Previous Table of Contents Next


Chapter 3
Fundamentals of Transmission Systems: Technologies and Applications

It was early declared by Professor Morse, and by other distinguished investigators of the nature and powers of the electric current, that neither the ocean itself, nor the distance to be traversed, presented any insuperable obstacle to the laying of submerged oceanic lines from continent to continent, and the confident prophecy that such lines would eventually be undertaken was freely uttered and discussed in learned circles.

Our First Century or the One Hundred Great and Memorable Events in the History of Our Country during the One Hundred Years of its Existence, 1876

Information is of considerably increased value if it can be conveyed clearly to others. This is a basic principle that is well understood in this information age. The conveyance, or transmission, of information across a distance necessarily involves some form of transmission medium. In this chapter, the terms transmission medium, transmission system, and transmission facility will be used interchangeably, as indeed they are in the real world. The selection of physical transmission media which serve to transport that information is critical to its successful conveyance. In interactive communication, the medium can be critical to the message.

This section addresses all transmission media commonly used in traditional voice, data, video, and image networks, whether analog or digital in nature. Those media can be grouped into two distinctive categories, the first of which includes all wired media, also referred to as conducted, guided, or bounded media. The second category includes all traditional wireless media, also referred to as radiated, unguided, or unbounded.

Wired transmission systems employ physical media which are tangible. In other words, they can be seen, felt and perhaps even smelled and tasted (the last two options are not recommended, as the results might well be unpleasant). Also known as conducted systems, wired media generally employ a metallic or glass conductor which serves to conduct, or carry on, some form of electromagnetic energy. For example, twisted pair and coaxial cable systems conduct electrical energy, employing a copper medium; fiber optic systems conduct light, or optical, energy, generally using a glass conductor. The term guided media refers to the fact that the signal is contained within an enclosed physical path. Finally, bounded media refers to the fact that some form of shield, cladding, and/or insulation is employed to bind the signal within the core medium, thereby improving signal strength over a distance and enhancing the performance of the transmission system in the process. Twisted pair (both unshielded and shielded), coaxial and fiber optic cable systems fall into this category.

Wireless transmission systems do not make use of a physical conductor, or guide, to bind the signal. Therefore, they are also known as unguided or unbounded systems. Rather than relying on electrical energy, such systems generally make use of radio waves; hence the term radiated often is applied to wireless transmission. Finally, such systems employ electromagnetic energy in the form of radio or light waves that are transmitted and received across space, and are referred to as airwave systems.

Each specific transmission system is distinguished by unique properties, limitations (or lack thereof), and appropriate applications. The application to be supported, clearly, must be of primary consideration in designing a network and in selecting the transmission medium, assuming one has the freedom of choice. As the application must support users effectively, it places certain demands on the network and on the transmission medium.

Traditional transmission systems addressed in this chapter include twisted copper wire, coaxial cable, microwave, satellite, infrared and fiber optics. The order of discussion is roughly chronological. Cellular radio, packet wireless and other wireless networks and specific applications are discussed in Chapter 12.

Frequency Spectrum

While human voice frequencies mostly are in the range of 100 Hz to 8,000 Hz; the energy in the speech spectrum peaks at approximately 500 Hz, with most articulation being at higher frequencies. The human ear can distinguish signals as low as 20 Hz and as high as 20 kHz, and is most sensitive in the range of 1,000 Hz to 3,000 Hz. Public switched telephone networks, as we have discussed previously, provide reliable, raw, voice-grade bandwidth of 4 kHz; with 3,300 Hz (200 Hz to 3,500 Hz) usable for signal transmission. This range of frequencies provides a band of intelligibility which is considered to be good, although not complete. In an electrical cable system, the range of carrier frequencies depends on the nature of the medium and the requirements of the applications supported. For instance, twisted pair can support bandwidths of 10 Hz to 105 Hz, and coaxial cable of 106 Hz to 108 Hz. The actual range of frequencies supporting a given communication are known as a passband, which is accomplished through the use of band-limiting filters [3-1], [3-2], and [3-3].


Previous Table of Contents Next