CertiGuide to A+ (A+ 4 Real)  9  Chapter 13: Basic Networking Terminology

Circuit Switching 1 ISDN

Modem (modulator-demodulator) - is a device that interprets digital and analog signals by modulating and demodulating the signal, enabling data transmission over voice-grade telephone lines. At the source, digital signals are converted to a form suitable for transmission over analog communication facilities. At the destination, these analog signals return to their digital form.

The official name of such networks is PSTN (Public Switched Telephone Network); they sometimes in addition named POTS (Plain Old Telephone Service). However, these networks are rarely used for construction modern WAN because of their low speeds of access. Such kind of WAN is applied for not strict users, entering in a network from their houses or for those network accesses who are taking place on business trip.

The speed of connection is limited to 33,6kbps, if digital connection is not applied. In the latter case, speed runs up to 56Kbps. However, in USA and Canada the Federal Communications Commission (U.S. Government agency that supervises, licenses, and controls electronic and electromagnetic transmission standards) has entered restriction until 53Kbps on speed of transfer of telephone lines. The quality of communication through the analog modem depends on the involved lines and used equipment.

Circuit Switching 1 ISDN