A system and method for performing rate adaptation and multiplexing of constant bit rate (CBR) client data for transmission over a Metro Transport Network (MTN) by defining a plurality of plurality of generic mapping procedure (GMP) thread frames for a respective stream of two or more streams of 64B/66B-encoded blocks of CR client data, defining a plurality of pseudo-Ethernet packets, mapping the plurality of GMP thread frames into consecutive pseudo-Ethernet packets, assembling a stream of GMP multiplexing frames comprising the consecutive pseudo-Ethernet packets, inserting a fixed number of idle blocks between the consecutive pseudo-Ethernet packets of the stream of GMP multiplexing frames and inserting an MTN path overhead (POH) frame into the stream of GMP multiplexing frames to generate a stream of GMP multiplexing rate adapted frames.