ver CSU/DSU Router Athens Figure 7.3 A multipoint leased-line network Circuit-Switched Networks SDH/SONET backbone Dark fiber backbone The following sections describe these approaches. A DDS uses leased lines that operate at either 56Kbps or 64Kbps, depending on whether the network uses T-, E-, or J-carrier infrastructure. Figure 7.4 illustrates the equipment associated with a DDS network. Two pieces of equipment are located at the customer site: the DTE (e.g., host computers, routers) and DCE (e.g., the data service unit [DSU]), which are connected to one another via a physical interface (e.g., an RS-232, a V.35). The data access line (i.e., the 56Kbps or 64Kbps facility) runs from the DCE to a specialized DDS hub that is a digital circuit switch. (When this service was introduced, local exchanges were analog, so in order to introduce DDS into a metropolitan area, the telcos had to put into the exchange environment a specific digital circuit-switched hub for those services.) The DDS hubs connect into the digital transport network; destination cities attach to the network the same way, using the same equipment configurations. A DSU is a device that connects various DTE via RS-232 or V.35 interfaces with the digital services that offer 56Kbps or 64Kbps access. DSUs can be used to accom- modate DDS, Frame Relay, and ATM facilities. When a DSU is combined with a channel service unit (CSU), it interfaces with services at the 64Kbps level, as well as n X 64Kbps, up to either T-1, E-1, or J-1 capacities. (These levels are described in detail in Chapter 4, "The PSTN.") The DSU converts the binary data pulse it receives from the DTE to the line-encoding format required by the network. Within Long-Distance Digital Channel Physical Interface: Physical Interface: RS-232 or V.35 RS-232 or V.35 Data Access Lines Figure 7.4 DDS equipment Chapter 7 Wide Area Networking the computer, the one bits are positive voltages, and the zero bits are no voltages or low-level voltages. The ones density rule says that if more than 15 zeros 