Directed (fast) advertising timeout and scanning at low power

[Eliot Stock]

We have an nRF51822 peripheral device that is bonded with one and only one central. The central is always an iOS or Android phone running our app, which is scanning at "low power", whatever that means for each mobile platform.

For Android (at least 5.0 through 7.1), low power means:

private static final int SCAN_MODE_LOW_POWER_WINDOW_MS = 500;
private static final int SCAN_MODE_LOW_POWER_INTERVAL_MS = 5000;

(See: android.googlesource.com/.../ScanManager.java

(I'm not aware of the equivalent values for iOS, but perhaps someone can enlighten me)

So an Android central of ours will only have the radio on 10% of the time. And, as explained here (devzone.nordicsemi.com/.../) each scan is on a different one of three channels. So we would need to be advertising for 15s in order to be reasonably sure of a connection and have scanning coincide with advertising at least once on each channel. Am I right so far?

If so, it would seem that directed advertising (fast) in this situation is pretty much useless because it's so short. How long is it, actually? I know it's really intended for situations when you've just lost a connection and the central is in fact doing higher powered scanning. Just looking for confirmation I've got this right.

[Petter Myhre]

I can't tell you how you can be reasonable sure that a connection is established in a certain amount of time. You will have to tune the scan window, scan interval, advertising interval to fit your application, I don't know your power budget, I don't know how long a connection establishment can take before the customer thinks it's too long, I don't know how much interference and packet loss there will be in the environment where the device are going to connect.