SDH communication network is critical for the WARMAP system (a Power System Security and Stability Defense System) which transmits data and control command among different layers of control stations and devices. Figuring out the impact of communication failure on WARMAP will ensure the proper functioning of WARMAP. Firstly, this paper introduces the framework and workflow of WARMAP. And then, through a typical provincial level WARMAP system, WARMAP's demand on real-time performance and reliability requirement of SDH network are summarized. Following that, the impact of communication interruption, latency and bit error on WARMAP and their consequences on power system stability are analyzed in detail. Two case studies are performed to demonstrate the impact of communication latency on power system transient angle margin and the relationship between communication latency and the amount of tripped generations. The future research directions have been pointed out at the end of the paper.