# 802.11 Beacon frame transmitter - mobile devices **URL:** \ **Category:** GNU Radio\ **Created:** [November 6, 2014, 8:44am UTC](https://www.ruby-forum.com/t/802-11-beacon-frame-transmitter-mobile-devices/232947 "2014-11-06T08:44:13Z")\ **Posts on this page:** 5\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [November 6, 2014, 8:44am UTC](https://www.ruby-forum.com/t/802-11-beacon-frame-transmitter-mobile-devices/232947/1 "2014-11-06T08:44:13Z")
Greetings After creating beacon frames transmitter based on usrp i decided to check its performance. Im sending a beacon frames and correctly decoding it by my wlan card (in monitor mode - screen attached). [http://gnuradio.4.n7.nabble.com/file/n51215/beacon.png](http://gnuradio.4.n7.nabble.com/file/n51215/beacon.png) Any ideas why i cant find this network on any mobile device (smartphone, laptop)? Best, Ernest – View this message in context: [http://gnuradio.4.n7.nabble.com/802-11-Beacon-frame-transmitter-mobile-devices-tp51215.html](http://gnuradio.4.n7.nabble.com/802-11-Beacon-frame-transmitter-mobile-devices-tp51215.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![ernest\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/779978/32.png) [@ernest\_s](https://www.ruby-forum.com/u/ernest_s)\ **Post date:** [November 6, 2014, 9:28am UTC](https://www.ruby-forum.com/t/802-11-beacon-frame-transmitter-mobile-devices/232947/2 "2014-11-06T09:28:58Z")
Can’t remember exactly but is there some form of CRC checking with the beacon? Because in monitor mode, CRC checking is typically disabled. R ---
**Author:** ![ernest\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/779978/32.png) [@ernest\_s](https://www.ruby-forum.com/u/ernest_s)\ **Post date:** [November 6, 2014, 10:20am UTC](https://www.ruby-forum.com/t/802-11-beacon-frame-transmitter-mobile-devices/232947/3 "2014-11-06T10:20:54Z")
I think yes. Rest of 802.11 beacon in my area also have CRC (while in monitor mode - Windows Network Monitor). – View this message in context: [http://gnuradio.4.n7.nabble.com/802-11-Beacon-frame-transmitter-mobile-devices-tp51215p51217.html](http://gnuradio.4.n7.nabble.com/802-11-Beacon-frame-transmitter-mobile-devices-tp51215p51217.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![ernest\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/779978/32.png) [@ernest\_s](https://www.ruby-forum.com/u/ernest_s)\ **Post date:** [November 6, 2014, 7:33pm UTC](https://www.ruby-forum.com/t/802-11-beacon-frame-transmitter-mobile-devices/232947/4 "2014-11-06T19:33:20Z")
> > Any ideas why i cant find this network on any mobile device (smartphone, > > laptop)? It is probably the difference between passive vs active scanning. To save time and consequently power, most battery powered wifi devices use active scanning where the station sends a probe request and expects to get a probe response from the AP (or beaconing device). Typical dwell times in active scanning are 20-40ms / channel while passive (listen only) scans are usually more like 110 ms / channel, since the default beacon interval of many APs is 100ms. If you have linux on your laptop, you might try ‘iw wlan0 scan freq $FREQ passive’ Jason ---
**Author:** ![ernest\_s](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/779978/32.png) [@ernest\_s](https://www.ruby-forum.com/u/ernest_s)\ **Post date:** [November 7, 2014, 11:11am UTC](https://www.ruby-forum.com/t/802-11-beacon-frame-transmitter-mobile-devices/232947/5 "2014-11-07T11:11:20Z")
So, following your idea, after waiting a larger period of time, passive scanning should “catch” my beacon right? (becouse when entering new area, or getting in range of unknown AP, there are no information to send direct probe request by mobile device). I think that mobile device is performing some kind of “check” that wlan card in monitor mode don’t, rather than im facing active/passive scanning problem. Best – View this message in context: [http://gnuradio.4.n7.nabble.com/802-11-Beacon-frame-transmitter-mobile-devices-tp51215p51230.html](http://gnuradio.4.n7.nabble.com/802-11-Beacon-frame-transmitter-mobile-devices-tp51215p51230.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com).