Backoff Timing and Retry Budget

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

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Ada ADA · CALCULATION AUDIT

Backoff Timing and Retry Budget

On the default 2.4 GHz CSMA/CA ladder a node’s first attempt uses BE = 3 and waits up to 2.24 ms before a single CCA; each busy result widens the window until the default ceiling caps one wait at 9.92 ms. With macMaxCSMABackoffs = 4, a worst-case channel-access failure stacks four busy waits into 26.88 ms, while macMaxFrameRetries = 3 permits 4 transmit attempts when ACKs go missing. This audit rebuilds every window from the 20-symbol backoff unit and asks whether those figures promise delivery, or only bound the timing-and-energy budget until measured CCA counts and ACK failures are captured.

Companion to the chapter 802.15.4 MAC Operation — every number here comes from that chapter.

Ada: 802.15.4 operation claims are timing physics plus a failure ledger. The chapter already gives enough numbers to check the default CSMA/CA wait windows without inventing traffic rates, current draw, or packet sizes.

  • The backoff unit is 20 symbols, and each 2.4 GHz symbol is 16 us, so 20 x 16 us = 320 us.
  • The first attempt uses BE = 3, giving 2^3 = 8 choices: 0 through 7 backoff periods.
  • The first-attempt maximum wait is 7 x 320 us = 2240 us = 2.24 ms; the mean random wait is (0 + 7) / 2 x 320 us = 1120 us = 1.12 ms.
  • After one busy CCA, BE = 4, so the maximum wait is (2^4 - 1) x 320 us = 15 x 320 us = 4800 us = 4.8 ms.
  • Once BE reaches the default ceiling 5, the maximum wait is (2^5 - 1) x 320 us = 31 x 320 us = 9920 us = 9.92 ms.
  • With macMaxCSMABackoffs = 4, a worst-case channel-access failure can draw four busy-CCA waits at BE = 3, 4, 5, 5: (7 + 15 + 31 + 31) x 320 us = 84 x 320 us = 26880 us = 26.88 ms, before counting CCA time and firmware overhead.
  • macMaxFrameRetries = 3 is a different budget: one original transmission plus 3 retransmissions gives 1 + 3 = 4 possible transmit attempts when ACKs are missing.

The audit conclusion is bounded: the default 2.4 GHz backoff ladder can add tens of milliseconds during repeated busy-channel decisions, and ACK loss can spend up to four transmit attempts. Those numbers support the timing-and-energy review question, not a delivery guarantee, until measured CCA counts, ACK failures, queue delays, and current traces are captured on the real device.

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