An encoder and an encoding method are suitable for use in an optical communication system. A concatenated coding scheme is used, in which the source data are encoded by means of an outer encoder to produce outer encoded data, and the outer encoded data are encoded by means of an inner encoder to produce inner encoded data, which are transmitted over a communications medium, such as an optical fibre. The inner encoder acts to produce inner encoded data in a format which occupies the space occupied one or more (for example, two) frames as defined in a standard, in this case the ITU-T G.709 standard. The inner encoder forms a product code from two sets of codewords, for example Extended Hamming codes. More particularly, at least one of the two sets of codewords is a shortened code, in order that the inner encoded data exactly occupies a plurality of frames as defined in the standard.