529w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-01.png?w=150&h=113 150w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-01.png?w=300&h=226 300w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-01.png 699w" sizes="(max-width: 529px) 100vw, 529px"></a><span style="text-decoration:underline;"><span style="color:#00ffff;text-decoration:underline;">Inter Frame Spaces</span></span><br>
After each frame transmission 802.11 protocol require an idle period on the medium called Inter Frame Space (IFS). The length of the IFS is depend on previous frame type, following frame type, access category, coordination function in use & PHY type as well. The purpose of an IFS is both to provide a buffer between frames to avoid interference as well as to add control and to prioritize frame transmissions.</p>
<p><span style="text-decoration:underline;"><span style="color:#00ffff;text-decoration:underline;">SIFS (Shortest Inter Frame Space)</span></span><br>
SIFS are used within all of the different coordination functions. For 802.11-2007, SIFS is the shortest of the IFSs and is used prior to ACK and CTS frames as well as the second or subsequent MPDUs of a fragment burst. However, with 802.11n, a shorter IFS (RIFS) was introduced.</p>
<p>SIFS for 802.11b/g/n (2.4 GHz) = <span style="color:#00ff00;">10μS</span><br>
SIFS for 802.11a/n/ac (5 GHz) = <span style="color:#00ff00;">16μS</span></p>
<p><span style="text-decoration:underline;"><span style="color:#00ffff;text-decoration:underline;">RIFS (Reduced Inter Frame Space)</span></span><br>
RIFS were introduced with 802.11n to improve efficiency for transmissions to the same receiver in which a SIFS-separated response is not required, such as a transmission burst (CFB-Contention Free Burst)</p>
<p>RIFS = <span style="color:#00ff00;">2μS </span></p>
<p>802.11n standard use RIFS & Block Acknowledgement (mandatory in 802.11n). RIFS is used only when Block ACK is enabled. When Block ACK are used, data frames of a CFB may send consecutively without interruption by ACK. At the end of CFB, Tx Station will simply send BAR (BlockACKRequest) & receiving a single Block Acknowledgement (BA).</p>
<p>Below shows the use of RIFS during a 802.11n frame transmission. (page 260 CWAP study guide). Note that AIFS used initially as of QoS Data frames.</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-07.png"><img data-attachment-id="5268" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-07/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-07.png?w=529" data-orig-size="524,250" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-07" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-07.png?w=529?w=300" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-07.png?w=529?w=524" class="aligncenter size-full wp-image-5268" src="./values_files/cwap-contention-07.png" alt="CWAP - Contention-07" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-07.png 524w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-07.png?w=150 150w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-07.png?w=300 300w" sizes="(max-width: 524px) 100vw, 524px"></a><span style="text-decoration:underline;"><span style="color:#00ffff;text-decoration:underline;">DIFS (Distributed Inter Frame Space)<br>
</span></span>When a STA desires to transmit a data frame (MPDU) or management frame (MMPDU) for the first time within a DCF network, the duration of a DIFS must be observed after the previous frame’s completion. The duration of a DIFS is longer than both the SIFS and PIFS.</p>
<p><span style="color:#ff00ff;">DIFS = SIFS + 2x SlotTime</span></p>
<p>SlotTime for 802.11a/n/ac (5 GHz) = <span style="color:#00ff00;">9μS</span><br>
SlotTime for 802.11g/n (2.4 GHz – HT or ERP) = <span style="color:#00ff00;">9μS</span> with <span style="color:#00ffff;">short preamble</span><br>
SlotTime for 802.11g/n (2.4 GHz – HT or ERP) = <span style="color:#00ff00;">20μS</span> with <span style="color:#00ffff;">long preamble</span><br>
SlotTime for 802.11b/g/n (2.4 GHz – DSS ) = <span style="color:#00ff00;">20μS</span></p>
<p><span style="text-decoration:underline;"><span style="color:#00ffff;text-decoration:underline;">EIFS (Extended Inter Frame Space)</span></span><br>
The EIFS value is used by STAs that have received a frame that contained errors. By using this longer IFS, the transmitting station will have enough time to recognize that the frame was not received properly before the receiving station commences transmission. If, during the EIFS duration, the STA receives a frame correctly (regardless of intended recipient), it will resume using DIFS or AIFS, as appropriate</p>
