Some communication systems transmit and receive data over redundant communication links, in order to improve error performance and provide protection against equipment failures and adverse channel conditions.  For example, U.S. Pat.  No.  6,611,942, whose disclosure is incorporated herein by reference, describes a method of protecting the transmission of cells in a telecommunication system.  On the transmitter side, two identical flows of cells are transmitted on two distinct physical links.  Cells serving as markers, and thus delimiting blocks of cells or sets of blocks of cells, are inserted regularly into each of the flows at the transmitter.  On the receiver side, the two flows of cells are received and the block or group of blocks from the flow having the fewer transmission errors is selected. 
U.S. Pat.  No.  5,631,896, whose disclosure is incorporated herein by reference, describes a path switching method without bit loss.  The same digital line signals on a working path and on a protection path are continuously monitored independently for bit errors.  If a bit error occurs in the working path and no bit error occurs in the protection path, a switching trigger is produced and a switching operation from the working path to the protection path is performed on a data block basis.  Only correct data are transferred to downstream apparatuses.  The method uses data blocks of one frame length with an indicator for bit error checking placed at the beginning or top of the block.