A method for communication includes receiving in a first communication interface input frames, which include data symbols that were derived by encoding respective characters, and further include one or more synchronization symbols having no corresponding characters. The characters from the data symbols are recovered, and the recovered characters are transmitted to a second communication interface by mapping the characters into communication frames and discarding the synchronization symbols. The characters are extracted from the communication frames in the second communication interface by synchronizing to the input frames independently of the synchronization symbols. The input frames are reconstructed in the second communication interface, including the data symbols and the synchronization symbols, by re-encoding the extracted characters.