<p><span style="color:#ff00ff;">EIFS (in DCF) = SIFS + DIFS + ACK_Tx_Time</span></p>
<p>EIFS 802.11b/g/n devices using DSS = <span style="color:#00ff00;">364μS<br>
</span> EIFS 802.11g/n devices using OFDM = <span style="color:#00ff00;">160μS</span><br>
EIFS 802.11a/n devices (5GHz) = <span style="color:#00ff00;">160μS</span></p>
<p><span style="color:#ff00ff;">EIFS (in EDCA) = SIFS + AIFS[AC] + ACK_Tx_Time</span></p>
<p>Below diagram show the arbitration process after receipt of a corrupted frame, where all other stations (except the transmitter) wait for EIFS.</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-10.png"><img data-attachment-id="5269" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-10/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-10.png?w=529" data-orig-size="492,240" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-10" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-10.png?w=529?w=300" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-10.png?w=529?w=492" class="aligncenter size-full wp-image-5269" src="./values_files/cwap-contention-10.png" alt="CWAP - Contention-10" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-10.png 492w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-10.png?w=150 150w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-10.png?w=300 300w" sizes="(max-width: 492px) 100vw, 492px"></a><span style="color:#ff00ff;">Near/Far Problem:</span> Due to side effect of EIFS, stations near to AP could cause problem to stations at Far(hence called Near/Far problem). When data send between AP & near by stations, they can use high data rates where far stations cannot be demodulate & interpret as corrupted frame. So far stations stay quiet for EIFS, while the near station will be allowed to use DIFS or AIFS. The use of DIFS will give nearby station higher priority & get more opportunity to transmit while far station remain quiet.</p>
<p><span style="text-decoration:underline;"><span style="color:#00ffff;text-decoration:underline;">PIFS (PCF Inter Frame Spaces)</span></span><br>
PIFS are used by STAs during the contention-free period (CFP) in PCF mode. Because PCF has not been implemented in 802.11 devices, you will not see PIFS used for this purpose. In order to gain priority over other STAs during contention, the AP can transmit a Channel Switch Announcement (802.11h) frame after observing a PIFS</p>
<p><span style="color:#ff00ff;">PIFS = SIFS + SlotTime</span></p>
<p>Below chart summarize SIFS,DIFS,PIFS & SlotTime values described earlier.</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-11.png"><img data-attachment-id="5270" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-11/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-11.png?w=529&h=328" data-orig-size="599,372" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-11" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-11.png?w=529&h=328?w=300" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-11.png?w=529&h=328?w=529" class="aligncenter size-full wp-image-5270" src="./values_files/cwap-contention-11.png" alt="CWAP - Contention-11" width="529" height="328" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-11.png?w=529&h=328 529w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-11.png?w=150&h=93 150w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-11.png?w=300&h=186 300w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-11.png 599w" sizes="(max-width: 529px) 100vw, 529px"></a><span style="text-decoration:underline;"><span style="color:#00ffff;text-decoration:underline;">AIFS (Arbitration Inter Frame Space)</span></span><br>
The AIFS shall be used by <span style="color:#ff00ff;">QoS STAs</span> to transmit all data frames (MPDUs), all management frames (MMPDUs), and the following control frames: PS-Poll, RTS, CTS (when not transmitted as a response to the RTS), BlockAckReq, and BlockAck (when not transmitted as a response to the BlockAckReq).</p>
<p>The number of slot times used in the AIFS is called the<span style="color:#ff00ff;"> Arbitration Inter Frame Space Number</span> (<span style="color:#00ffff;">AIFSN</span>). 802.11e specifies 4 access categories (<span style="color:#00ffff;">AV_VO</span> : <span style="color:#ff00ff;">Voice</span>, <span style="color:#00ffff;">AC_VI</span> : <span style="color:#ff00ff;">Video</span>, <span style="color:#00ffff;">AC_BE</span> : <span style="color:#ff00ff;">Best Effort</span> & <span style="color:#00ffff;">AC_BK</span> : <span style="color:#ff00ff;">Background</span>). <span style="color:#ff00ff;">Voice & Video</span> category use <span style="color:#00ff00;">2 slottimes</span> by default. <span style="color:#ff00ff;">Best Effort</span> category use <span style="color:#00ff00;">3 slottimes</span> where as <span style="color:#ff00ff;">Background</span> traffic use <span style="color:#00ff00;">7 slottimes</span> by default.</p>
