Dynamic voltage scaling (DVS) is a technique that varies the supply voltage and clock frequency based on the computation load to provide desired performance with the minimal amount of energy consumption. It has been demonstrated as one of the most effective low power system design techniques, in particular for real time systems. Previously, there are works on both ends of the DVS systems: the ideal variable voltage system which can change its voltage with no physical constraints, and the multiple voltage system which has a number of discrete voltages available simultaneously. In this paper, we study the DVS systems between these two extreme cases. We consider systems that can vary the operating voltage dynamically under various real-life physical constraints. Based on the system's different behavior during voltage transition, we define the feasible DVS system and the practical DVS system. We build mathematical model to analyze the potential of DVS on energy saving for these different systems. Finally, we simulate the behavior of a secure wireless communication networks with DVS systems. The results show that DVS results in energy reduction from 36% to 79%, and the real life DVS systems can be very close to the ideal system in energy saving.