Traditional Transmission Media

Transmission media are the physical pathways that connect computers, other devices, and people on a networkthe highways and byways that comprise the information superhighway. Each transmission medium requires specialized network hardware that has to be compatible with that medium. You have probably heard terms such as Layer 1, Layer 2, and so on. These refer to the OSI reference model, which defines network hardware and services in terms of the functions they perform. (The OSI reference model is discussed in detail in Chapter 5, "Data Communications Basics.") Transmission media operate at Layer 1 of the OSI model: They encompass the physical entity and describe the types of highways on which voice and data can travel.

It would be convenient to construct a network of only one medium. But that is impractical for anything but an extremely small network. In general, networks use combinations of media types. There are three main categories of media types:

• Copper cable Types of cable include unshielded twisted-pair (UTP), shielded twisted-pair (STP), and coaxial cable. Copper-based cables are inexpensive and easy to work with compared to fiber-optic cables, but as you'll learn when we get into the specifics, a major disadvantage of cable is that it offers a rather limited spectrum that cannot handle the advanced applications of the future, such as teleimmersion and virtual reality.
• Wireless Wireless media include radio frequencies, microwave, satellite, and infrared. Deployment of wireless media is faster and less costly than deployment of cable, particularly where there is little or no existing infrastructure (e.g., Africa, Asia-Pacific, Latin America, eastern and central Europe). Wireless is also useful where environmental circumstances make it impossible or cost-prohibitive to use cable (e.g., in the Amazon, in the Empty Quarter in Saudi Arabia, on oil rigs).
• There are a few disadvantages associated with wireless, however. Historically, wireless solutions support much lower data rates than do wired solutions, although with new developments in wireless broadband, that is becoming less of an issue (see Part IV, "Wireless Communications"). Wireless is also greatly affected by external impairments, such as the impact of adverse weather, so reliability can be difficult to guarantee. However, new developments in laser-based communicationssuch as virtual fibercan improve this situation. (Virtual fiber is discussed in Chapter 15, "WMANs, WLANs, and WPANs.") Of course, one of the biggest concerns with wireless is security: Data must be secured in order to ensure privacy.
• Fiber optics Fiber offers enormous bandwidth, immunity to many types of interference and noise, and improved security. Therefore, fiber provides very clear communications and a relatively noise-free environment. The downside of fiber is that it is costly to purchase and deploy because it requires specialized equipment and techniques.

This chapter focuses on the five traditional transmission media formats: twisted-pair copper used for analog voice telephony, coaxial cable, microwave and satellite in the context of traditional carrier and enterprise applications, and fiber optics. (Contemporary transmission solutions are discussed in subsequent chapters, including Chapter 11, "Optical Networking," and Chapter 16, "Emerging Wireless Applications.") Table 2.1 provides a quick comparison of some of the important characteristics of these five media types. Note that recent developments in broadband alternatives, including twisted-pair options such as DSL and wireless broadband, constitute a new categorization of media.

The frequency spectrum in which a medium operates directly relates to the bit rate that can be obtained with that medium. You can see in Table 2.1 that traditional twisted-pair affords the lowest bandwidth (i.e., the difference between the highest and lowest frequencies supported), a maximum of 1MHz, whereas fiber optics affords the greatest bandwidth, some 75THz.

Another important characteristic is a medium's susceptibility to noise and the subsequent error rate. Again, twisted-pair suffers from many impairments. Coax and fiber have fewer impairments than twisted-pair because of how the cable is constructed, and fiber suffers the least because it is not affected by electrical interference. The error rate of wireless depends on the prevailing conditions, especially weather and the presence of obstacles, such as foliage and buildings.

Yet another characteristic you need to evaluate is the distance required between repeaters. This is a major cost issue for those constructing and operating networks. In the case of twisted-pair deployed as an analog telephone channel, the distance between amplifiers is roughly 1.1 miles (1.8 km). When twisted-pair is used in digital mode, the repeater spacing drops to about 1,800 feet (550 m). With twisted-pair, a great many network elements must be installed and subsequently maintained over their lifetime, and they can be potential sources of trouble in the network. Coax offers about a 25% increase in the distance between repeaters over twisted-pair. With microwave and satellite, the distance between repeaters depends on the frequency bands in which you're operating and the orbits in which the satellites travel. In the area of fiber, new innovations appear every three to four months, and, as discussed later in this chapter, some new developments promise distances as great as 4,000 miles (6,400 km) between repeaters or amplifiers in the network.

Security is another important characteristic. There is no such thing as complete security, and no transmission medium in and of itself can provide security. But using encryption and authentication helps ensure security. (Chapter 9, "IP Services," discusses security in more detail.) Also, different media types have different characteristics that enable rapid intrusion as well as characteristics that enable better detection of intrusion. For example, with fiber, an optical time domain reflectometer (OTDR) can be used to detect the position of splices that could be the result of unwanted intrusion. (Some techniques allow you to tap into a fiber cable without splices, but they are extremely costly and largely available only to government security agencies.)

Finally, you need to consider three types of costs associated with the media types: acquisition cost (e.g., the costs of the cable per foot [meter], of the transceiver and laser diode, and of the microwave tower), installation and maintenance costs (e.g., the costs of parts as a result of wear and tear and environmental conditions), and internal premises costs for enterprises (e.g., the costs of moves, adds, and changes, and of relocating workers as they change office spaces).

The following sections examine these five media typestwisted-pair, coaxial cable, microwave, satellite, and fiber opticsin detail.