A network device includes a substitutor and a transmitter. The substitutor receives input columns of concurrently received input symbols. Each of the input columns includes one input symbol from each of a plurality of parallel input lanes. The substitutor generates output columns corresponding to the input columns, wherein each of the output columns includes one output symbol for each of a plurality of parallel output lanes. The substitutor replaces the output symbols of a selected column of the output columns with alignment symbols. The selected column is immediately followed by a second column, and the second column is immediately followed by a third column. The substitutor replaces the output symbols of the second column with disposable symbols, and replaces the output symbols of the third column with boundary symbols. The transmitter drives data onto a communications medium in response to the output symbols generated by the substitutor module.