.4 A stored program control be tolerated before the transmission is greatly hampered. Hence, we will be able to build connections to meet a particular service class and thereby aid in ensuring the proper performance of an application. Customer premises equipment (CPE) circuit switches include PBXs. In the PSTN, circuit switches include the local exchanges with which subscribers access the network, the tandem or junction switches that interconnect numbers of local Chapter 3 Establishing Communications Channels exchanges throughout a metro area, the toll or transit switches used for national long-distance communications, and the international gateways used for cross- country communications. A large number of vendors sell these circuit switches, as well as more specialized niche products. A fourth generation of switches-optical networking switches-is emerging now (see Chapter 12, "Broadband Access Alternatives"). Often, these optical net- working elements are referred to as wavelength routers or optical switches. The idea is to be able to provision a very high-speed path, at OC-48 (i.e., 2.5Gbps), end to end across a network of dense wavelength division multiplexers. This will be increasingly important in providing communications interfaces to the high-speed switches that have become available. With circuit switches, the ratio of performance to cost doubles approximately every 80 months to 40 months (i.e., normally the performance improves every 80 months, although sometimes new generations are created more rapidly-every 40 months). Major architectural changes in circuit switches occur relatively infrequently. Network switches are responsible for doing all the work of setting up and tear- ing down calls, as well as for addressing and providing the features requested. They provide a very high level of functionality on a very centralized basis within the net- work, and that enables the end stations to be very cheap and very dumb (e.g., a single-line telephone). Again, when intelligence was extremely expensive, there w