Peripheral Latency Governs Both Power and Timeout

Ada re-derives this chapter’s own numbers step by step, at full precision

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Ada ADA · CALCULATION AUDIT

Peripheral Latency Governs Both Power and Timeout

A BLE peripheral on a 30 ms connection interval with peripheral latency 4 has a supervision-timeout floor of (1 + 4) × 30 × 2 = 300 ms, so a 2 s timeout is legal but a 200 ms one is not, while the same latency stretches its idle listening gap to 250 ms. Push latency to 9 on a 1 s interval and the gap and the timeout floor jump to 10 s and 20 s. This audit re-derives each and asks whether the shared (1 + latency) factor means a power tweak that lets the radio sleep longer silently widens how long a dead link goes undetected.

Companion to the chapter BLE Field Debugging and Internals — every number here comes from that chapter.

Ada: The Under-the-Hood section uses the same (1 + peripheral_latency) multiplier twice – once for the idle listening gap and once for the supervision-timeout floor. Let me confirm both, because that shared factor is the reason you cannot tune one without moving the other.

Idle listening gap, (1 + latency) x interval:

  • 50 ms interval, latency 4: (1 + 4) x 50 = 250 ms.
  • 1 s interval, latency 9: (1 + 9) x 1,000 = 10,000 ms = 10 s.

Supervision-timeout floor, (1 + latency) x interval x 2:

  • 30 ms interval, latency 4: (1 + 4) x 30 x 2 = 300 ms, so a 2 s timeout is valid while a 200 ms timeout is below the floor and illegal.
  • 1 s interval, latency 9: (1 + 9) x 1,000 x 2 = 20,000 ms = 20 s, so a 10 s timeout – half the floor – is invalid for that latency.

The design meaning is the coupling: raising latency to let the radio sleep longer multiplies the idle gap and the minimum legal supervision timeout by the identical (1 + latency) factor, so a power tweak that widens the sleep window silently widens how long a dead link goes undetected – which is exactly why a high-latency sensor is forced into a long supervision timeout.

Every number above is taken from the chapter’s own material and re-derived step by step.