Supplement 39 to ITU-T G-series Recommendations provides information on the background and methodologies used in the development of optical interface Recommendations such as Recommendations ITU-T G.957, ITU-T G.691 and ITU-T G.959.1. This revision adds further information on modulation formats; clarifies power budget parameters for systems with optical amplifiers; provides further information on optical signal-to-noise ratio (OSNR) measurement; updates information on concatenation of non-linear effects; adds bit error ratio (BER) estimation technique based on constellation; and makes other miscellaneous corrections.