<p>Below is the formula to calcluate AIFS for a given Access Category (AC)</p>
<p><span style="color:#ff00ff;">AIFS[AC] = AIFSN[AC] × SlotTime + SIFSTime</span></p>
<p><span style="text-decoration:underline;color:#00ffff;">Contention Window/Backoff Timer</span><br>
After a STA has observed an idle wireless medium with carrier sense (CS) for the appropriate IFS interval (DIFS, EIFS, or AIFS). To contend for medium access after the IFS, each station selects a backoff value called random backoff period and is selected at random by the STA from a window of possible values called a contention window (CW) calculated using the below formula where x is a value increments with each failed frame.</p>
<p><span style="color:#ff00ff;">CW = 2^x -1</span></p>
<p>For <span style="color:#ff00ff;">DSS</span> based networks <span style="color:#00ff00;">x starts at 5</span> which resulting CW of<span style="color:#00ff00;"> 31</span>, for <span style="color:#ff00ff;">OFDM</span> based networks,<span style="color:#00ff00;"> x starts at 4</span> which result in a CW of <span style="color:#00ff00;">15</span>. In <span style="color:#ff00ff;">both DSS & OFDM</span> x values stops incrementing at <span style="color:#00ff00;">10</span> which result CW of <span style="color:#00ff00;">1023</span>. Below table summarize these values for DCF network.</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-05.png"><img data-attachment-id="5263" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-05/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-05.png?w=529" data-orig-size="459,296" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-05" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-05.png?w=529?w=300" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-05.png?w=529?w=459" class="aligncenter size-full wp-image-5263" src="./values_files/cwap-contention-05.png" alt="CWAP - Contention-05" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-05.png 459w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-05.png?w=150 150w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-05.png?w=300 300w" sizes="(max-width: 459px) 100vw, 459px"></a>In EDCA medium access (QoS stations & AP), each AC has a CW_min & CW_max. These default values are derived from the following formula.</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-06.png"><img data-attachment-id="5266" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-06/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-06.png?w=529" data-orig-size="498,310" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-06" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-06.png?w=529?w=300" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-06.png?w=529?w=498" class="aligncenter size-full wp-image-5266" src="./values_files/cwap-contention-06.png" alt="CWAP - Contention-06" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-06.png 498w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-06.png?w=150 150w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-06.png?w=300 300w" sizes="(max-width: 498px) 100vw, 498px"></a>For example, in OFDM aCWmin is 15. This result <span style="color:#ff00ff;">CWmin for AC_VO =3</span> {[(15+1)/4] – 1} & <span style="color:#ff00ff;">CWmax for AC_VO=7</span> {[(15+1)/2]-1}. In the same way <span style="color:#ff00ff;">CWmin for AC_VI=7</span> {[(15+1)/2] -1} & <span style="color:#ff00ff;">CWmax for AC_VI=15</span>. For <span style="color:#ff00ff;">AC_BE & AC_BK</span> those values are <span style="color:#ff00ff;">15</span> & <span style="color:#ff00ff;">1023</span> respectively.</p>
