Model: available SNR = 81 dB − insertion loss (20 dB·√(f/100 MHz)·L/100 m) − crosstalk penalty (per category, scaling with length). Required SNR = Shannon bound 10·log₁₀(2^η−1) for spectral efficiency η = payload-per-pair ÷ bandwidth, plus an implementation margin (modulation loss, FEC overhead). The model captures crosstalk within the cable only; the certified reach limits additionally account for alien crosstalk from neighbouring cables in the same bundle, so a PASS also requires the length to be within the standard’s certified reach for that cable — which is why the verdict can fail even when the raw margin looks positive.