Chapter 3. Transmission Media: Characteristics and Applications

Chapter 3. Transmission Media: Characteristics and Applications

Transmission media are the physical pathways that connect computers, other devices, and people on a network the 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 terms are used to 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 6, "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 the voice and data can travel.

It would be convenient if we could construct a network of only one medium. But that's impractical for anything but an extremely small network. So, in general, we use combinations of media types. There are three main categories of media types:

         Cable Types of cable include unshielded twisted-pair (UTP), shielded twisted-pair (STP), and coaxial cable. Cable is inexpensive compared to the other media types, 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 will not be able to handle the truly advanced applications of the future.

         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 (for example, Asia Pacific, Latin America, eastern and central Europe). Wireless is also useful where environmental circumstances make it impossible or cost-prohibitive to use cable (for example, the Amazon, the Empty Quarter in Saudi Arabia, oil rigs). The disadvantage of wireless is that it provides slow data rates.

Prevailing Conditions and Network Diversity

No one of the three categories of media types can be considered best. Each is useful in different situations, and often you need to take advantage of a number of media types. When you are building a new network or upgrading an old one, your choice of media type should be based on the prevailing conditions.

The saying "don't put all your eggs in one basket" can be applied to networking. Having multiple pathways of fibers in and out of a building is not always enough. You need diversity terrestrial and nonterrestrial facilities combined because in disastrous events such as earthquakes, floods, and fires, if one alternative is completely disabled, you may be able to rely on another medium.

         Fiber optics Fiber offers enormous bandwidth and immunity to many types of interferences and noise. 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.

You can assess various parameters to determine which media type is most appropriate for a given application. This chapter focuses on the five most commonly used transmission media formats: twisted-copper pair, coaxial cable, microwave, satellite, and fiber optics. Table 3.1 provides a quick comparison of some of the important characteristics of these five media types.

The frequency spectrum in which the medium operates directly relates to the bit rate you can obtain with the medium. You can see in Table 3.1 that twisted-pair affords the lowest frequency spectrum, a maximum of 1MHz, whereas fiber optics affords 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 suffer 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.

Yet another characteristic that you need to evaluate is the distance required between the repeaters. This is a major cost issue for those constructing and operating the networks. In the case of twisted-pair deployed as an analog telephone channel, the distance between the amplifiers is roughly 1.1 miles (1.8 kilometers). When twisted-pair is used in digital mode, the repeater spacing drops to about 1,800 feet (550 meters). With twisted-pair, a great many network elements must be installed and subsequently maintained over their lifetime, and they can be a potential source 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, we see new innovations 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 kilometers) between the repeaters or amplifiers in the network.

Table 3.1. Transmission Media Characteristics
Media Type	Frequency Spectrum	Performance Error Rate	Distance Between Repeaters	Security	Cost
Twisted-pair	1MHz	Poor to fair (10 5)	Short (1.2 miles/2 km)	Poor	Low
Coaxial	1GHz	Good (10 7 to 10 9)	Short (1.5 miles/2.5 km)	Good	Moderate
Microwave	100GHz	Good (10 9)	Medium (up to 45 miles/72 km)	Poor	Moderate
Satellite	100GHz	Good (10 9)	Long (up to 22,3000 miles/36,000 km)	Poor	Moderate to high
Fiber	75THz	Great (10 11 to 10 13)	Long (up to 4,000 miles/6,400 km)	Good	Moderate to high

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. 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 potential leaks that could be the result of unwanted intrusion.

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

The following sections examine twisted-pair, coaxial cable, microwave, satellite, and fiber optics in detail.