The present invention provides frame-interleaving systems and methods for Optical Transport Unit K (OTUK) (i.e. Optical Transport Unit 4 (OTU4)), 100 Gb / s Ethernet (100 GbE), and other 100 Gb / s (100 G) optical transport enabling multi-level optical transmission. The frame-interleaving systems and methods of the present invention support the multiplexing of sub-rate clients, such as 10×10 Gb / s (10 G) clients, 2×40 Gb / s (40 G) plus 2×10 G clients, etc., into two 50 Gb / s (50 G) transport signals, four 25 Gb / s (25 G) transport signals, etc. that are forward error correction (FEC) encoded and carried on a single wavelength to provide useful, efficient, and cost-effective 100 G optical transport solutions today. In one exemplary configuration, a 100 G client signal or 100 G aggregate client signal carried over two or more channels is frame-deinterleaved, followed by even / odd sub-channel FEC encoding and framing. In another exemplary configuration, a 100 G client signal or 100 G aggregate client signal carried over two or more channels is received and processed by a single 100 G FEC framer, followed by frame-deinterleaving into two or more sub-rate channels.