What the ACK Costs Over a Year

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

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Ada ADA · CALCULATION AUDIT

What the ACK Costs Over a Year

Sending one CoAP message every 60 s for a year — 525,600 messages — a confirmable (CON) exchange spends 1.0 mAs of radio charge because it waits out a 100 ms ACK, while a non-confirmable (NON) message spends exactly half. On a 220 mAh CR2032 that works out to 1.16 years of life for CON against 1.88 years for NON. This audit re-runs the arithmetic at full precision and asks whether the panel’s headline saving really is 63%, and where every bit of it comes from.

Companion to the chapter CoAP Resource API Design — every number here comes from that chapter.

Ada: The “Putting Numbers to It” panel concludes that NON gives “63% longer battery life” than CON. The chain of steps is sound, but let me re-run it at full precision to see where the saving actually comes from — and to pin the final percentage.

Per message, the only difference between the two types is the 100 ms ACK reception that a CON exchange waits for:

  • CON: (50 ms x 10 mA) + (100 ms x 5 mA) = 500 + 500 = 1000 mA-ms = 1.0 mAs, which is 1.0 / 3.6 = 0.2778 uAh.
  • NON: 50 ms x 10 mA = 500 mA-ms = 0.5 mAs = 0.1389 uAh — exactly half, because the ACK window is gone.

Over a year of one message every 60 s, that is 365 x 24 x 60 = 525,600 messages:

  • CON radio: 525,600 x 0.2778 uAh = 146.0 mAh; plus sleep 0.005 mA x 24 x 365 = 43.8 mAh; total 189.8 mAh.
  • NON radio: 525,600 x 0.1389 uAh = 73.0 mAh; plus the same 43.8 mAh sleep; total 116.8 mAh.

On a 220 mAh CR2032 that is 220 / 189.8 = 1.16 years for CON against 220 / 116.8 = 1.88 years for NON. The gain is 1.88 / 1.16 - 1 = 0.62, so about 62% longer life — the panel’s “63%” is the same result carried through the rounded 190 and 117 mAh totals.

The design meaning is that the entire battery advantage traces to one deleted line: the 100 ms ACK reception, which is fully half of each message’s radio charge. That is why the chapter reserves CON for actuator commands and alerts, where confirmed delivery is worth burning the second half of the radio budget, and hands periodic telemetry to NON, where a dropped reading is superseded by the next one anyway.

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