说明：  Voice over IP (VoIP) is undoubtedly a powerful and innovative communication tool. Compared with the public switched telephone network (PSTN), VoIP off ers the benefi t of reducing communication and infrastructure cost. This makes it possible for everyone to easily keep in touch with family, friends, and clients around the world. Furthermore, VoIP has the potential to create new and att ractive communication tools by integrating with various applications, such as Web systems, presentation soft ware, and photo viewers. In the near future, we expect VoIP to provide a rich multimedia communication service.