 Computer Networks: The Physical Layer2.2 Transmission Media
Highlights
2.2.1 Magnetic MediaDisks and tapes are one of the most popular methods of transmitting data from one computer to another fast. The problem is that the source and destination have to be very close together for this to be reasonable. If you need to transmit your data to another town in less time than it would take to drive there, then another method of transfer will be needed.2.2.2 Twisted PairTwisted Pair wiring (Cat 3 and Cat 5) are popular methods of transferring data. They are especially prevalent in the LAN environment. The twists allow the signal to travel further than it could on a regular copper wire. The more twists per centimeter, the further the signal can travel. This is why Cat 5 wire (with more twists) is preferred over Cat 3 wire. Twisted pair wires consist of two strands of copper twisted together; the wires are unshielded, which is why Twisted Pair wire is also called Unshielded Twisted Pair (UTP).2.2.3 Baseband Coaxial Cable
Reading through this material is relatively easy. It describes the
technical details of coaxial cable, which is an insulated copper wire
covered with a mesh conductor, with a coating of plastic on top of that.
The reason it is made like that is to provide a combination of high
bandwidth with low noise. Although it has been replaced by fiber
optics in the long-distance telephone industry, the cable industry still uses
coaxial cable.
This diagram of a coaxial cable comes from Novell's Network Primer. 2.2.4 Broadband Coaxial CableIn networking, the term "broadband" refers to any cable that uses analog transmission. The reason broadband cables are used is to get longer distances, but amplifiers are needed. These amplifiers transform the cable into a unidirectional cable. If two-way communication is needed, two cables will be needed (one going one way, the other goes the other way).2.2.5 Fiber Optics
Although computing technology is rapidly advancing, it is not gaining
ground nearly as fast as communication technology is. Fiber optics is
one of the advances that has propelled communication technology into the
future at high speeds. Communication over fiber optics requires a source
(of light), a line (transmission medium = fiber), and a destination (to detect
the light). The light stays within the fiber line because of the angle at
which the light hits the surface of the fiber line. Instead of passing through
the fiber's surface (like a window), the light bounces off of it (like a mirror).
The light propagates down the fiber line because it continually reflects off
the surface from the inside; the light never escapes the fiber line until the
receiver detects it.
Like copper, fiber optics suffers problems when transmitting over a distance. Attenuation (a weakening of the power of a signal) occurs, as well as dispersion (the spreading out of light waves over a distance). The discovery of solitons has helped wipe out the problem of dispersion, though. A fiber cable is heavily insulated like coax, but it has several differences. The core of the cable is a glass strand, which is surrounded by a thick glass covering, which is then covered by plastic. When compared to copper for its overall purposes, fiber wins because it is lighter, higher bandwidth, easier to install, harder to tap, and the signal stays stronger longer than in copper. The only drawback to fiber at this point in time is the lack of familiarity among the engineering community with the fiber technology compared to the copper. This diagram of a fiber optic cable comes from Novell's Network Primer.
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