Beacon Timing and Reserved Airtime

Beacon Timing and Reserved Airtime

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

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Ada ADA · CALCULATION AUDIT

Beacon Timing and Reserved Airtime

Twelve shelf sensors report by contention while two safety sensors must each land one status frame inside a 100 ms control window, so the coordinator reserves one Guaranteed Time Slot for each sensor. At BO = 6, SO = 3 every slot is 7.68 ms, so two one-slot reservations consume 15.36 ms — 12.5% of the active superframe — yet the beacon interval itself stays at 983.04 ms. This audit re-derives that two-sensor schedule and asks whether reserving a GTS actually meets the 100 ms deadline, or only protects airtime while the roughly one-second beacon interval never gets shorter.

Companion to the chapter 802.15.4 Collisions and Scheduling — every number here comes from that chapter.

See the relationship before changing it

The figure reads from left to right. The blue card is reserved gts slots. The middle card applies this page's rule. The green card is reserved airtime. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

The retained audit below checks several chapter fixtures. This model keeps those stated values fixed and changes only reserved gts slots, so the numeric fixture does not switch without explanation.

Reserved GTS slots changes reserved airtime An input card leads through the rule reserved airtime = slots x 7.68 ms to the reserved airtime result. INPUT PAGE INPUT APPLY THE RULE predict calculate check units OUTPUT RESULT
Walk the arrows. Reserved slots protect active airtime but do not shorten the beacon interval.

Derive the baseline in four named moves

  1. 1

    Name the input. The chapter baseline is 2 slots.

  2. 2

    Name the relationship. reserved airtime = slots x 7.68 ms

  3. 3

    Substitute with units. 2 x 7.68 ms = 15.36 ms

  4. 4

    Read the result. Keep the unit beside the value. Use it only inside the technical boundary on this page.

Predict, then change reserved gts slots

Try Predict the direction of reserved airtime = slots x 7.68 ms. Test another reserved gts slots, then compare reserved airtime.

2 slots
Chapter baseline
Reserved airtime

Observe Reserved slots protect active airtime but do not shorten the beacon interval. Reset reserved gts slots to 2 and compare reserved airtime.

Explain Reserved slots protect active airtime but do not shorten the beacon interval.

Check yourself

What should you do before trusting a moved-control result?
Answer: Predict its direction, apply the shown relationship, keep the units, and reset to the worked baseline.
What does this small model leave out?
Answer: Only reserved gts slots moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only reserved gts slots. It is not a field prediction.

Ada: The beacon schedule is not just a MAC feature; it is a clock budget. Keep the symbol arithmetic visible before accepting a low-power or bounded-latency claim.

  • Base superframe: 960 symbols * 16 us/symbol = 15360 us = 15.36 ms.
  • Low-duty cycle, BO = 6, SO = 0: BI = 960 * 2^6 = 61440 symbols; 61440 * 16 us = 983040 us = 983.04 ms; SD = 960 symbols = 15.36 ms; active fraction 15.36 / 983.04 = 0.015625 = 1.5625%; inactive time 983.04 - 15.36 = 967.68 ms.
  • Slot-sizing case, BO = 6, SO = 3: SD = 960 * 2^3 = 7680 symbols; 7680 * 16 us = 122.88 ms; slot time 122.88 / 16 = 7.68 ms.
  • Two one-slot GTS reservations consume 2 * 7.68 = 15.36 ms, which is 2 / 16 = 12.5% of the active superframe and leaves 122.88 - 15.36 = 107.52 ms for the rest of the active-period work.

The physics check is the conclusion: a 983.04 ms beacon interval can be excellent for sleeping nodes, but it cannot guarantee that either sensor starts its reserved slot inside the stated 100 ms control window unless the schedule changes. A GTS protects airtime; it does not make the beacon interval shorter.

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

TryCompute BI, SD, and one-slot duration for BO 6 and SO 3 before checking the deadline.
ObserveTwo GTS slots consume 15.36 ms, but the beacon interval remains 983.04 ms.
ExplainReserved airtime prevents CAP contention; it does not guarantee entry to the slot within 100 ms.
Technical boundaries. The audit assumes ideal 16 microsecond symbols, a fixed beacon schedule, and one slot per safety sensor. It does not model clock drift, missed beacons, queueing, frame retries, CAP traffic, coordinator failure, or rescheduling.
Audit result

The two slots fit the active superframe, yet BI = 983.04 ms exceeds the 100 ms control window, so this schedule cannot prove that deadline.