<p>For an initial attempt at a frame transmission, the CW is the aCWmin value.For each attempted retransmission, the CW is increased exponentially until the CW reaches the aCWmax. In this case, the CW remains at the aCWmax until a successful frame sequence is performed or the number of attempted retransmissions reaches the max retry count. Below diagram shows this concept</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-03.png"><img data-attachment-id="5260" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-03/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-03.png?w=529&h=410" data-orig-size="549,426" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-03" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-03.png?w=529&h=410?w=300" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-03.png?w=529&h=410?w=529" class="aligncenter size-full wp-image-5260" src="./values_files/cwap-contention-03.png" alt="CWAP - Contention-03" width="529" height="410" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-03.png?w=529&h=410 529w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-03.png?w=150&h=116 150w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-03.png?w=300&h=233 300w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-03.png 549w" sizes="(max-width: 529px) 100vw, 529px"></a><span style="text-decoration:underline;"><span style="color:#00ffff;text-decoration:underline;">TXOP- Transmit Opportunity</span></span><br>
EDCA introduce this TXOP which is a time period where one device, called TXOP holder has unfettered acccess to the channel for data transmission. The data frame transmissions within a TXOP are called a “<span style="color:#ff00ff;">contention free burst -CFB</span>” During a TXOP, only the data that makes up a CFB and the ACK for that data may access the channel.</p>
<p>802.11e standard defines default TXOP limit value for each AC, but values can be configured on AP. TXOP limit are set in intervals of <span style="color:#00ff00;">32µs</span> (microseconds). Default TXOP is <span style="color:#00ff00;">47</span> for <span style="color:#ff00ff;">AC_VO</span> (47×32=<span style="color:#00ff00;">1504µs</span>) for OFDM. It is <span style="color:#00ff00;">94</span> for <span style="color:#ff00ff;">AC_VI</span> (94×32=<span style="color:#00ff00;">3008µs</span>) . Note that for <span style="color:#ff00ff;">AC_BE & AC_BK</span> always TXOP set to <span style="color:#00ff00;">0</span>,<em><span style="color:#ff00ff;"> in other words those traffic category always has to send one frame at at time</span></em> (no CFB).</p>
<p>Here is the default settings (AIFSN, CW_min, CW_max, x value, TXOP, AIFS) of those four access categories.</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-02.png"><img data-attachment-id="5257" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-02/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-02.png?w=529&h=147" data-orig-size="756,211" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-02" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-02.png?w=529&h=147?w=300" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-02.png?w=529&h=147?w=529" class="aligncenter size-full wp-image-5257" src="./values_files/cwap-contention-02.png" alt="CWAP - Contention-02" width="529" height="147" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-02.png?w=527&h=147 527w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-02.png?w=150&h=42 150w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-02.png?w=300&h=84 300w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-02.png 756w" sizes="(max-width: 529px) 100vw, 529px"></a>Here is a beacon frame of a 802.11a radio where you can see those default settings as well.</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-08.png"><img data-attachment-id="5256" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-08/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-08.png?w=529&h=828" data-orig-size="746,1169" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-08" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-08.png?w=529&h=828?w=191" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-08.png?w=529&h=828?w=529" class="aligncenter size-full wp-image-5256" src="./values_files/cwap-contention-08.png" alt="CWAP - Contention-08" width="529" height="828" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-08.png?w=529&h=828 529w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-08.png?w=96&h=150 96w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-08.png?w=191&h=300 191w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-08.png 746w" sizes="(max-width: 529px) 100vw, 529px"></a>As explained these EDCA value can be configurable on AP. Here is on my Cisco WLC (3850) I have changed the default setting to optimized Voice & Video. “<span style="color:#ff00ff;">wmm-default</span>” is the default setting in this WLC.</p>
<pre>3850-1(config)#<span style="color:#00ffff;">ap dot11 5ghz shutdown</span>
3850-1(config)#ap dot11 5ghz edca-parameters ?
