Queue-length monitoring arrangement for detecting consistency between

Abstract

A shared-buffer-memory-based asynchronous transfer mode (ATM) switch module (1) is duplicated (2) and operates in active-standby mode for fault-tolerance. Following failure and repair of a module, contents of the two modules are resynchronized as follows. When the synchronizing operation is begun contents of the memory of the repaired standby module are cleared, all writes to the active module's memory are also made to the standby module's memory, and the system monitors the overwriting of the contents of the active module's memory that existed at the time the synchronizing commences. This is done by a function (FIG. 4), which sets a flat (64) in a queue-length-counter monitor (60) for every active-module queue-length counter (200) that reaches a count of zero, to indicate that its corresponding buffer-memory queue (100) has been emptied. When all monitor flags have been set in response to each queue-length counter having reached a count of zero, this indicates that the original contents of the active module's memory have been overwritten and that the memories again have identical contents, and consequently that resynchronization of the two memories has been achieved and fault-tolerance has been re-established.