Chapter 7. Wide Area Networking

Chapter 7. Wide Area Networking

A wide area network (WAN) is a group of computer networks that are 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, and no one else has access to it; also, you know who the users are. The 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, many customers migrate to Frame Relay services. 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 as compared to leased lines. Another WAN alternative to leased lines is Asynchronous Transfer Mode (ATM), which is perhaps the best solution, especially in environments that have intensive multimedia applications or other high-bandwidth applications. Virtual private networks (VPNs) are increasingly being used in WANs as well.

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 we began to introduce options designed for data, we needed to introduce into networks specialized equipment that was meant for such service (see Figure 7.1). The end user needed data terminal equipment (DTE) at the customer premise, as well as a physical interface to data communications, or data channel, equipment (DCE). From that DCE there would be an access link into a specific access node, designed to facilitate the data service in question (for example, a digital switching hub for a digital data service [DDS] over leased lines, a unique X.25 packet switch for X.25 services).

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 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 is to provide an integrated broadband infrastructure that minimizes the range of required equipment that needs to be maintained on an ongoing basis.

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 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.