This Supplement is not a Recommendation and does not have the status of a standard. In case of conflict between the material contained in this Supplement and the material of the relevant Recommendations, the latter always prevails. This Supplement should not be used as a normative reference; only the relevant Recommendations can be referenced. This Supplement describes design and engineering considerations for unamplified and amplified single-channel and multichannel digital optical line systems supporting PDH, SDH and OTN signals in intra-office, inter-office and long-haul terrestrial networks. One intent of this Supplement is to consolidate and expand on related material that is currently included in several Recommendations, including [ITU-T G.955], [ITU-T G.957], [ITU-T G.691], [ITU-T G.692] and [ITU-T G.959.1]. This Supplement is also intended to allow a better correlation of the specifications of the fibre, components and system interface Recommendations currently developed in ITU-T Study Group 15 by Question Groups 5, 7 and 6, respectively.