The Cost of Listening Every Second

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

ADA · CALCULATION AUDIT

The Cost of Listening Every Second

A soil-moisture node sends only one useful reading per minute, yet wakes every second to check whether the channel is active, and each check holds the receiver on for 20 ms. Across a minute that is 60 channel checks costing 1.2 seconds of listening before a single sensor byte has moved. This audit asks the question that arithmetic invites: does the chapter’s 1.2-second figure actually check out, and what duty cycle and hourly listening cost does it imply?

Companion to the chapter Common WSN Design Mistakes — every number here comes from that chapter.

Ada: The soil-moisture node sends one useful reading a minute but wakes every second to check the channel, holding the receiver on for 20 ms each time. The chapter says that is 1.2 seconds per minute of listening - let me verify it and turn it into a duty cycle.

• Waking once a second gives 60 channel checks per minute, and each keeps the radio on for 20 ms = 0.020 s: 60 x 0.020 = 1.2 s of listening every minute. The figure checks out.
• As a duty cycle that is 1.2 / 60 = 0.02 = 2% of the time spent receiving before a single sensor byte has moved - and 1.2 x 60 = 72 s, or 1.2 minutes, of idle listening every hour.
• Set that against the workload of one short reading per minute: the radio burns roughly 1.2 s of listening for every reading it actually sends.

So the energy villain here is the wake schedule, not the traffic. Halving the check rate to every two seconds would halve idle listening to 0.6 s/min - which is why the chapter files this under idle listening and aims the repair at the listen-and-wake policy rather than the reporting rate.

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