rter pay- back period for the deployment. They also facilitate quick response to growth in services and competition. With ever-increasing interest in high-speed broadband access, DLCs could be a tool for shortening loop length, thereby bringing more bandwidth to the customer. Consequently, some of the additional changes that have occurred with the newer generations of DLCs also provide interfaces for SDH/SONET or optical fibers. However, the vast majority of DLCs deployed are incompatible with the xDSL ser- vices. It is imperative that the proper generation of DLC be deployed in order to meet the customer's demand for broadband residential access via twisted-pair. T-1/E-1 Local Exchange T-E-J-Carrier Analog T-3/E-3 SDH/SONET Lines Cross-Connect OC-1, ,OC-3 Trunks Digital Switch T-1/E-1 Cross- Trunks Connect Figure 4.6 DLC architecture Chapter 4 The PSTN DCSs DCSs, developed in 1981 and officially introduced in 1985, basically auto- mated the process of circuit provisioning and replaced the use of manual patch panels. The key feature of DCSs is called drop and insert, which refers to the capa- bility of the cross-connect to exchange channels from one facility to another. It is used to implement appropriate routing of traffic, to reroute around congestion or failure, and to allow customers to dynamically reconfigure their own networks. Generally, drop and insert keeps communications paths in place for continuous use over a period of months, or sometimes even years, but it does allow change as demand warrants. Essentially, a DCS is a computer system with a variety of software databases that describe first-choice routes and alternate routes (see Figure 4.7). If channel 7 normally goes out over line 1 and then goes out over trunk 1, but trunk 1 fails, the DCS can consult its alternate routing table, which might say to reroute that partic- ular line over trunk 2. A reconfiguration can take place in a matter of minutes. DCSs provide for different levels of switching. You can switch between DS-3s and DS-3s or between E-3