The Public Switched Telephone Network

Public Switched Telephone Network: When two computers in a company or organization next to each other need to exchange data, it is often easier to connect one cable to another. Local networks work this way. However, when distances are large or many computers or cables have to cross a public road or another public right of way, the cost of laying private cables is often prohibitive.

These devices, including the public, switched telephone network (PSTN), were generally designed many years ago for a different purpose: to transmit the human voice in a more or less recognizable way. Their ability to be used in computer communications is often negligible. To see the scale of the problem, consider that inexpensive and inexpensive cable linking two computers can transfer data at speeds of 1 Gbps or more. On the other hand, the typical ADSL, an incredibly fast alternative to the telephone modem, runs at about 1 Mbps. The difference between them lies in the difference between flying in the plane and traveling.

However, since the telephone system is closely linked to computer networks (vast territory), it is worthwhile to devote time to its detailed study. It turns out that the limiting factor for the network is the “last mile” to which customers are connected, not the connection lines and switches of the telephone network. This situation is changing with the gradual deployment of fiber optics and digital technology at the edge of the network, but it will take time and money. During this long wait time, computer system developers, who are used to working with systems that offer at least three orders of magnitude of performance improvement, have spent a great deal of time and effort searching for effective use of the system telephone network.

1. Structure of the Telephone System
2. The Politics of Telephones
3. The Local Loop: Modems, ADSL, and Fiber
4. Trunks and Multiplexing
5. Switching

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