Telecom was born out of the need that occurs in ordinary life to communicate messages over distances at a reasonable cost. This was an economic need in the most basic sense; survival depended upon the reliable transfer of the message. For example, in the case of a battlefield situation, the reliability of having messages delivered by runners crossing battle lines would be relatively low and the cost would be high; multiple runners would be required and many of them would lose their lives.
Once ways of encoding messages had been invented and a basic understanding of electrical circuits was developed, the way was paved for electrical communications. Electrical wires could be strung between two distant points. Through the process of opening and closing switches, or making and breaking contacts, hisses and pops could be sent and then interpreted as different signals. Using such a means, any desired message could be sent without actually having to carry the physical message. Furthermore, delivery could be fast and more or less private, and receipt could be verified.
To use this astounding technological advance in conveying messages, all one needed was a wire strung between the two distant points, a simple mechanism having the appropriate switches, and a trained person at each end. Samuel Morse invented such a mechanism in 1837. He later invented a scheme for encoding messages, and then, under contract with the government of the United States of America, built the first telegraph system in 1843.
The distances over which signals can be transmitted over electrical circuits without excess quality degradation are limited. More distant places could be reached by means of relaying. A mechanism for automatically relaying messages was introduced, also with Morse's involvement, soon after the introduction of the first telegraph system, thus allowing a single pair of operators to handle the entire message transfer. Acknowledgments could then be sent from the most distant receiving end to the sending end, thus verifying message delivery. Obviously, much could be gained from automating the process of repeating the messages at intermediate points.
This point-to-point system between the end points can be thought of as a basic communications channel. Among its many properties, a channel has a transmitter-receiver pair at each end and a capacity, which defines the volume of information that can be conveyed over the channel.
Once the system is in place to take care of necessary communications, any excess capacity could be used for messages having lower priority; that is, messages for which people would place a lower value of delivery. Spreading the cost among more messages would then lower the per-message cost, and this would increase the amount demanded. To further increase the amount demanded, the per-message cost would have to be lowered. The first step along the way was to raise the number of messages that could be sent simultaneously over the same wire from one to two. Next the capacity could be raised to four, and so forth. The practice of using the same transmission system for multiple simultaneous communications is called multiplexing, and the system that implements multiplexing is called a multiplexing system.
A multiplexing system supports a number of communication channels, each of which carries the equivalent of one conversation. A multiplexing system consists of two terminal equipments, one for each end of the connection, and an electrical transmission circuit that interconnects the terminal equipments. Each terminal equipment has a drop side, which faces individual channels, and a line side, which faces the transmission circuit between the two terminal equipments. In addition, each terminal equipment has a multiplexor, which combines signals of individual channels for transmission over the line side, and a demultiplexer, which separates the signal received over the line side for transmission over the individual channels of the drop side. The input (output) circuitry associated with each channel at each multiplexer is called an input (output) port.
The initial multiplexing systems were based on frequency division multiplexing. In a frequency division multiplexing system, there is a collection of single frequency signals called carriers. The signal for each call to be carried, for example, the signal from a telegraph transmitter, enters the multiplexor via one of the ports on the drop-side of the multiplexor. This signal, which is called the baseband signal, modulates one of the carriers. The multiplexor then combines the collection of modulated carriers into a single electrical signal that is sent over the transmission line connecting the multiplexing equipment. At the receiving end, the demultiplexer separates the combined line signal, through the use of demodulating and filtering circuitry, into the signals that belong to each of the individual channels and then routes the signal to the correct output port.
Because frequency division multiplexing systems are based on carrier signals, they are called carrier systems. A carrier system is analogous to the radio broadcast system, where each radio station uses a different carrier frequency and the listener tunes to the desired station. The electrical signal representing the voice messages and music broadcast over each radio channel is the baseband signal. And, when we tune into 870 on the radio dial, we are tuning to the station whose carrier frequency is 870 KHz. The airways provide the communication link between the radio stations and the radios.
Thus, a carrier system consists of terminating equipment that is used to combine and separate signals and an electrical circuit of some kind between its end points. Individual signals enter the multiplexing systems via input ports and exit from the opposite end via output ports. Multiple parallel communication channels can be achieved over the same electrical circuit through the use of carrier systems.
Generally speaking, if a carrier system is designed to interconnect switches over a great distance, then that carrier system is called a long-haul carrier system, otherwise, it is called a short-haul carrier system. Both short- and long-haul carrier systems can have a variety of capacities in terms of the number of calls that they can handle simultaneously. In fact, the capacities of the various carrier systems range from a few circuits to many thousands of circuits.
With increased multiplexing capability, a natural improvement would be in the area of message entry and delivery. Once a machine could fill the available channels, one would want to increase the number of channels by using better circuitry at the end points or better wires and so forth. This process could continue indefinitely with advances in wire manufacturing technology as well as transmitter and receiver circuitry, including the circuitry in repeaters at intermediate points. This explains, to a large extent, the tremendous investment in basic and applied research in so many different areas of technology by the telecommunications industry.
With increased message carrying capability, the per-message cost would drop and quantity demanded would increase. This, too, would go on in a continuous cycle. In addition, it is not hard to see that significant savings in cost might be achieved by replacing wires by radio transmission and receiving equipment. For example, to send signals across a large body of water, it may be less costly to use radios than to develop and deploy underwater cables. Guglielmo Marconi developed such a radio system in the mid-1890s.
Once a carrier system is in place, it is natural to separate the location of the end-user equipment from that of the carrier system's terminating equipment. The user's equipment is then connected to the carrier system through an access line, which is often referred to as a local loop.
So far, then, we have a point-to-point system for transmitting messages. This system consists of the end-user equipment, a local loop at each end, and a carrier system. The carrier system, in turn, consists of its terminating equipment, repeaters needed between the terminating equipment, and electrical circuits interconnecting the repeaters and the terminating equipment.
In the case of telegraphy, the objective of the system is to convey messages from some source to some destination, much as the objective of the postal service is to deliver letters. The primary difference is that the objective is accomplished electronically, and therefore, more quickly. The service model is that the system takes in messages at one end and delivers them to the other end; the service itself is message delivery.
