This paper presents the testing methodology for out-of-step protection system operation validation. The protection under consideration is a wide-area measurementbased system that consists of several parts: intelligent electronic devices, GPS measurement synchronization and communication network. The entire protection system must be tested in the laboratory before installation on site. The problem is, that communication network is hardly available at the laboratory testing stage and, at the same time, the communication network is a critical part of the system, which directly influences the entire system operation. To overcome this problem, the communication network emulator was elaborated that allows to test the entire protection system in real-time and in the presence of various, potentially vulnerable conditions.