This Supplement describes interfaces between an optical transport network (OTN) framer device and an optical module with or without digital signal processor (DSP) (module framer interfaces (MFIs)). The MFIs described in this Supplement carry optical transport unit k (OTUk; k = 3, 4), optical transport unit 25 or 50 with Reed-Solomon (RS) forward error correction (FEC) (OTU25-RS or OTU50-RS), optical transport unit Cn (OTUCn), or flexible optical transport unit n (FlexO-n(e)) signals. Electrical parameters for these interfaces can use specifications provided in the relevant clauses of the OIF-CEI implementation agreement (IA) [b-OIF-CEI].