Abstract: The dynamic functional behavior of geographically distributed fast packet switching systems, including those which accommodate high-priority circuit switched traffic and low-priority packet switched traffic, are tested in real-time by sending test packets from one or more source nodes through the system to specific destinations that comprise a test packet analyzer. The test packets have the same structure as the data packets, but in their payload portion carry the entire information required to perform the testing. The nature of that test information depends on the characteristics of a set of predefined system errors the verification system is supposed to identify.