The authors propose a novel method for test sequence generation based on UIOS which only needs a minimal verification part. They also prove that the UIOv method (or revised unique input/output method) has the same applicability as the characteristic set method. The UIOv method can find a test sequence for any minimal finite-state machine. The proposed method has the same applicability as the UIOv method. In addition, the method uses a simple test on a given protocol specification to decide whether a verification part is needed for detecting transfer faults, the method can generate a minimal number of input/output sequences for a verification part. A more efficient algorithm for generating UIOSs and signature sets is presented. It is proven that the upper bound on the length of a UIOS is (n-1)*n rather than n/sup n/, where n is the number of states in a finite-state machine.>