Abstract: A technique for compensating for skew between parallel digital data bitstreams resulting from transmission over a path, not requiring the sending of a training sequence or excessive down-time, is effected by sending a parallel reference bitstream carrying a sequence of samples taken from the individual data bitstreams. It is found that correlation of the reference bitstream with the data bitstreams is sufficient to identify the skew amount. This can then be signalled or preferably corrected at the receive end or pre-corrected at the transmit end of the path. Signalling can also be superimposed by multiplexing the reference bitstream with signalling information with little or no additional overhead. This can be particularly useful to shut down a laser in an optical transmission path if a broken optical fibre is detected that would otherwise pose a health hazard to personnel.

Abstract: Within a transmission channel in a frequency domain transmission medium, a plurality of non-contiguous channel portions are selected for data transmission. The selected channel portions are separated in frequency by unused channel portions. A transmitter generates a data frame of predetermined length for transmission, comprising a payload and an error detection portion. The frame length is equal to the length which would map for transmission into a symbol in the transmission channel if the entire channel were selected for transmission. The payload contains a header and a data portion, into which the transmitter inserts data bytes and a number of padding bytes. Block coding error correction is performed on the padded payload to generate error detection bytes to fill the error detection portion.