al equipment (DTE) at the customer premises, as well as a physical interface to data communications (or channel or cir- cuit-terminating) equipment (DCE). From that DCE there would be an access link into a specific access node, designed to facilitate the data service in question (e.g., a digital switching hub for a digital data service [DDS] over leased lines, a unique X.25 packet switch for X.25 services, a Frame Relay switch for Frame Relay, an ATM switch for ATM, an IP router for IP, or the latest entrant-a multiservice pro- visioning platform [MSPP] that supports multiple protocols). Despite the fact that there are numerous WAN options, all of which can offer various cost-efficiencies or performance improvements, the many separate net- works in use translate into high costs associated with the overall infrastructure- for both the end user and the operator. One goal of WANs today is to integrate voice, data, and video traffic so that it runs through a common platform (in terms of access nodes) and through a common core network (in terms of the transport infrastructure). For example, the goal of ATM was to provide an integrated broad- band infrastructure that minimizes the range of required equipment that must be maintained on an ongoing basis. Similarly, IP networks are today seen as the road to converged (what we used to refer to as integrated) networks. Access Access Transport Network Digital Interface Figure 7.1 Data service components Circuit-Switched Networks All the various WAN options can be put into two major categories of data net- works: circuit-switched and packet-switched networks. This chapter discusses the categories and characteristics of traditional WANs. It covers the use of circuit- switched WAN options-leased lines and Integrated Services Digital Network (ISDN)-as well as packet-switching WAN options-X.25, Frame Relay, and ATM. Given their significant impact, Internet and IP services are covered in separate chapters (see Chapters 8 and 9). Circuit-Switched Networks There are two main types o