Demultiplexing and multiplexing method and apparatus

A method of demultiplexing client data from a constant bit rate (CBR) optical client serial link into lower bit-rate CBR signals for transmitting over a plurality of wavelength division multiplexed (WDM) optical channels. The method comprises mapping blocks of bytes from a client frame received from the optical client serial link into respective CBR transport frames for transmitting over the respective WDM optical channels and such that the number of bytes in the client frame is the same as the number of bytes in the plurality of respective CBR transport frames. Modifying a number of non-payload bytes from the client frame to carry CBR transport frame control data in each said CBR transport frame.