Recommendation ITU-T G.959.1 provides physical layer inter-domain interface (IrDI) specifications for optical networks which may employ wavelength division multiplexing (WDM). The IrDIs within the optical transport network (OTN) are provided by unidirectional, point-to-point, single and multichannel line systems. Their primary purpose is to enable transversely compatible interfaces to span the boundary between two administrative domains. The IrDI specifications include intra-office, short-haul and long-haul applications, without line amplifiers. This version of this Recommendation includes single-channel interfaces suitable for FOIC1.1-RS with PAM4 100G. Amendment 1 to Recommendation ITU-T G.959.1 (2024) includes an updated definition of OTSi and defines new terms T-OTSi, R-OTSi and OTScSi. Amendment 2 to Recommendation ITU-T G.959.1 (2024) includes the addition of 4×FOIC1.2-RS to application codes 8R1-4D1F and 8I1-4D1F.