A wide area network (WAN) is a group of computer networks connected over long distances by telecommunications links, which can be either wireline or wireless. A number of WAN links can be used, each of which was developed to address specific requirements in data communications. To meet specific network and application needs, a number of WAN techniques (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 circuit and use only a part of it, you may still pay for the full 45Mbps. The main disadvantage 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 customers 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 applications. Most recently, IP-based networks have taken an increasingly important role, 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 Infrastructures," 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 terminal equipment (DTE) at the customer premises, as well as a physical interface to data communications (or channel or circuit-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 entranta multiservice provisioning platform [MSPP] that supports multiple protocols).
Figure 7.1. Data service components
Despite the fact that there are numerous WAN options, all of which can offer various cost-efficiencies or performance improvements, the many separate networks in use translate into high costs associated with the overall infrastructurefor 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 broadband 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.
All the various WAN options can be put into two major categories of data networks: circuit-switched and packet-switched networks. This chapter discusses the categories and characteristics of traditional WANs. It covers the use of circuit-switched WAN optionsleased lines and Integrated Services Digital Network (ISDN)as well as packet-switching WAN optionsX.25, Frame Relay, and ATM. Given their significant impact, Internet and IP services are covered in separate chapters (see Chapters 8 and 9).
Part I: Communications Fundamentals
Telecommunications Technology Fundamentals
Traditional Transmission Media
Establishing Communications Channels
Part II: Data Networking and the Internet
Data Communications Basics
Local Area Networking
Wide Area Networking
The Internet and IP Infrastructures
Part III: The New Generation of Networks
Broadband Access Alternatives
Part IV: Wireless Communications
Wireless Communications Basics
WMANs, WLANs, and WPANs
Emerging Wireless Applications