ess the local exchange and to replace combined voice and data trunks. When you are using it to access the PSTN, you have to work within the subscribed standards. For example, with T-1 you get 24 channels at 64Kbps per channel, so if you have only a little bit of data to send, you can't reallocate your extra bandwidth to another application. You are stuck with static bandwidth allo- cation (see Figure 7.8), either 24 64Kbps channels or 1 1,536Kbps channel. A second way in which you can use T-, E-, or J-carrier facilities is to build a pri- vate network. For example, if you use a T-1/E-1 to tie together two of your own locations, you can basically do as you wish with that pipe. You're in control of it, and you have the intelligent equipment at either end that can manage it on your behalf. For example, you can dynamically assign bandwidth; you can allocate only as much capacity as is necessary for an application, which more efficiently uses the capacity available on the digital facility. When you use a T-1/E-1 to build a private network, you can also perform dynamic alternative routing. For example, in Figure 7.9, the primary route between Los Angeles and New York City is the direct diagonal line between them. But say there is a problem and that link fails. The mul- tiplexer in Los Angeles will be smart enough to know to reroute its highest-priority traffic through Denver to get it to New York. And in Denver, it may reduce the second-priority traffic to third priority in order to make room for the incoming high-priority traffic from Los Angeles. When the primary link is recovered, the net- Static Bandwidth Allocation 64Kbps 64Kbps 64Kbps 64Kbps 64Kbps Dynamic Bandwidth Allocation Available Figure 7.8 Static versus dynamic bandwidth allocation Circuit-Switched Networks Denver New York City Primary route Alternative route Los Angeles Dallas Figure 7.9 Dynamic alternative routing work will revert to the original routing mechanisms. Dynamic alternative routing is useful in the face of congestion, failure, and a customer