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Fundamentals of Telecommunications Tutorial

4. Transmission Media

There are four types of media that can be used in transmitting information in the telecommunications world:

  • copper wire
  • coaxial cable (actually an adaptation of copper wire)
  • fiber
  • wireless

In days of old, copper wire was the only means of transmitting information. Technically known as unshielded twisted pair (UTP), this consisted of a large number of pairs of copper wire of varying size in a cable. The cable did not have a shield and therefore the signal—primarily the high-frequency part of the signal—was able to leak out. Also, the twisting on the copper pair was very casual, designed as much to identify which wires belonged to a pair as to handle transmission problems. However, this is the way it was done, and for voice communications it was quite satisfactory. Consequently, there are millions of miles of copper in the PSTN—miles that must be used.

Not only did the copper cable itself have limitations, but things were done to this cable to make it even more unsuitable for high-speed data transmission. These actions primarily took two forms:

  • loading—Load coils were frequently added to loops longer than 18,000 feet. These load coils were essentially low-pass filters. That is, they passed without attenuation all voice frequencies but effectively blocked frequencies above the voiceband. This is disastrous for data communications, which depend on high frequencies to achieve the desired speed of transmission.
  • bridge taps—A bridge tap is any unterminated portion of a loop not in the direct talking path. A bridge tap may be a used cable pair connected at an intermediate point or an extension beyond the customer. For example, a drop wire that provided a second line to a home is left in place even after the second set of customer premises equipment (CPE) is removed. Records of this were not always kept and assigning a particular copper pair to a high-speed data circuit is far from a sure thing. Bridge taps do nasty things to data transmission.

Coaxial cable consists of a single strand of copper running down the axis of the cable. This strand is separated from the outer shielding by an insulator made of foam or other dielectrics. Covering the cable is a conductive shield. Usually an outer insulating cover is applied to the overall cable—this has nothing to do with the carrying capacity of the cable. Because of the construction of the cable, obviously coaxial in nature, very high frequencies can be carried without leaking out. In fact, dozens of TV channels, each 6 MHz wide, can be carried on a single cable.

The fact that a coaxial cable—or coax—can support a tremendous bandwidth has not been lost on the CATV folk. A leader of the CATV industry said, some years ago, "We have more bandwidth by accident than the telephone people have on purpose." Indeed, that is correct; piggybacking a telephone channel on a coax cable is no challenge at all.

Fiber is the third transmission media, and it is unquestionably the transmission medium of choice. Whereas transmission over copper utilizes frequencies in the megahertz range, transmission over fiber utilizes frequencies a million times higher. This is another way of saying that the predominant difference between electromagnetic waves and light waves is the frequency. This difference, in turn, permits transmission speeds of immense magnitudes. Transmission speeds of as high as 9.9 Gbps have become commonplace in the industry today. At this speed the entire fifteen-volume set of Encyclopedia Britannica can be transmitted in well under one second.

Laying fiber, on a per-mile basis, still costs somewhat more than laying copper. However, on a per-circuit basis there is no contest; fiber wins hands down. However, if a local loop is being laid to a residence, there is little justification to installing fiber—there will never be a need for more than one or two or three circuits. This realization has led to a transition in our thinking. Shortly after the commercialization of fiber, we talked about fiber-to-the-home (FTTH). It was then realized that there was little need to install fiber for a final several hundred yards, so the industry shied away from fiber-to-the-curb (FTTC). In such a system, fiber would carry a plurality of channels to the "curb," whereupon they would be broken down and applied to the copper drop leading to the home. In many cases even this was overkill, and fiber-to-the-neighborhood (FTTN) is now being used. The message is clear: apply fiber when it is economical to do so, and otherwise rely on copper.

One final approach is being used in many areas, and it often proves workable. This is a combination of fiber and coax or, as it is known, hybrid fiber/coax (HFC). As we have seen, coax has a greater bandwidth than copper but a smaller one than fiber. Also, in some 60 percent of the homes in the United States, coax in the form of CATV goes to the home; tying fiber to coax for the final several hundred yards makes technological sense.

Fiber comes in several forms; the two predominant ones are multimode and single-mode (see Figure 4). As can be seen, the total strand diameter for both is about 125 microns (a micron is a millionth of a meter). However, the ultrapure glass that forms the core transmission medium is between 50 and 62.5 microns for the multimode fiber and about 8 to 10 microns for the singlemode fiber. One would think that the multimode fiber would have a greater carrying capacity; however, just the opposite is true. With single-mode fiber, only one ray or mode can travel down the strand, and this makes for a simpler job in regenerating the signal at points along the span. In fact, single-mode fiber makes up the majority of today's long-distance network.

Figure 4. Optical Fiber Sizes

Figure 4

The tremendous capacity of fiber certainly makes for more efficient communications; however, placing so much traffic on a single strand makes for greater vulnerability. Most of the disruptions in the long-distance network are a result of physical interruption of a fiber run. It is called backhoe fade.

Wireless communications is the final option as a transmission medium. This can take several forms: microwave, synchronous satellites, low-earth-orbit satellites, cellular, personal communications service (PCS), etc. Some of these will be described in more detail later. In every case, however, a wireless system obviates the need for a complex wired infrastructure. In the case of synchronous satellites, transmission can take place across oceans or deserts. With microwave there is no need to plant cable, and in mountainous territories this is a significant advantage. Cellular and PCS afford mobility. There are advantages and disadvantages to each.

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Table of Contents

Definition and Overview

1. History and Regulation of the Telephone Industry

2. Network

3. Switching Technology

4. Transmission Media

5. Transmission Technology

6. Broadband Access and Service

7. Wireless

Self-Test

Correct Answers

Acronym Guide

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