Control symbols taking the form {k1-k2-k2-k1} are inserted in a serial stream including m bit data words. k1 and k2 are each predefined m bit control words differing from the m bit data words. The Hamming distance between k1 and k2 is at least 2. Such control symbols may be robustly detected in the presence of a one bit error in the symbol, or a data word immediately preceding or following the symbol. The m bit words may be 8B/10B encoded data, or defined control words. The control symbols may be used for data delineation, stream synchronization, transmitter/receiver synchronization or for other control signalling.