custom-voice Enable Custom Voice parameters for 802.11a
<span style="color:#ff00ff;">optimized-video-voice</span> Enable combined video-voice-optimized parameters for 802.11a
optimized-voice Enable non-spectralink voice-optimized parameters for 802.11a
svp-voice Enable SpectraLink Voice Priority (SVP) parameters for 802.11a
<span style="color:#ff00ff;">wmm-default</span> Enable WMM default parameters for 802.11a
3850-1(config)#<span style="color:#00ffff;">ap dot11 5ghz edca-parameters optimized-video-voice</span>
3850-1(config)#<span style="color:#00ffff;">no ap dot11 5ghz shutdown</span>
3850-1#<span style="color:#00ffff;">show ap dot11 5ghz network</span> | <span style="color:#ff00ff;">in EDCA</span>
EDCA profile type check : <span style="color:#ff00ff;">optimized-video-voice</span>
</pre>
<p>Once you change this, you can see AP advertising these values on WMM parameters & you can see values change to give priority for Voice & Video.</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-04.png"><img data-attachment-id="5262" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-04/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-04.png?w=529&h=205" data-orig-size="551,214" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-04" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-04.png?w=529&h=205?w=300" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-04.png?w=529&h=205?w=529" class="aligncenter size-full wp-image-5262" src="./values_files/cwap-contention-04.png" alt="CWAP - Contention-04" width="529" height="205" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-04.png?w=529&h=205 529w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-04.png?w=150&h=58 150w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-04.png?w=300&h=117 300w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-04.png 551w" sizes="(max-width: 529px) 100vw, 529px"></a>Here is a beacon frame capture showing those modified values.</p>
<p><a href="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-09.png"><img data-attachment-id="5254" data-permalink="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/cwap-contention-09/" data-orig-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-09.png?w=529&h=815" data-orig-size="727,1121" data-comments-opened="1" data-image-meta="{"aperture":"0","credit":"","camera":"","caption":"","created_timestamp":"0","copyright":"","focal_length":"0","iso":"0","shutter_speed":"0","title":"","orientation":"0"}" data-image-title="CWAP – Contention-09" data-image-description="" data-medium-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-09.png?w=529&h=815?w=195" data-large-file="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-09.png?w=529&h=815?w=529" class="aligncenter size-full wp-image-5254" src="./values_files/cwap-contention-09.png" alt="CWAP - Contention-09" width="529" height="815" srcset="https://mrncciew.files.wordpress.com/2014/10/cwap-contention-09.png?w=529&h=815 529w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-09.png?w=97&h=150 97w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-09.png?w=195&h=300 195w, https://mrncciew.files.wordpress.com/2014/10/cwap-contention-09.png 727w" sizes="(max-width: 529px) 100vw, 529px"></a>Reference<br>
1. CWAP Official Study Guide – Chapter 7<br>
2. <a href="http://www.cwnp.com/uploads/802-11_arbitration.pdf">802.11 Arbitration</a> – CWNP White Paper<br>
3. <a href="https://mrncciew.files.wordpress.com/2014/10/802-11-qos-overview.pdf">802.11-QoS-Overview</a></p>
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<h3 id="comments-title">
<span>10</span> thoughts on “CWAP – 802.11 Medium Contention” </h3>
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<li class="comment even thread-even depth-1 parent highlander-comment" id="comment-11335">
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<img alt="" src="./values_files/13192b8c9af09f06b0f600219176d9e3" class="avatar avatar-32 grav-hashed grav-hijack" height="32" width="32" id="grav-13192b8c9af09f06b0f600219176d9e3-0"> <span>Franklin</span> <em>said:</em> </p>
<p class="comment-date">
<a href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/#comment-11335"><time pubdate="" datetime="2014-11-06T16:40:18+00:00">November 6, 2014 at 4:40 pm</time></a> </p>
