> There are there PHY layer delay like CSMACA for 802.11 > Now when STA sends data there is an ACK or BA for that Data. > If WLAN security is enabled then extra header is added. Along with data 802.11MAC header and other OSI layer header are there. > When STA sends UDP packets, it’s not only data. Let’s assume STA is sending UDP packets to AP and got 100Mbps throughput in Iperf. This is called theoretical PHY Data Rate.īut when you measure throughput using Iperf or Ixia chariot, it will be less than 144.4Mbps. ♥♥ If you have any doubts or query please let me know in comment section or send mail at Octoat 10:16 pmĢ×2 11n HT20 Data Rate is 144.4Mbps. This helps STA to be updated with surrounding APs and other purpose like roaming. Scanning is very important part for STA feature. It’s observed that sometimes hidden ssid AP replies to broadcast probe request but does not share AP’s SSID inside probe response. Screenshot for directed probe request for above discussed 2 nd scenario. In this case also before connecting to AP, STA may send one directed probe request to AP to check if AP is alive. In another way we can say, STA must know the hidden ssid name.Īnother scenario where STA was previously connected to one AP (Ex: Android Wi-Fi) and now again wants to reconnect to same AP. That means, to get AP details of a hidden ssid, STA has to send directed probe request. So, according to our scenario if STA sends directed probe request to AP3 then only AP3 replies with probe response. ♣ What is special about directed probe request? When do you need it?ĭirected probe request is having below address structure Or there can be mixture of active or passive scanning. So battery operated devices should go for passive scanning but the device which needs scan list in short time and does not care about more power consumption should go for active scanning. ♦Passive scan saves more battery as no frame is transmitted in the air. But passive scan slower as STA has to wait for beacon in each channel. ♦Active scan is quick as STA sends Probe Request and get probe response. Here are the advantages and disadvantages of these two scanning method. ♣ Which SCANNING is better Active scan or passive scan? But we can take any android phone Wi-Fi and observe that it does passive scan maximum times. Each vendor may have their own command to put device into passive scanning mode. ♣ How to trigger Passive scanning in STA?Īs per knowledge, I have not seen any open source command to put STA into passive scanning mode. *For passive scan we do not see any packet in sniffer capture* After completing all supported channel STA collects SCAN results. Then STA moves to next supported channel (Ch: 1,2,3,…11 or 36…165) and repeat the same.Ĥ. ![]() ![]() STA waits certain time on one supported channel and receive beacon frame broadcasted by AP.ģ. If STA is in passive scanning mode then STA does not send any frame in air.Ģ. There are many other methods which can be exposed by vendor.ġ. ♦ Use below command (wlan0 is Wi-Fi interface name) This is implementation depended that STA can do scan on all channels or only one channel or non-overlapping channels only.įor testing purpose there are some general methods to trigger active on STA. At last STA collects all AP probe response and make scan list. If STA supports 5Ghz, then STA also perform active scan starting from channel 36 to 165 (Depending on supported channel).ĥ. STA receives probe response from AP2 when probe request was sent on channel 11. Now, STA continues to send probe request on channel 2,3,4…11 (or 12,13 if supported). So, AP1 replies with probe response frame to STA.Ĥ. Now AP1, AP2, AP3 can see this broadcast probe request. Let’s think our STA sends broadcast probe request.Ģ. This type of probe request is called directed probe request. ♦ Probe request can be unicast to particular AP. This means whoever gets this probe request, may (Except hidden SSID AP ex: AP3) reply with probe response to STA. ♦ In general Probe request is broadcast packet. STA sends probe request on first channel 1 of 2.4Ghz band. Let’s assume there are three APs (2.4Ghz) and one client/STA is there.ĪP3 ġ. There are two types of scanning are there ♣ How many types of SCANNING are there in 802.11? In this article we will discuss on STA scanning feature only. ![]() In general scanning is performed by client or station to get list of APs. So STA can proceed for connection if needed. If STA finds required AP in scan list, then it can be sure that target AP is alive. SCANNING is a mechanism to find out APs within range. Tags : active scan directed probe request hidden ssid passive scan wlan scan
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