whether deployed over public or private networks) have been developed and become popular over the years. Leased lines offer the greatest network management control. With leased lines, a known amount of bandwidth is provisioned to you, and no one else has access to it; also, you know who the users are. One disadvantage of leased lines, depending on the carrier, is that you often pay for capacity not used. So if you lease a DS-3 cir- cuit and use only a part of it, you may still pay for the full 45Mbps. The main dis- advantage is that leased lines are very costly; you pay a premium for the comfort of having control over your own destiny. To reduce the costs associated with leased lines, in the late 1990s, many cus- tomers migrated to Frame Relay services and today are choosing to implement converged networks based on IP. Frame Relay was introduced in the early 1990s and was largely designed as an application for LAN-to-LAN interconnection. Because numerous subscribers share its virtual circuits, Frame Relay offers great cost-efficiency compared to leased lines. Another WAN alternative to leased lines is Asynchronous Transfer Mode (ATM), which has been perhaps the best solution in environments that have intensive multimedia or other high-bandwidth applica- tions. Most recently, IP-based networks have taken an increasingly important role, Chapter 7 Wide Area Networking offering the benefits of a converged network and relatively low cost. Virtual private networks (VPNs) are being used more and more in WANs as well, with increased emphasis on IP-based VPNs. (See Chapters 8, "The Internet and IP Infrastruc- tures," and 9, "IP Services," for discussions of IP and VPNs.) A data service is a digital service offered for data communications at subscriber locations. Remember that data communication was essentially an add-on to the public switched telephone network (PSTN). As options designed for data were introduced, networks needed specialized equipment meant for such service (see Figure 7.1). The end user needed data termin