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<p>How to understand DIFS, PIFS and SIFS work with good example?</p>
<p class="reply-link"><a rel="nofollow" class="comment-reply-link" href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/?replytocom=11335#respond" onclick="return addComment.moveForm( "comment-11335", "11335", "respond", "5246" )" aria-label="Reply to Franklin">Reply</a></p>
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<li class="comment byuser comment-author-nayarasi bypostauthor odd alt depth-2 highlander-comment" id="comment-11351">
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<img alt="" src="./values_files/e6bfc472b18c36eb851ccc502bfb036d" class="avatar avatar-32 grav-hashed grav-hijack" height="32" width="32" id="grav-e6bfc472b18c36eb851ccc502bfb036d-0"> <span><a href="https://mrncciew.wordpress.com/" rel="external nofollow" class="url">nayarasi</a></span> <em>said:</em> </p>
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<a href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/#comment-11351"><time pubdate="" datetime="2014-11-07T09:25:55+00:00">November 7, 2014 at 9:25 am</time></a> </p>
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<p>Here is another good document explain this. See if that helps</p>
<p><a href="http://www.ieee802.org/1/files/public/docs2008/avb-phkl-802-11-qos-overview-0811-1.pdf" rel="nofollow">http://www.ieee802.org/1/files/public/docs2008/avb-phkl-802-11-qos-overview-0811-1.pdf</a></p>
<p>Rasika</p>
<p class="reply-link"><a rel="nofollow" class="comment-reply-link" href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/?replytocom=11351#respond" onclick="return addComment.moveForm( "comment-11351", "11351", "respond", "5246" )" aria-label="Reply to nayarasi">Reply</a></p>
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<img alt="" src="./values_files/5f21ac2f61313ab07f5c04b2cbefab0d" class="avatar avatar-32 grav-hashed grav-hijack" height="32" width="32" id="grav-5f21ac2f61313ab07f5c04b2cbefab0d-0"> <span>Sai Pavan</span> <em>said:</em> </p>
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<a href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/#comment-22637"><time pubdate="" datetime="2016-04-07T09:21:23+00:00">April 7, 2016 at 9:21 am</time></a> </p>
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<p>Great Post and blog. Will definitely follow the blog.</p>
<p class="reply-link"><a rel="nofollow" class="comment-reply-link" href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/?replytocom=22637#respond" onclick="return addComment.moveForm( "comment-22637", "22637", "respond", "5246" )" aria-label="Reply to Sai Pavan">Reply</a></p>
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<img alt="" src="./values_files/e6bfc472b18c36eb851ccc502bfb036d" class="avatar avatar-32 grav-hashed grav-hijack" height="32" width="32" id="grav-e6bfc472b18c36eb851ccc502bfb036d-1"> <span><a href="https://mrncciew.wordpress.com/" rel="external nofollow" class="url">nayarasi</a></span> <em>said:</em> </p>
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<a href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/#comment-22692"><time pubdate="" datetime="2016-04-11T15:09:54+00:00">April 11, 2016 at 3:09 pm</time></a> </p>
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<p>Thanks Pavan.</p>
<p class="reply-link"><a rel="nofollow" class="comment-reply-link" href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/?replytocom=22692#respond" onclick="return addComment.moveForm( "comment-22692", "22692", "respond", "5246" )" aria-label="Reply to nayarasi">Reply</a></p>
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<img alt="" src="./values_files/f6346ac53ea9e44bb0702598afb85635" class="avatar avatar-32 grav-hashed grav-hijack" height="32" width="32" id="grav-f6346ac53ea9e44bb0702598afb85635-0"> <span>Mike</span> <em>said:</em> </p>
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<a href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/#comment-23839"><time pubdate="" datetime="2016-07-07T09:40:51+00:00">July 7, 2016 at 9:40 am</time></a> </p>
