Two Budgets That Both Look Comfortable

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.

See the relationship before changing it

The figure reads from left to right. The blue card is site loss allowance. The middle card applies the page rule. The green card is usable link margin. Walk the arrows once: set the input, apply the rule, then read the result with its unit.

Site loss allowance changes usable link margin An input card leads through the rule usable margin = 29 dB measured margin - site losses to the usable link margin result. INPUT PAGE INPUT APPLY THE RULE predict calculate check units OUTPUT RESULT
Walk the arrows. Enclosure and fading losses spend the comfortable-looking margin before deployment.

Derive the baseline in four named moves

  1. 1

    Name the input. The chapter baseline is 20 dB.

  2. 2

    Name the relationship. usable margin = 29 dB measured margin - site losses

  3. 3

    Substitute with units. 29 - 20 = 9.00 dB

  4. 4

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

Predict, then change site loss allowance

Try Predict the direction of usable margin = 29 dB measured margin - site losses. Test another site loss allowance, then compare usable link margin.

20 dB
Chapter baseline
Usable link margin

Observe Enclosure and fading losses spend the comfortable-looking margin before deployment. Reset site loss allowance to 20 and compare usable link margin.

Explain Enclosure and fading losses spend the comfortable-looking margin before deployment.

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 site loss allowance moves here. Field effects named in the technical boundary stay fixed.

Honesty note. This teaching model changes only site loss allowance. It is not a field prediction.

TrySelect Check derivation for the 100 m link, its 29 dB nominal margin, and a four-frame message at 98% success per frame.
ObserveSite allowances cut link margin from 29 to 9 dB, while chained success falls from 98% to about 92.2%.
ExplainAdditive 20 dB site loss spends RF headroom, and multiplying 0.98 four times compounds frame-failure opportunity across the tunneled transaction.

Ready: use the stated baseline inputs, then compare each displayed result.

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.

Technical boundaries. ISA100 channel hopping, correlated retransmissions, routing churn, and stochastic fading remain outside the fixed link-loss and independent four-frame reliability budgets.