Chapter 10 Transmitting Media

专业英语

Chapter 10 Transmitting Media

In the previous Chapter, we have an overview of some of the most important access network technologies in the Internet. As we described these technologies, we also indicate the physical transmitting media used. Understanding how these media function will help you sort out the various rates for them and determine which is the most appropriate in a given situation. In this subsection we provide a brief overview of these and other transmission media that are commonly employed in the Internet.

Transmitting media fall into two categories: guided media and unguided media. With guided media, the waves are guided along a solid medium, such as a fiber-optic cable, a twisted-pair copper wire, or a coaxial cable. With unguided media, the waves propagate in the atmosphere and in outer space, such as Satellite Radio Channels.

10.1 Some popular guided transmitting media

10.1.1.twisted pair

The least-expensive and most commonly used transmission medium is twisted-pair wire. For over one-hundred years it has been used by telephone networks. In fact, more than 99 percent of the wired connections from the telephone handset to the local telephone switch use twisted-pair wire. Most of us have seen twisted-pair in our homes and work environments. Twisted pair consists of two insulated copper wires, each about 1mm thick, arranged in a regular spiral pattern. The wires are twisted together to reduce the electrical interference from similar pairs close by.

10.1.2. coaxial cable

Coaxial cable consists of two copper conductors, but the two conductors are concentric rather than parallel. （see Fig.10.1）With this construction and a special insulation and shielding, coaxial cable can have higher bit rates than twisted pair. Coaxial cable comes in two varieties: baseband coaxial cable and broadband coaxial cable.

Fig.10.1 the Structure of coaxial cable

10.1.3.fiber optics

An optical fiber is a thin, flexible medium that conducts pulses of light, with each pulse representing a bit. A single optical fiber can support tremendous bit rates, up to tens or even hundreds of gigabits per second. They are immune to electromagnetic interference, have very low signal attenuation up to 100 kilometers. These characteristics have made fiber optics the preferred long-haul guided transmission media, particularly for overseas links. Many of the long-distance