The Installed-Link Margin
Ada re-derives this chapter’s own numbers step by step, at full precision
ADA · CALCULATION AUDIT
The Installed-Link Margin
A deployed 10 m, 2.44 GHz link starts with 0 dBm transmit power and a −95 dBm receiver, giving a comfortable 35 dB margin in line of sight — but routing it through three drywall partitions and one brick wall adds 24 dB of loss and cuts the margin to just 11 dB. A stronger parent recovers 7 dB, and a node reporting every five minutes turns three retries into 36 extra attempts an hour. This audit rebuilds the received-power ledger and asks whether that remaining headroom can survive walls, doors, and carts, or whether battery life stays unproven until those attempts are tied to measured radio-on time.
Companion to the chapter 802.15.4 Deployment Practices — every number here comes from that chapter.
One path-loss ledger, one retry warning, ~4 minutes
The physics is not hidden in the join result. Ada keeps the deployed link as a received-power ledger, then asks whether the remaining dB headroom can survive walls, doors, carts, and retries.
Rebuild the received-power ledger
The chapter’s 10 m, 2.44 GHz example starts with 0 dBm transmit power, near-0 dBi antennas, about 60 dB path loss, and −95 dBm receiver sensitivity.
line_of_sight_margin = -60 - (-95) = 35 dB
wall_loss = 3 x 4 dB + 12 dB = 24 dB
through_walls_rx = -60 - 24 = -84 dBm
through_walls_margin = -84 - (-95) = 11 dB
Compare parents before accepting
The parent comparison uses the same sensitivity, so the stronger parent can be checked without inventing a new radio model.
parent_b_margin = -77 - (-95) = 18 dB
extra_headroom = 18 - 11 = 7 dB
Parent B still needs LQI and retry evidence, but the link-budget math shows why it is the better first commissioning candidate.
Turn retries into a power gate
A node that reports every five minutes sends 12 reports per hour. Three extra retries per report become a measurable radio-on burden.
extra_retry_attempts_per_hour = 12 x 3 = 36
The chapter does not state current or airtime for each retry, so Ada stops at the count. Battery life can be claimed only after those 36 attempts/hour are tied to measured radio-on time and current.
Every number above is taken from the chapter’s own installed-link example and re-derived step by step.