A power budget is confused with a complete network approval

A fictional link model supplies a transmitter output of minus3 dBm and receiver sensitivity of minus28 dBm. A designer reports25 dB of available budget and immediately declares the complete network suitable. No engineering margin, wavelength compatibility, component loss inventory, bandwidth or dispersion evidence has been considered. Another person adds the two negative power values and treats the result as a loss. This written exercise transmits no light and approves no installed network. Explain the correct arithmetic and its scope, distinguish dBm from dB and identify why an apparently positive power budget does not settle every condition needed for a real working link.