Modems and ISDN

Published on Dec 17, 2015


The modern telephone system is designed to carry the sound of a human voice talking normally. The engineers save money by cutting out the high a low pitch sounds that occur only in music, or sounds that last for a very short period of time.

Modems convert data into sound so that it can be sent through the phone system. Just as the telegraph sent data by alternating dots and dashes, the earliest modems sent data by alternating the sound on a phone between two tones. However, modern modems move much larger amounts of data by sending combinations of tones at different frequencies. If you have access to all possible frequencies, there is an enormous amount of data you can carry.

Each TV channel is about 3 Megabits of data per second, and a cable system carries nearly 100 channels. However, the telephone system is designed to only handle the frequencies of the human voice, and the maximum amount of data it can theoretically carry is 4000 bytes per second per phone line. Trying to squeeze as much data as possible into the available frequencies makes the signal vulnerable to "noise" that is introduced from a bad connection, "crosstalk" from other phone conversations in the same bundle of wires, and external electrical sources.

Computer chips made it possible to build a "smart" modem that transmits a block of data, waits for an acknowledgment, and retransmits the block if it something went wrong the first time. Then data compression was added. In current use, the information actually exchanged between the two modems may be much more complicated and sophisticated than the data that either computer sees.


Digital phone equipment is used everywhere except at your home or office. Since the limit on data transmission is caused by the conversion of sound to a digital signal, the obvious direct solution to the problem is to extend digital signaling all the way to the home. A T1 line would provide this type of connection. It carries 24 phone circuits each with 8000 bytes per second, or a single circuit of around 1.5 Million bits per second. Prices vary depending on location. In Connecticut, they start at around $400 per month. An international standard called ISDN has been established for an less expensive personal digital service. An ISDN line carries two digital phone circuits on a single pair of wires, and in Connecticut the starting price is $50 per month.

This is called a Basic Rate Interface or BRI circuit. Because it is designed for use in a home or small office, ISDN allows the lines to be shared by voice telephones, fax machines, and computers. If an incoming phone call originates from another ISDN device, the digital "ring" signal identifies the type of device that places the call. Only the fax machine would answer an incoming fax. An incoming voice call would ring the phone. However, only in Europe do they generally use native ISDN devices. The BRI has two logical phone lines. They may have two phone numbers, but today phone companies are running out of numbers.

A better practice is for them to share one number. If one line is busy and a second call comes in, it rings on the second line. A conventional telephone line runs from the pole to a distribution point in the building. It may then be wired to many different jacks. More than one phone can be connected to the line. However, an ISDN line has to provide high speed and low noise, so it can only be wired to one jack and, initially, connect to one device.