What the 10% S-MAC Duty Cycle Actually Saves

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

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Ada ADA · CALCULATION AUDIT

What the 10% S-MAC Duty Cycle Actually Saves

S-MAC lets a node sleep most of the time, then wake for a 200 ms listen window inside each 2 second frame — a 10% duty cycle, with the radio off for the remaining 1.8 seconds. The chapter pairs that schedule with classic TelosB-style current traces of about 20 mA radio-on versus about 1.8 mA MCU-on/radio-off, noting that idle listening may cost almost as much as useful transmission. This audit asks the question that pairing invites: what average current does the 10% S-MAC schedule actually deliver, and how much of the saving depends on that 1.8 mA sleep figure being real?

Companion to the chapter WSN Duty Cycling — every number here comes from that chapter.

Ada: This section pairs two facts I can check together: the S-MAC schedule (200 ms listen in a 2 s frame) and the TelosB current scale (~20 mA radio-on versus ~1.8 mA MCU-on/radio-off, with idle listening costing almost as much as transmitting).

  • Duty cycle: 200 ms / 2000 ms = 0.10, the chapter’s 10%; the radio is off for 2000 - 200 = 1800 ms = 1.8 s.
  • Current contrast: turning the radio off drops the draw by 20 / 1.8 = 11.1x. And because listening (~20 mA) is nearly the transmit current, a receiver that stays awake pays almost a full transmit’s worth of current for zero delivered bytes.

To price the schedule, take the 1.8 s off-window at the chapter’s own 1.8 mA MCU-on/radio-off figure (an illustrative upper bound – a true deep-sleep mode draws far less):

  • I_avg = (0.2 s x 20 mA + 1.8 s x 1.8 mA) / 2 s = (4.0 + 3.24) / 2 = 3.62 mA.

Against a node that simply listened the whole frame at 20 mA, that is already a 20 / 3.62 = 5.5x reduction – and a real sleep floor would push it lower still. The design meaning is that the 90% radio-off fraction, not a faster transmit, is what turns a 20 mA always-listening radio into a milliamp-scale average; but the win only materializes if the “off” state is genuinely low-current, which is why the chapter insists the sleep floor be measured, not assumed.

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