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<p>Great post, this accompanied with some CWDP practice questions is helping me wrap my head around 802.11 contention. One thing that is not clear to me is why AIFS is longer than DIFS? I understand that EDCA is an extension of DCF, however I don’t understand how they interoperate within a single BSS. By enabling EDCA are you directing clients that DIFS (DCF) is no longer used and only AIFS (EDCA) interframe spacing is being observered? If that is true, are non 802.11e compliant devices unable to join the BSS when EDCA is enabled? I guess I don’t understand how (and if) DCF clients interoperate with EDCA clients when DIFS is shorter than AIFS. Thanks! Mike</p>
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<img alt="" src="./values_files/40dcea0c2dac53258abe5c9ddae97705" class="avatar avatar-32 grav-hashed grav-hijack" height="32" width="32" id="grav-40dcea0c2dac53258abe5c9ddae97705-0"> <span><a href="http://gravatar.com/vlado0302" rel="external nofollow" class="url">Vladirmir</a></span> <em>said:</em> </p>
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<a href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/#comment-23980"><time pubdate="" datetime="2016-07-18T02:39:07+00:00">July 18, 2016 at 2:39 am</time></a> </p>
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<p>Hi,</p>
<p>Thank you for and educational article.</p>
<p>I have a question. </p>
<p>This is a Maximum throughput calculation for 802.11a:</p>
<p> MTa (8k * Ldata)/(kTdata+Tack+Tdifs+Tsifs+(CWmin-1)Tslot/2).</p>
<p>Would you be able to tell me what is the maximum throughput formula for 802.11n please?</p>
<p>Thank you.</p>
<p>Regards,</p>
<p>Vladimir</p>
<p class="reply-link"><a rel="nofollow" class="comment-reply-link" href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/?replytocom=23980#respond" onclick="return addComment.moveForm( "comment-23980", "23980", "respond", "5246" )" aria-label="Reply to Vladirmir">Reply</a></p>
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<li class="comment byuser comment-author-mali77 even thread-even depth-1 highlander-comment" id="comment-24927">
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<img alt="" src="./values_files/e236292ee7c8c596f0d942db4364eadd" class="avatar avatar-32 grav-hashed grav-hijack" height="32" width="32" id="grav-e236292ee7c8c596f0d942db4364eadd-0"> <span><a href="http://gravatar.com/mali77" rel="external nofollow" class="url">mali77</a></span> <em>said:</em> </p>
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<a href="https://mrncciew.com/2014/10/12/cwap-802-11-medium-contention/#comment-24927"><time pubdate="" datetime="2016-11-12T03:38:41+00:00">November 12, 2016 at 3:38 am</time></a> </p>
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<p>This is great been going crazy trying to understand this in depth. In short we basically have:<br>
1- CCA<br>
2- NAV<br>
3- DIFS<br>
4- CW<br>
5- Tx</p>
<p>NAV portion was confusing me a bit with regards to DCF and was checking this link out:<br>
<a href="http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Mobility/emob41dg/emob41dg-wrapper/ch5_QoS.html#wp1021972" rel="nofollow">http://www.cisco.com/c/en/us/td/docs/solutions/Enterprise/Mobility/emob41dg/emob41dg-wrapper/ch5_QoS.html#wp1021972</a></p>
<p>per this link I do not see any mention of NAV in any of the steps. Did they just miss that step in there?</p>
<p>Thank you</p>
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<p>Pingback: <a href="http://news.mali77.com/index.php/2017/02/my-ccna-wireless-journey/" rel="external nofollow" class="url">My CCNA Wireless Journey | » Ali's Technology Blog</a></p>
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<p>Pingback: <a href="http://robrobstation.com/2016/04/17/wi-fi-csmaca-going-deep/" rel="external nofollow" class="url">Wi-Fi CSMA/CA – Going Deep – ROBROBSTATION</a></p>
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<p>Pingback: <a href="https://netgab.net/web/2017/12/05/cisco-wlc-edca-timers/" rel="external nofollow" class="url">Cisco WLC: EDCA timers - NetGab - The daily networking madness</a></p>
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