f circuit-switched WANs: those based on leased lines and customer premises equipment (CPE) to manage the leased lines and those based on ISDN, including both Basic Rate Interface (BRI) and Primary Rate Inter- face (PRI). Networks Based on Leased Lines Leased lines can be configured in two different ways. The first approach uses point- to-point leased lines, as shown in Figure 7.2. In this approach, a communications London Server CSU/DSU Router San Francisco 56/64Kbps Server Server CSU/DSU 56/64Kbps CSU/DSU Router Router CSU/DSU CSU/DSU Server 56/64Kbps Paris CSU/DSU Router Athens Figure 7.2 A point-to-point leased-line network Chapter 7 Wide Area Networking link joins two nodes-and only two nodes. The good thing about this setup, of course, is that there is no contention. The two devices always have an available communications path. The disadvantage becomes pronounced when the network starts to grow, either in the number of devices or in the distance between the loca- tions. Because leased lines are calculated on a mileage-sensitive basis, the cost increases as the network scope increases. To increase the cost-efficiency of a leased-line network, you can use the second approach: a multipoint leased-line network. A multipoint leased-line network has a shared communications facility, and multiple nodes vie for access to the commu- nications link. The advantage of this configuration is that it combines mileage, so the overall monthly cost associated with the leased line is reduced. The disadvan- tage is that it requires some type of intelligent scheme that helps determine which device gets to use the communications pathway at what time. Figure 7.3 shows a multipoint leased-line network. Leased lines can be configured on a point-to-point or multipoint basis by using a number of approaches, including these: DDSs, which essentially use 56Kbps or 64Kbps leased lines T-, E-, or J-carrier backbone San Francisco London Server CSU/DSU Router Server 56/64Kbps 56/64Kbps Server Router CSU/DSU CSU/DSU Router Paris 56/64Kbps Ser