Two Budgets That Both Look Comfortable

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

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Ada ADA · CALCULATION AUDIT

Two Budgets That Both Look Comfortable

This chapter makes two reassuring “it will be fine” claims. A 100 m industrial link shows a healthy 29 dB of margin above the receiver’s -95 dBm floor, and a single wireless frame succeeds 98% of the time. Yet enclosure, pipe-rack, and fading losses of 12 + 8 dB cut that link to just 9 dB, and a tunneled message chaining four frames drops success toward 0.922 — so do these two comfortable-looking budgets actually survive once real losses stack up?

Companion to the chapter ISA100.11a Fundamentals — every number here comes from that chapter.

Ada: This chapter makes two “it will be fine” claims — an RF link-margin budget and a frame-reliability budget — and both deserve a check, because both shrink faster than they first appear.

  • Link budget at 100 m: free-space loss is 40.05 + 20 x log10(100) = 40.05 + 40 = 80.05 dB, about 80 dB. Received level is 10 + 2 + 2 - 80 = -66 dBm, which sits -66 - (-95) = 29 dB above the -95 dBm receiver floor.
  • But the enclosure, pipe-rack, and fading allowances take 12 + 8 = 20 dB, leaving 29 - 20 = 9 dB. A comfortable-looking 29 dB is really 9 dB of usable margin once the site is honest.
  • Frame-reliability budget: one tested frame succeeds with probability 0.98. A tunneled request that expands into two fragments over a two-hop path depends on roughly four frame transmissions, so the no-retry success is 0.98^4 = 0.922368, about 0.922. That means 1 - 0.922 = 0.078, or about 7.8% of messages need at least one retry.

The audit conclusion is that both numbers reproduce exactly and both carry the same warning: a 29 dB link and a 98% frame each look safe in isolation, yet stacking real losses drives margin to 9 dB and chaining four frames drops success to 92%. Reserve dB headroom and reserve retries — a scheduled industrial link must budget both, not assume either.

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