Physical Media

Physical Media

In the earlier subsection, we gave general idea of some of the most important network access technologies in the Internet. As we explained these technologies, we also indicated the physical media used. For instance, we said that HFC uses a combination of fiber cable and coaxial cable. We said that dial-up 56 kbps modems and DSL use twisted-pair copper wire. And we said that mobile access networks use the radio spectrum. In this section we provide a brief overview of these and other transmission media that are frequently used in the Internet.

In order to describe what is meant by a physical medium, let us consider the brief life of a bit. Consider a bit traveling from one end system, through a series of links and routers, to another end system. This poor bit gets kicked around and transmitted many, many times! The source end system first transmits the bit and soon thereafter the first router in the series receives the bit. The first router then transmits the bit, and soon thereafter the second router receives the bit; and so on. In this way, our bit, when traveling from source to destination, passes through a series of transmitter-receiver pairs. For each transmitter-receiver pair, the bit is sent by propagating electromagnetic waves or optical pulses across a physical medium. The physical medium can take many shapes and forms and does not have to be of the same type for each transmitter-receiver pair along the path. Examples of physical media contain twisted-pair copper wire, coaxial cable, multimode fiber-optic cable, terrestrial radio spectrum, and satellite radio spectrum. Physical 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 in a wireless LAN or a digital satellite channel.

But before we get into the features of the various media types, let us say a few words about their costs. The actual cost of the physical link (copper wire, Fiber-optic cable, and so on) is often quite minor compared with other networking costs. Particularly, the labor cost linked with the installation of the physical link can be orders of magnitude higher than the cost of the material. That's why, many builders install twisted pair, optical fiber and coaxial cable in every room in a building. Even if only one medium is primarily used, there is a good chance that another medium could be used in the near future, and so money is saved by not having to lay additional wires in the future.



Tags

end system, router, lan

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