CS601 Assignment 3 Solution, June 2010

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a. Is the transmission medium a part of the physical layer? Why or why not?

Solution:

The transmission medium is the physical path between transmitter and receiver in a data transmission system.
The term transmission medium can also refer to the technical device which employs the material substance to transmit or guide the waves. Thus an optical fiber or a copper cable can be referred to as a transmission medium.
Coaxial Cable, one example of a transmission medium
Electromagnetic radiation can be transmitted through an optical media, such as optical fiber, or through twisted pair wires, coaxial cable, or dielectric-slab waveguides. It may also pass through any physical material which is transparent to the specific wavelength, such as water, air, glass, or concrete. Sound is, by definition, the vibration of matter, so it requires a physical medium for transmission, as does other kinds of mechanical waves and heat energy. Historically, various aether theories were used in science and thought to be necessary to explain the transmission medium. However, it is now known that electromagnetic waves do not require a physical transmission medium, and so can travel through the "vacuum" of free space. Regions of the insulative vacuum can become conductive for electrical conduction through the presence of free electrons, holes, or ions.
b. What is the significance of the twisting in twisted-pair cable?

Solution:

They are twisted together for the purposes of canceling out electromagnetic interference (EMI) from external sources; for The significance of twisting in twisted pair cable is as follows:
Instance, electromagnetic radiation from unshielded twisted pair (UTP) cables, and crosstalk between neighboring pairs.
 It reduces the cross-sectional are exposed to magnetic fields, so reduces noise pickup on the line. The twisting also evens out the neg/pos exposure, minimizing common-mode noise.
 If the two wires are twisted around each other at regular intervals (b/w 2& 12 twists per foot), each wire is closer to the noise source for half the time and is away for the other half
 Twisting does not always eliminate the impact of noise but it does significantly reduce it
 With twisting, therefore the cumulative effect of the interference is equal on both wires
 Each section of wire has a Load of 4 when it is on the top of the twist and 3 when it is on the bottom
 The total effect of the noise at the receiver is therefore 0 (14-14)

c. What is the purpose of cladding in an optical fiber? Discuss its density relative to the core.

Solution:

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Optical fibers are used to transmit light over distances of up to several hundred kilometers. A typical optical fiber consists of a cylindrical glass core, just a few micrometers in diameter, surrounded by a layer of a slightly different type of glass known as the cladding. Light in the core travels slightly slower than light in the cladding and this property tends to keep any light sent into the core from one end of the fiber from leaking out, until it reaches the far end. The cladding is a glass sheath that surrounds the core. The cladding acts like a mirror, reflecting light back into the core. The cladding itself is covered with a plastic coating and strength material when appropriate.

It gives strength to the fiber. In practical fibers, the cladding is usually coated with a tough resin buffer layer, which may be further surrounded by a jacket layer, usually plastic. These layers add strength to the fiber but do not contribute to its optical wave guide properties. Rigid fiber assemblies sometimes put light-absorbing ("dark") glass between the fibers, to prevent light that leaks out of one fiber from entering another. This reduces cross-talk between them.
d. How does sky propagation differ from line-of-sight propagation?

Solution:

Youre probably talking about bouncing AM radio waves, and lower frequencies off of the ionosphere, vs. higher frequencies radio waves, like microwaves, requiring line of sight from a transmitter to the receiver. The ionosphere, because its excited plasma, has a unique permittivity that reflects EM waves that are below a certain frequency. Waves that have a low frequency will be reflected back to the earth and can propagate greater distances around the earth when compared to the limits of line of site propagation. Of course now we have satellites.
There are various layers on the atmosphere. D - E - F1 and F2 layers. The F1 layer is opaque to low frequency radio waves (HF).
The medium frequency wave will propagate to the F2 layer.
E layer propagation works much the same way but reflects VHF and UHF (90 -800 MHz). VHF is really 30 MHz up to 300Mhz, but above 90 MHz the E layer reflects the signals better. Sporadic E is a densely ionized layer of Nitrogen atoms that are ionize\zed by high energy particles from the solar wind. Excited Oxygen Helium and Hydrogen atoms also plasmate too. These pasmated atoms (highly excited atoms) propagate UHF and microwaves, too.

Modulation types have no bearing on propagation, though Single Sideband (SSB) suppressed carrier with a bandwidth of 3khz and CW (Continuous Wave - 200 - 800Hz) and a narrow band with a narrow IF pass-band will be detected by the envelope modulation detector before any other modulation types at further distances.

The D layer will break down when the sun falls below the horizon, and medium wave signals will bounce of the F1 and F2 layers and travel further.