The wide area measurement system (WAMS) is gradually becoming an important guarantee of security and stability in smart transmission grids. In the utility industry, WAMS is sometimes referred to as a PMU (phasor measurement unit) network, as synchronized PMUs are the most crucial elements in collecting real-time monitoring information. A PMU network provides much better observability and controllability in smart grid operations. However, like any other physical system, a PMU network itself can fail. The consequences of PMU network failure are serious and can include a large blackout. Therefore, the reliability of PMU networks should be quantitatively evaluated and assured.