Method. Span loss = L × α + extra. Line OSNR =
58 + Pch − Lspan − NF − 10·log10(N) (0.1 nm reference). Delivered OSNR combines line and transmitter in parallel: 1/OSNR = 1/OSNR_line + 1/OSNR_Tx. The LD-OSNR floor subtracts the end-of-life derate. OPP = floor − ROSNR. Margin = delivered OSNR − ROSNR − sum of penalties, reported at beginning of life (nominal) and end of life (floor).
OPP allocation — where the budget goes
Method. Total link loss = fiber (len × α) + connectors (count × each) + splices (count × each) + other components. Power budget = Tx output − Rx sensitivity. Margin after loss = power budget − total loss. Net margin = margin after loss − held system/repair margin. Maximum reach = (power budget − fixed losses − system margin) / α.