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    <p>I'm developing the application for Android 4.3 (Nexus 4&amp;7) using BLE and from my observations scanning returns the same device multiple times if there was no SCAN REQUEST send back to the peripheral.</p> <p>Device may advertise in 2 ways: passive and active. In passive mode the peripheral device is just advertising all of it's data and doesn't listen after sending periodic packet. It's just sending, sleeping, sending, sleeping... In active mode the sensor also advertises but the message is as short as possible. After sending it it switches to listening for some very short time. When scanned detects the short message, it immediately sends SCAN REQUEST command to the peripheral and gets response with more details. As far as I can see Android doesn't sent SCAN REQUEST multiple times during one scanning.</p> <p>Let's assume that we have 2 devices in range. One is f.e. Nordic's nRF Temp sensor (passive advertising) and one other connectible device. I've received the following scan response:</p> <pre><code>11-10 21:32:54.281: D/BluetoothAdapter(13468): startLeScan(): null 11-10 21:32:54.281: D/BluetoothAdapter(13468): onClientRegistered() - status=0 clientIf=4 11-10 21:32:54.321: D/BluetoothAdapter(13468): onScanResult() - Device=CD:61:1A:A8:BC:BE RSSI=-94 11-10 21:32:55.122: D/BluetoothAdapter(13468): onScanResult() - Device=CB:32:81:CF:FD:00 RSSI=-61 11-10 21:32:56.414: D/BluetoothAdapter(13468): onScanResult() - Device=CB:32:81:CF:FD:00 RSSI=-62 11-10 21:32:57.715: D/BluetoothAdapter(13468): onScanResult() - Device=CB:32:81:CF:FD:00 RSSI=-61 11-10 21:32:59.016: D/BluetoothAdapter(13468): onScanResult() - Device=CB:32:81:CF:FD:00 RSSI=-63 11-10 21:33:01.609: D/BluetoothAdapter(13468): onScanResult() - Device=CB:32:81:CF:FD:00 RSSI=-63 11-10 21:33:02.901: D/BluetoothAdapter(13468): onScanResult() - Device=CB:32:81:CF:FD:00 RSSI=-63 11-10 21:33:04.212: D/BluetoothAdapter(13468): onScanResult() - Device=CB:32:81:CF:FD:00 RSSI=-62 11-10 21:33:04.282: D/BluetoothAdapter(13468): stopLeScan() </code></pre> <p>As you can see the connectible device showed up just once while the other one 7 times.</p> <blockquote> <p>Another question I would like to resolve is how rapidly the scanning can be switched on and off. Scanning is a power intensive operation, yet the broadcast of data will happen periodically on a fairly precise, real time timer. As a result, it would be a great optimization if the scan can be switched on and off, such that the broadcast and scan are synchronized, with the scanner shut down the other 90%+ of the time. This will likely need to be tested experimentally.</p> </blockquote> <p>Scanning frequency depends on the device. Furthermore advertising is usually done on 3 channels: 37, 38 and 39 to increase probability of being found. However this might be quite good idea to get advertising packets from 'active' devices multiple times.</p>
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    1. POAndroid 4.3: BLE: Filtering behaviour of startLeScan()
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    1. USphilips77
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    1. COI've tested the same case on Samsung S4 with official 4.3 update and I receive updates for both devices multiple times. Actually I got updates from 61:1A:A8:BC:BE much often than the later one. So in conclusion it depends on what device are you using. Not good at all for developers :/
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    2. COThanks for commenting, this is really useful information. It is interesting and useful that the device returns multiple scan results when using the passive advertising mode. In this application, ideally I'd like Android to behave like the passive scanner with active advertisement -- so that I can have a device broadcasting data, as well as allowing a connection to change settings.
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      1. USmattthebaker
    3. CORegarding switching the Android Scanner on and off: I succeeded in my test application to switch the startBleScan on and off, and receive active advertisement packets at a rate of 20Hz. There is unfortunately some latency in the phone when switching on and off, so the scanning process ends up being active 80% to 90% of the time. At lower packet rates this method may be viable as an "Observer" hack.
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