Recent Information and Communication Technology (ICT) advances have enabled power system applications using measurement signals across the Wide Area Network (WAN). The application control performance relies on the quality of data delivery service. However, the characteristics of Quality of Service (QoS), such as latency, packet loss, and packet jitter, are unavoidable. It is a trend to take QoS metric requirement into the consideration of controller design. But, how to ensure the application receiving the data within the designed tolerant range is another challenge. This paper presents ongoing work on a novel Stateful Data Delivery Service (SDDS) for power system application to address the challenge from the side of the communication infrastructure. The SDDS monitors the QoS performance on-line and identifies the signals which satisfy the requirement for the application to use. As a proof of concept, a Power Oscillation Damping (POD) controller is connected to the SDDS. The result shows the improvement in robustness of the POD controller by application of the SDDS. The paper also shows the feasibility of applying SDDS to WAC applications.