Abstract: ATM test cells are generated and inserted into an ATM combined data stream in such a manner that a desired average insertion rate is maintained. The desired average insertion rate is obtained by enabling generation of and insertion of test cells into each idle cell of the ATM combined data stream so long as the actual number of test cell being inserted does not exceed a number that should have been inserted.

Abstract: In a duplicate active-standby memory unit arrangement, a resynchronization initiation determination is made on a cell-interval-by-cell-interval basis based on the equality of cell contents of corresponding queues in the active and standby memory units. If an inequality of the cell contents of the corresponding queues in the active and standby memory units occurs during any cell interval, resynchronization may be initiated. In an exemplary embodiment of the invention, the lengths of data in corresponding queues, i.e., the number of cells stored in the corresponding queues, in the active and standby memory units are compared on a cell-interval-by-cell-interval basis. If the number of cells stored in any of the queues is different than the number of cells its corresponding queue in the other of the memory units during any cell interval, resynchronization of the memory units is initiated.

Abstract: An errorless line protection apparatus involves receipt of data from an active line and a standby line. A determination is made as to whether the data from the active line leads or lags the data on the standby line. A switching system directs the leading data to a lead channel containing a controllable amount of time delay up to the maximum amount that the leading data is expected to led the lagging data. Lagging data is directed to a lag channel. A selected one of the data from the lead channel or the lag channel is delivered to an output line.