Unlike computing, communication made tremendous advances before the advent of the integrated circuit. Communication engineers developed the analog telephone, which transmits the waveform of human voice, into a worldwide system displacing the telegraph. Humans desire their conversations over phones to be a close approximation of live conversations between two people. Consequently, phone conversations require a continuously open channel and people want their messages to be transmitted as quickly as they are through the air between two individuals. Also, most individuals use their phones at infrequent intervals. The phone system was designed to meet these requirements.
Why didn't the phone companies simply hardwire every phone to every other phone? Because if the phone companies had connected each phone to every other phone in the system the number of connections would have increased as the square of the number of phones. Just imagine the number of wires there would be into your home or apartment. Since most phones are only used a tiny fraction of the time and most people only want to be connected to a fraction of the other numbers, the phone companies chose to connect phone conservations through telephone exchanges (switches).
The system is designed with a tradeoff between the number of switches and the number of calls which can be processed at one time. For example, if you have 100 telephones and you hook each with every other phone, you need 10000 connections. If you create a switch with 10 input and 10 output lines, ten telephone conversations are possible at one time between any ten pairs of phones. If the switch has 20 input and output lines, twenty pairs of phones can be linked at any one time. To design systems which meet the estimated demand for phone conversations almost all the time, the Bell system developed a branch of operations research called queuing theory.
To initiate a phone conversation, the two parties must be connected in a phone channel through the switch, which remains continuously open until disconnected by one of the parties. At first phone calls were manually placed by human telephone operators manning the first type of telephone exchanges (switches). Later communication engineers developed automatic switchboards which could connect phones by dialing. The reason for developing switches is a simple matter of economics. Obviously, creating automatic switches to replace manual switches greatly reduces the need for labor to operate the phone system.
Currently, the telephone system is in the process of switching from analog to digital. The switch to digital communications is promoted by the advances in microelectronics. These advances in integrated circuit technology can be used to build ever cheaper and more powerful communication devices. For example, today telephone switchboards are simply special purpose computers with software to provide services such as call waiting. Also, with digital communications, errors in communication can be more easily controlled than with analog communication. The telephone system is currently a mix between analog and digital technologies. Currently, long distance is generally digital while local is analog.
Between switches either within a city or between cities, it is not efficient to string a separate phone line for each phone conversation. Instead, phone conversations are multiplexed, that is combined, so that many phone conversations can be carried in a single higher frequency of the electromagnetic spectrum. Over time, the demand for communication capacity has constantly increased in phone traffic, data communications, and now image communication such as television and teleconferencing. Consequently, communication engineers have moved to harness higher and higher frequencies in the electromagnetic spectrum.
The latest is laser light in optical fibers, which represents an 10^6 = 1,000,000 increase in capacity of over microwave communications. Because these optical fibers can fit in existing copper cable ducts and they have a much greater capacity than the copper cable which they replace, their use is expanding very rapidly. Through the utilization of railroad rightaways, several optical fiber networks have been constructed to connect the various major communication centers.
A laser light-optical fiber communication system consists of encoding and decoding devices, a transmitter, silica fibers, repeaters, and a receiver. Through applied research the capacity of such systems is increasing 10 fold every four years, and by 1992 it was possible to transmit 100,000 billion bits of information 10 kilometers per second. A basic problem in any communication system is attenuation, which is the disintegration of the signal as it travels down the communication channel. To restore the signal to its original strength amplifiers (repeaters) must be placed at regular intervals in the system. The increase in capacity has been obtained by (1) increasing the purity of the optical fiber, (2) developing optical repeaters, (3) developing better laser generators, and (4) experimenting with various pure and mixed frequencies of laser light. For example, an optical repeater is created by doping the optical fiber with the rare earth element erbium. This is a major advance over previous repeaters which necessitated converting the light beam from light to electricity, amplifying the electrical signal, and then converting the electrical signal back to light.
Finally, the phone systems have had to develop elaborate billing systems to charge individual customers for each long distance call.