Figure 1 depicts the current scheme for connecting dial-up Internet users to ISPs using the PSTN. The figure shows a typical network topology. The central-office (CO) switches are connected to tandem switches via IMTs. In some cases, the CO switches are directly connected via interoffice trunks.

Figure 1. Dial-Up Internet Users' Connections to ISPs Using PSTN

Figure 1 shows two typical paths in a representative PSTN for a dial-up connection. In the first scheme, calls from Internet users go through an originating local CO switch (ingress Class 5), a regional tandem switch (Class 4) and a terminating local CO switch (egress Class 5) and are terminated via either multiline hunt group (1 Mb) phone lines or integrated services digital network (ISDN) primary-rate interface (PRI) at the ISP site or a data-networking provider site. This means that for the duration of an Internet session between a user and an ISP, which is data-oriented and does not require circuit switching, two circuits on each involved switch are busy and tied exclusively to this call. In the second scheme, dial-up calls go through directly from an originating CO switch (ingress) to a terminating CO switch (egress) via interoffice trunk.

The telephony network was designed for voice calls with an average mean connect time of two or three minutes. Statistics show that the average call made by Internet users is between 20 and 40 minutes. As more people use their telephone lines for Internet access, telephone companies are forced to purchase more switch trunk ports and add more IMTs to avoid congestion in their networks. Failure to make these upgrades limits access not only for ISP calls but also for all of the voice calls originating and terminating on the involved switches. Based on one estimate, a large local-exchange carrier (LEC) spends about $30 million per year in additional costs just for Internet-related load balancing.