It should be appreciated that designers are usually concerned, not with particular circuits or links, but with plant that will be used for the establishment of many links.
ITU-T G.226-1993 Referenced Document
ITU-T G.222 NOISE OBJECTIVES FOR DESIGN OF CARRIER-TRANSMISSION SYSTEMS OF 2500 km
ITU-T G.223 ASSUMPTIONS FOR THE CALCULATION OF NOISE ON HYPOTHETICAL REFERENCE CIRCUITS FOR TELEPHONY