Telecommunications means communication that takes place over some distance (from the Greek word Tele, which means far away). The distances involved may be small, as is the case with communications that take place between people working in the same office building, or they may be vast, as is the case with the communications that occur between a deep space probe and its mission controllers on Earth. Communicating over long distances has been a challenge throughout history. In ancient times, runners were used to carry messages between distant locations. Other methods used have included drums (used for thousands of years to send messages, and for ceremonial and religious purposes), smoke signals and signal beacons (visible for many miles if visibility is good), the heliograph (used to send signals by reflecting the light of the sun), and semaphore (a method of signaling using two flags held in various positions by the signaler). Modern telecommunications can probably be considered to have started with the invention of the telegraph in 1832, which exploited the properties of electricity and electromagnetism discovered in the 19th century. The telegraph operated over long distances using a simple electrical circuit. An operator at one end of the connection repeatedly makes and breaks an electrical contact using a telegraph key, and the resulting intermittent bursts of current are used to produce a series of audible signals at the other end which are interpeted and transcibed by a second operator.
A telegraph key and sounder
In the 1870s, Alexander Graham Bell was credited with the invention of the telephone, a device that could transmit speech along a wire by varying the voltage in an electrical circuit using sound. The invention was a result of Bell's attempts to improve the performance of the telegraph. Sound is the result of differences in pressure in the air around us caused by vibrations. A microphone uses these small differences in pressure to vary the resistance of an electrical circuit, constantly changing the amount of current flowing through it. The current flowing through the circuit thus becomes an analogue of the sound waves picked up by the microphone. The public switched telephone system (PSTN) that subsequently evolved was originally intended only for voice transmission, but as the end of the twentieth century approached, the installation of fibre optic trunk lines and fully automated digital excghanges have enabled the PSTN to carry vast amounts of digital data.
In the latter half of the nineteenth century, British physicist James Clerk Maxwell predicted that moving electrons will create electromagnetic waves that can propagate through free space, a theory that was later proved by German physicist Heinrich Hertz. By attaching an antenna to an electrical circuit, electromagnetic waves can be broadcast and received by a receiver some distance away. In 1901, Marconi successfully broadcast a radio message from Cornwall in the UK to Canada, a distance of over three thousand kilometres. The behaviour of electromagnetic waves varies with frequency. Today, much of the electromagnetic spectrum, including radio, microwave, infra-red, and visible light, are used for both short-range and long range wireless communications.
The telecommunications industry continues to develop new technologies and to deliver new services, but many of the principles that underpinned the early development of telephony and radio communications are just as relevant today as they have ever been. These pages examine some of the fundamental characteristics of transmission lines, the application of analogue and digital signalling techniques. They will also examine communication system architectures, explain the importance of communication protocols, and provide an in-depth look at concepts such as modulation and multiplexing.