in a row are transmitted, the network may lose synchronization, which means transmission errors could occur. Therefore, the DSU performs a bipolar variation-it alternates the one bits as positive and as negative voltages. The DSU also supplies the trans- mit and receive logic, as well as the timing. The CSU provides a means to perform diagnostics. DDS facilities can be used for LAN interconnection, access to the Internet, and remote PC access to local hosts. One thing to bear in mind with a traditional DDS approach is that it is a leased-line service. If the leased line goes down, the network is out of service, and recovering the service can be a lengthy process, depending on how many network providers are associated with the link end to end. The mere process of troubleshooting and identifying within whose network the problem lies can often lead to resolution times of 24 to 48 hours-or even more, if network managers do not cooperate with one another. If you rely on a DDS network for crit- ical applications, you need a backup in case the leased line fails at some point, and the best backup option is generally switched digital access, a dialup option in which facilities are allocated based on demand rather than associated with a specific cus- tomer all the time. Switched digital access supports transmission rates of 56Kbps, 64Kbps, 384Kbps, and 1,536Kbps. Another potential backup is ISDN, which is described later in this chapter. T-, E-, or J-Carrier Backbone In the 1980s, as networks, traffic, and applications were evolving and growing, customers saw a rise in the amount of data traffic they were carrying, and they began to institute various data services in response. This resulted in a hodgepodge of single-purpose networks based on leased lines that included unique universes of equipment or specific applications. For example, assume that your company owns the networks shown in Figure 7.5. Both point-to-point and multipoint leased lines are being used to provide connec- tivity between LANs in various cities around