PDF of this tutorialThe PSTN as it exists today presents a number of scaling points that provide obstacles to rapid growth. Unplanned network loading, as with the increases in dial-up Internet call hold times, does not follow the more predictable population migration or growth formula that traditionally allowed carriers to upgrade and expand their networks over time. “Hot spots” emerge as these scaling points are stressed under load.
Figure 3. Public Switched Telephone Network (PSTN)
Class-5 Switch
An integral part of the PSTN, the Class-5 switch is an extremely complex and expensive piece of equipment that resides within the local exchange or central office (CO). Although all Class-5 switches perform the same tasks within the network, there are two types of Class-5 switches from a connection standpoint. The Class-5 switch at the CO to which the dial-up Internet call is made—the point at which the traffic makes ingress to the PSTN—is an “ingress” switch. The Class-5 switch at the CO to which an Internet service provider (ISP) is connected—where the Internet traffic egresses the PSTN—is an “egress” switch. Anything on the network before a switch is considered to be “line-side” or “preswitch,” whereas anything after the ingress Class-5 switch is considered “trunk-side.” Hot spots can occur as a result of unplanned network loading on both the line side and trunk side of the PSTN.
Class-4 Switch
Another major component within the PSTN is the Class-4 tandem switch, which is responsible for intermediate connection of trunk lines between the station originating the call and the destination. Like the Class-5 switch, the Class-4 tandem switch has capacity limitations of its own. Certainly one of the primary scaling points in the PSTN, any necessity to rapidly increase the amount of traffic or number of trunks a tandem facility must manage may cause congestion, creating an additional hot spot.
From a network management perspective, not only is each component complex and expensive, it is also proprietary equipment with very few suppliers and long procurement lead-times. This scenario does not lend itself to economical, rapid scaling to meet new demand for subscribers or for additional network load due to increasing dial-up Internet call hold times. Implied within these major PSTN facility descriptions is that, regardless of where on the network the particular component resides, increased load stresses each element's capability and capacity—the more unplanned the load or stress, the more problematic for the carrier and the more likely that facility will become a network hot spot. Moreover, any investment the carrier makes to mitigate the problem reaps no increased revenue due to flat-rate access.
To date, the majority of efforts devoted to easing congestion have targeted the trunk side of the PSTN, attempting to off-load the network and alleviate the need for additional Class-4 tandem switches and Class-5 egress switches.
