The Efficiency Claim Reduces to Counting Bytes

The Efficiency Claim Reduces to Counting Bytes

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

foundations
math-foundations
calculation-audit
coap
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.

Try

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 isprotocol overhead. Calculate this case.

Observe

This audit asks whether the energy calculation adds anything, or whether the efficiency claim simply reduces to counting bytes. Check shows this.

Explain

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. Check confirms it.

See the relationship before changing it

The chapter figure below shows: CoAP versus HTTP overhead for a 5-byte temperature reading, comparing total on-wire bytes, daily radio energy, and TX-only CR2032 battery life.

Comparison chart for the chapter model: CoAP uses 45 bytes and 0.86 joules per day for one thousand daily readings, while HTTP uses 425 bytes and 8.16 joules per day, yielding 7.7 versus 0.8 years of TX-only CR2032 battery life.
CoAP versus HTTP overhead for a 5-byte temperature reading, comparing total on-wire bytes, daily radio energy, and TX-only CR2032 battery life.

Derive the baseline in four named moves

  1. 1

    Name the input. The chapter baseline is 425 bytes.

  2. 2

    Name the relationship. ratio = HTTP bytes / 45 CoAP bytes

  3. 3

    Substitute with units. 425 / 45 = 9.44 times

  4. 4

    Read the result. Keep the unit beside the value, then use the result only inside the technical boundary below.

Predict, then change http message size

Try Predict how energy ratio versus 45-byte coap responds when http message size moves. Calculate http message size; compare energy ratio versus 45-byte coap with that prediction.

425 bytes
Chapter baseline
Energy ratio versus 45-byte CoAP

Observe Return to 425 bytes. Recheck energy ratio versus 45-byte coap with http message size at its chapter value.

Explain Common current, voltage, count, and rate terms cancel.

Check yourself

What should you do before trusting a moved-slider result?
Answer: Predict its direction, apply the displayed relationship, keep the units, and compare the reset value with the chapter’s worked baseline.
What does this small model leave out?
Answer: Only http message size moves here. The energy ratio versus 45-byte coap calculation excludes field effects listed below.

Honesty note. The energy ratio versus 45-byte coap result holds http message size as the only changing input. It is not a field prediction.

Technical boundaries

The energy ratio holds the Constrained Application Protocol message at 45 bytes and uses identical current, voltage, message count, and radio rate for both protocols. It excludes link-layer framing, Internet Protocol fragmentation, security overhead, retransmissions, multicast loss, and server processing.

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.