's need to reconfigure capacities based on activities and personnel at given locations at random times. Another element that a customer can use in creating a comprehensive enterprise backbone is a digital cross-connect. This purpose of this device, introduced in 1985, was to automate the process of provisioning circuits-in essence, to replace the use of manual patch panels. The key feature of the digital cross-connect system (DCS) is its capability to "drop and insert," which means the cross-connect can exchange channels from one facility to another (refer to Figure 4.7 in Chapter 4). It is used to implement appropriate routing of traffic, to reroute around congestion or failure, or to allow customers to dynamically reconfigure their networks. With digital cross- connects, network configurations are defined entirely in software, which gives the customer great control and allows reconfigurations to be implemented in a matter of minutes rather than hours or days. The levels at which switching can be per- formed are at the DS-3/CEPT-3 level, DS-1/CEPT-1 level, and DS-0/CEPT-0 level. Sub-DS-0 and sub-CEPT-0 levels are also possible. This capability is also offered by some of the intelligent multiplexers-the T-1/E-1 and T-3/E-3 multiplexers-so the functionality can be bundled. The main applications for digital cross-connects are recovering after disasters, bypassing systems during scheduled maintenance, addressing peak traffic demand, and implementing temporary applications. For customers who need to support even more advanced applications-such as computer-aided design, three-dimensional modeling and simulation, visualization, and multimedia-the capacities of the T-, E-, and J-carrier infrastructure may not suffice. The next step is to migrate to the SDH/SONET signal hierarchy. Chapter 7 Wide Area Networking SDH/SONET Backbone As discussed in Chapter 4, SDH/SONET is the second generation of digital infra- structure, based on the use of fiber optics. With SDH/SONET, an enterprise today might subscribe to OC-1 (51Mbps) 