Do the "setForegroundScanPeriod" and "setForegroundBetweenScanPeriod" in the AltBeacon library match to the Scan Window and Scan Interval of the BLE standard? Also, does this refer to a scan event for each of the 3 advertising channels or will a scan of the 3 channels occur in a scan window?

I have a beacon advertising on a single channel (CH39) every 400ms and want to set my scan window and scan interval appropriately to ensure a maximum packet reception rate with those two options that Android let you control.

I am also open to try other suggestions for that matter.

Thanks,

Answer

The setForegroundScanPeriod and setForegroundBetweenScanPeriod settings of the Android Beacon Library are a high-level concept designed to control:

  1. battery usage

    The BLE scan will be stopped during the foregroundBetweenScanPeriod, allowing you to duty cycle the scanning to save battery. This is typically set to 0 for the foreground and a much higher value (say 5-15 minutes) for the equivalent background setting. The Background setting applies when the app is not in the foreground or the screen is off, and the Foreground setting applies when the app is visible on an illuminated screen.

  2. beacon search intervals

    By default, the foregroundScanInterval is set to 1100ms. This means that the library will look for beacons for 1100ms, keep track of the distinct list of all it detects in that period, then report them to the application using the library at the end of that interval (e.g. every 1100ms). This is similar to what iOS does with its CoreLocation API at a rate of 1000ms. The reason it defaults to 1100 instead of 1000 is because many early Android devices with BLE support could not detect more than one distinct advertisement per scan, so scanning had to be stopped every cycle and restarted to detect one again. The default cycle was set to be slightly more than 1000ms to avoid being closely synchronized with beacons advertising at a 1Hz rate.

The library's settings are not the same thing as the ScanWindow and ScanInterval of the BLE Standard. The library's settings are a much higher level concept.

The BLE standard concepts of ScanInterval and ScanWindow, as you state, control how quickly a device doing a BLE scan rotates between listening on each of the distinct BLE advertising channels. Unfortunately, Android APIs give you no direct control over these intervals -- they are baked into the firmware by the Android manufacturer. Further, Android gives you no APIs to determine what these are set to, or even which advertising channel the receiver was set to when an advertising packet was detected (something that has unfortunate impacts on RSSI measurements, see below.) The limited scan settings provided by Android are visible here. The SCAN_MODE_LOW_POWER vs. SCAN_MODE_LOW_LATENCY may affect these intervals (they do in the open source Android code), but again, Android manufacturers may adjust this at will. The Android Beacon Library generally uses SCAN_MODE_LOW_LATENCY except in specific background mode states.

In general, these BLE-Standard settings baked into the Android firmware will be different compared to the library's settings mentioned above. (The BLE spec says the ScanInterval can range from 2.5 ms to 10,240 ms) The periods, however, vary quite a bit between different Android models. You can see the open source Android definitions in this answer, which set the ScanInterval to 5000ms for SCAN_MODE_LOW_LATENCY, but keep in mind that each manufacturer may adjust the constants to their own liking.

Because the RSSI of an advertisement detection varies a small but significant amount between each advertising channel, you can usually derive the hard-coded ScanInterval of an Android device by plotting the RSSI of advertisements detected on an Android device from an advertiser that uses all advertising channels. Plotting this on a graph of RSSI vs. time will show a stair step pattern, where the width of each step is equal to the ScanInterval. On Samsung devices, the ScanInterval is close to the max allowed by the spec at about 10 seconds. My anecdotal testing suggests the baked-in setting from devices by other manufacturers are typically shorter.

The inability to control the channel hopping rate on Android means that 2/3 of your advertisements will not be detected, and on Samsung devices, you will typically go 20 seconds without any detections.

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    • Very good answer, but: your answer there contradicts this answer in my opinion. Even if you set highlevel time settings it won't really change what's going under the hood and by stopping scan and restarting you won't have the same effect. Or do I misunderstand something?
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    • To add to the confusion, the other answer you reference talks about Android OS constants that perform different (yet also related!) functions to the two concepts discussed in this Q&A. the other Android Scan Mode question than this one. The Scan Period and Between Scan Period are generally designed to work with the Android LOW_LATENCY Android Scan Mode. A full discussion is worth its own question for sure.
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    • I see. Do you know what is the difference between INTERVAL and WINDOW in those variables? I couldn't find any info regarding that it seems it is obvious to everonye. One of those variables is the setting for hopping the 3 advertisement channels, but what is the other? (for me it's a bit strange that low_power mode hops more frequently than low latency (if window=the setting I described))
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    • Oh, I've got it from here: ncbi.nlm.nih.gov/pmc/articles/PMC4327007 (3.1) So interval defines how often we hop channels, window sets just the actual window for listening inside interval. Now it makes sense as for low latency, window and interval are the same (at least in android code), in low power mode we only scan for 1/10 of the time(interval).

Warm tip !!!

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