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Posts : 638
Points : 2155
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Join date : 2010-05-15
Age : 30
Location : islamabad

PostSubject: CS601 3RD ASSIGNMENT SOLUTION   Wed Jun 30, 2010 6:39 am

CS 601

a. Is the transmission medium a part of the physical layer? Why or why

The transmission medium is the physical path between transmiter and receiver in
a data transmision 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?

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.

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
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?
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
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.
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