The Efficiency Claim Reduces to Counting Bytes

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

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Ada ADA · CALCULATION AUDIT

The Efficiency Claim Reduces to Counting Bytes

The chapter states a 9.4x overhead ratio between a 45-byte CoAP message and a 425-byte HTTP one, and claims it “translates directly to a 9.4x difference in radio energy.” Since only 5 of those 45 CoAP bytes are the actual reading, nearly the whole message is protocol overhead. This audit asks whether the energy calculation adds anything, or whether the efficiency claim simply reduces to counting bytes.

Companion to the chapter CoAP Methods and Multicast — every number here comes from that chapter.

Ada: The panel above makes a strong claim — that a 9.4x overhead ratio “translates directly to a 9.4x difference in radio energy.” I want to check whether the energy calculation adds anything, or whether it just re-states the byte count.

Confirm the two message sizes first:

  • CoAP: 4 + 8 + 28 + 5 = 45 bytes
  • HTTP: 200 + 40 + 5 + 180 = 425 bytes

Now the daily energy, at 1000 readings, 200 mA, 3 V, 250 kbps:

  • CoAP: 1000 x (45 x 8 / 250000) x 0.2 x 3 = 0.864 J/day
  • HTTP: 1000 x (425 x 8 / 250000) x 0.2 x 3 = 8.16 J/day
  • Ratio: 8.16 / 0.864 = 9.444

That ratio is identical to the byte ratio 425 / 45 = 9.444, and it is no coincidence: every other term in the energy formula — the reading count, the current, the voltage, the link rate — is the same for both protocols, so it cancels, and the energy ratio can only be the on-wire byte ratio. The battery-life figures ride the same line: 2430 / 0.864 = 2812 days is 7.7 years, and 2430 / 8.16 = 298 days is 0.8 years.

One more number exposes where the bytes go. The actual reading is 5 bytes of a 45-byte CoAP message — 5 / 45 = 11.1% — so nearly nine-tenths of every transmission is protocol overhead, not data.

The design meaning is that the whole CoAP-versus-HTTP energy argument collapses to a single question — how many bytes leave the antenna — because for a tiny, infrequent reading the payload is a rounding error and the header is the bill.

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