If ISDN can squeeze 144K bits per second (Kbps) into your telephone line, why can't a modem do the same thing? The reason lies in the evolution of the telephone network.
A given pair of wires connecting two parties for communication can carry electrical signals in one of two forms: analog or digital. An analog signal changes gradually through an infinite number of values. A digital signal changes instantly (in theory) between just two values. The human voice and musical instruments are examples of analog signals--both produce complex variations in frequency and amplitude. A light switch typifies a digital signal--it can be either on or off. Figure A shows the analog signal is converted to digital via sampling.
An analog signal's infinite number of variations makes it impossible to reproduce exactly. It will go only so far in copper wire: To go farther, the signal must be regenerated electronically with a device called a repeater. A repeater converts a weak input signal to a stronger output signal, unavoidably distorting it in the process. Each regeneration lessens the signal in the same way that photocopies of photocopies get worse with each iteration. In a large telephone network, the "copy of a copy" problem becomes very expensive to solve, requiring sophisticated equipment and costly cabling.
Digital signals, on the other hand, are easy to regenerate precisely. Because there are only two possible states for the signal, even a heavily eroded signal can be regenerated into an exact copy of the original. What's more, the cost (and complexity) of the equipment required to regenerate digital signals is trivial compared with that necessary to reproduce analog signals. Not surprisingly, telephone companies recognized this cost advantage long ago and have since converted all long-distance and some local transmission to digital signaling.
When a subscriber makes a long-distance call, the central office converts the analog signal to digital through a technique called sampling in which the state of the analog signal is captured about 8000 times per second. Each state is converted to an 8-bit binary number, and the resulting string of binary numbers becomes a digital data stream at 64Kbps. This digital stream is routed along the long-distance network and is regenerated as needed. Each regeneration produces an exact copy of the original digital data stream. When the destination central office receives the digital data stream, it reverses the sampling process and transmits the resulting analog signal to the receiving subscriber.
A modem can't get 64Kbps out of an analog line because the central office's signal sampling, at 8000 times per second, limits the bandwidth of the analog signal to about 3 KHz, which in turn invokes Shannon's Law (see the sidebar "The Truth About High-Speed Modems" on page 40), setting the practical speed limit for such a channel to about 30Kbps. Obtaining higher analog speeds requires higher fidelity in the audio signal, which would dictate faster sampling in the central office's analog-to-digital conversion and result in a digital data stream faster than 64Kbps. Telephone companies aren't about to replace all their voice digitizers with faster versions or upgrade all their digital circuits to carry channels faster than 64Kbps, so the probability of the phone system ever supporting faster analog signaling rates is nearly nonexistent.
This is where ISDN steps in. ISDN eliminates the need for voice digitizers in the central office. It carries the 64Kbps digital signal from the central office directly to the subscriber, and the subscriber can use it for voice or data. Advances in electronics make voice digitizing in the subscriber's phone practical, and direct digital attachment of computers eliminates the need for modems.