tion of the call, and the capacity provisioned on that path is yours and yours alone. Advantages and Disadvantages of Circuit Switching Circuit switching uses many lines to economize on switching and routing computation. When a call is set up, a line is dedicated to it, so no further routing calculations are needed. Chapter 3 Establishing Communications Channels Local Exchange Intermediate Switching Offices Trunk that stays in place Connection set up when call is placed Figure 3.1 A circuit-switched call Since they were introduced in the mid-1980s, digital cross-connect systems (DCSs) have greatly eased the process of reconfiguring circuit-switched networks and responding to conditions such as congestion and failure. DCSs create pre- defined circuit capacity, and then voice switches are used to route calls over circuits set up by these DCSs. DCSs are analogous to the old patch panels. You may have seen a main distribution frame (MDF) on which twisted-pair wiring is terminated. The MDF is a manual patch panel, and before DCSs were introduced, when it was necessary to reconfigure a network based on outage, congestion, or customer demand as a result of shifting traffic patterns, technicians had to spend days or even weeks manually making changes at the MDF. The DCS is a software patch panel, and within the software are databases that define alternate routes-alternate connections that can be activated if the network encounters a condition that requires some form of manipulation. DCSs are one of the elements of the PSTN that contribute to its reliability: When network conditions change, in a matter of minutes, a DCS can reconfigure the network around those changes. With such tools, the PSTN can offer five-nines reliability-in other words, 99.999% guaran- teed uptime. (DCSs are discussed in more detail in Chapter 4.) Circuit switching offers the benefits of low latency and minimal delays because the routing calculation on the path is made only once, at the beginning of the call, and no more delays are incurred subseque