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Dynamic voltage scaling (DVS) is a technique that varies the supply voltage and clock frequency, based on the computation load, to provide the desired performance with the minimal amount of energy consumption. It has been demonstrated as one of the most effective low-power system design techniques, particularly for real-time embedded systems. Most existing work are on two different system models that enable DVS: 1) the ideal DVS system that can change its operating voltage with no physical constraints; and 2) the multiple DVS system that has only a number of discrete voltages available. Although the ideal DVS system provides the theoretical lower bound on the system's energy consumption, it is the practicability of multiple DVS systems and the emergence of other DVS-enabled systems, which do not fit either model, that challenge system designers the following questions: Should DVS be implemented in the design or not? If so, how should DVS be implemented? In this paper, these questions are answered by studying the DVS-enabled systems that can vary the operating voltage dynamically under various real-life physical constraints. Based on the system's different behaviors during voltage transition, the optimistic feasible DVS system and the pessimistic feasible DVS system are defined. A mathematical model for each DVS-enabled system is built and their potential in energy reduction is analyzed. Finally, a secure wireless communication network with different DVS-enabled systems is simulated. The results show that DVS gives significant energy saving over system with fixed voltage. Interestingly, it is also observed that although multiple DVS system may consume more energy than the theoretical lower bound, the optimistic and pessimistic feasible DVS systems can achieve energy savings very close to the theoretical bound provided by the ideal DVS system.