al exchanges. If a circuit switch is blocked because it is carrying a long Internet session, people may not be able to get a dial tone. There are several solutions to this problem. For example, as discussed in Chapter 10, "Next-Generation Networks," we can apply intelligence in front of some exchanges SO that calls destined for ISPs can be diverted over a packet-switched network to the ISP rather than being completed on a circuit-switched basis through the local exchange. Yet another variable that's important to the design of the PSTN has to do with what it was designed to support. The capacities of the channels in the PSTN are of the narrowband generation-they are based on 64Kbps channels. The worldwide infrastructure to accommodate voice communications evolved to include a series of circuit switches. Different switches are used based on the locations to which they connect. The switches have a high degree of intelligence built into them, both for establishing the communications channels and for delivering the service logic to activate a growing array of features. In the traditional framework, the monolithic switches in the network had all the smarts. The switch manufacturer and the car- rier worked together very closely, and the carrier couldn't introduce new features and services into a particular area until a software release was available for the switch platform through which the neighborhood was being serviced. Thus, carri- ers were often unable to roll out new services and features because they hadn't yet received the new software releases from the switch manufacturers. Over time, we have separated the functions of switching and connection establishment from the functions involved in the intelligence that enables various services and features to be activated. The traditional PSTN is associated with highly developed, although not neces- sarily integrated, operational support systems (e.g., billing systems, provisioning systems, network management systems, customer contact systems, security sys- items). These