US Patent 7193964 Hitless protection switching

The invention provides a method and apparatus for providing hitless protection switching in a transmission systems which support a standardized implementation of virtual concatenation. A switching apparatus is arranged to receive data signals on at least two transmission paths and to output data from a selected one of said transmission paths. The apparatus is arranged to align the respective received data signals so that a selector mechanism can select between corresponding elements of the received data signals. This arrangement allows hitless switching to be performed since there is no loss or repetition of signal elements when switching occurs. In a preferred embodiment, data in respect of each frame of each data signal is stored and the selector mechanism selects between paths on a frame-by-frame basis by comparing quality information carried by the data signals. This arrangement offers a very low end to end Bit Error Rate (BER) performance. The invention is suitable for both synchronous and non-synchronous networks (for example, OTN) which implement standardized virtual concatenation.