Page 44 - DCAP406_DCAP_207_Computer Networks

Unit 3: Physical Layer

7. The twisting decreases the electrical noise immunity, and reduces the error rate of the data Notes
transmission.
8. A UTP cable contains from 2 to 4200 twisted pairs.

9. Coaxial cable is inherently an insecure transmission medium.
10. Local Area Networks can operate over coaxial cable to the 10BASE5, 10BASE2 and 10BASET
specifications

3.2.3 Optical Fiber

We have seen in the previous section that the geometry of coaxial cable significantly reduces the
various limiting effects, the maximum signal frequency, and hence the information rate that can
be transmitted using a solid conductor, although very high, is limited. This is also the case for
twisted lines. Optical fiber differs from both these transmission media in that it carries the
transmitted information in the form of a fluctuating beam of light in a glass fiber rather than as
an electrical signal on a wire. This type of transmission has become strong support for digital
network owing to its high capacity and other factors favorable for digital communication.

Figure 3.5: Fiber Optic Cable – General View

Fiber optic transmission systems are opto-electric in nature. In other words, a combination of
optical and electrical electromagnetic energy is involved. The signal originates as an electrical
signal, which is translated into an optical signal, which subsequently is reconverted into an
electrical signal at the receiving end. Thin glass fiber as shown in Figure 3.5 is very clear and
designed to reflect light internally for efficient transmission carries light with encoded data.
Plastic jacket allows fiber to bend (some!) without breaking. Light emitting diode (LED) or laser
injects light into fiber for transmission. Light sensitive receiver at other end translates light back
into data.
The optical fiber consists of a number of substructures as shown in Figure 3.6. In this case, a
cladding made of glass with lower refractive index surrounds a core made of glass, which carries
most of the light. This bends the light and confines it to the core. The core is surrounded by a
substrate layer (in some fibers) of glass, which does not carry light, but adds to the diameter and
strength of the fiber. A primary buffer coating and a secondary buffer coating to provide
mechanical protection cover all these.

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