Download guided media (twisted pair, coaxial cable, and optical fiber)

Question #2 (p.146-151): impairments w/ guided media (twisted pair, coaxial cable, and
optical fiber):
 Attenuation and attenuation distortion: a signal becomes weaker when
transmitted along any medium at a great distance.
 Delay distortion: occurs in guided media. It is caused by the fact that the velocity
of propagation of a signal through a cable is different for different frequencies.
For a signal with a given bandwidth, the velocity tends to be highest near the
center frequency of the signal and to fall off toward the two edges of the band.
Thus, various components of a signal will arrive at the receiver at different times.
 Noise: when information is transmitted in the form of an electromagnetic signal,
the received signal will consist of the transmitted signal, modified by attenuation
and the various distortions imposed by the transmission system, plus the addition
of unwanted electromagnetic energy that is inserted somewhere between
transmission and reception. The latter, undesired signals are referred to as noise.
Noise is the major limiting factor in communication system performance. There
are four kinds of noise: thermal noise, intermodulation noise, crosstalk, and
impulse noise.
 Thermal noise is due to thermal agitation of electrons in a conductor. It is
present in all electronic devices and transmission media and is a function of
temperature. Thermal noise is uniformly distributed across the frequency
spectrum and hence is often referred to as white noise. Thermal noise cannot
be eliminated.
 Intermodulation noise occurs when different frequencies share the same
transmission medium. Intermodulation reduces frequency at a level equal to
the sum or difference of the two original frequencies. For example,
intermodulation can produce 12,000 Hz frequency from the two separate
signals of 4000Hz and 8000 Hz.
 Crosstalk happens when you’re having a telephone conversation and you hear
another telephone conversation. It is the result of electrical coupling between
nearby cables or by the overlap of signals transmitted by antennas. Crosstalk
has the same or less magnitudes than thermal noise.
 Impulse noise: is not continuous, irregular pulse with high amplitude
generated by lightning, fault and flaws in the communication system. Impulse
noise is the primary source of error in digital communication.
Impairments w/ wireless transmission:
Free-space loss: data, when transmitted wirelessly, may be lost over a long distance
because signals disperse with distance.
Atmospheric absorption: Water vapor and oxygen contribute most to attenuation. At 22
GHz, water vapor causes greatest attenuation. At 15 GHz or less, the attenuation is less.
Multipath: Obstacles reflect signal creating multiple copies
Refraction: radio waves are bent when they go through the atmosphere.
Thermal noise: as described above
Question #3 explain the three types of media used for transmission of voice and data.
Give details on the nature of their use, suitability for an application, bandwidth, and
security. (p.158 –167).
Descriptions
Nature of use &
suitability
Bandwidth
Security
Price
Twisted Pair
Consists of two
insulated copper
wires arranged in a
regular spiral
pattern to minimize
the electromagnetic
interference
between adjacent
pairs
Most common for
both analog and
digital signals. Used
for telephones, local
area network for
personal computers
Digital link for ISN
Low frequency.
4 Mbps with long
distance
applications and 10
– 100 Mbps for
local area networks
Susceptible to
interference and
noise, easy to tap.
Least expensive and
readily available,
easy to work with,
and light weight
Coaxial Cable
A cable consisting
of one conductor
insulated and within
another conductor
of larger diameter
Use for cable TVs,
LAN, and telephone
Higher bandwidth:
400 – 600 MHz
Up to 10,800
conversations
Optical Fiber
Flexible medium
capable of
conducting an
optical ray. An
optical fiber has
three sections:
center, cladding,
and, and protective
jacket.
Used by telephone
companies for long
distance trunk lines
Used by private
companies for local
telecommunication
network
Very high
bandwidth: up to 2
Gbps
Can be tapped easily Immuned to
interference, very
hard to tap
Very expensive over Small size and light
long distance
weight, expensive,
and require high
skill installers
Question #4: Describe satellite communication. Give technical details. (p.168 – 172)
 Satellite communication consists of a satellite and a number of
transmitter/receivers, known as earth stations or ground stations. The satellite
receives a signal on one frequency (uplink), amplifies or repeats the signal, and
transmits it on another frequency (downlink). A single orbiting satellite will
operate on a number of frequency bands, called transponder channels or
transponders.
 There are two common configurations for satellite communication: point to point
between two ground-based antennas and between one-ground based transmitter
and a number of ground-based receivers.
 A satellite is required to be stationary with respect to the earth. It means that the
satellite rotation period must be equal to the earth rotation period.
 Applications of satellite communication include television, long distance
telephone, and private business networks.
 Television: a TV network program is transmitted to the satellite and then
broadcast down to a number of stations, which then distribute to the program to
individual viewers. The new satellite technology, direct broadcast satellite (DBS),
transmits satellite signals directly to home users.
Transmission Characteristics:
Optimum frequency range for satellite transmission is 1 to 10 GHz. Under 1 GHz, there
will be noise from natural sources like galactic, solar, and atmospheric noise, and humanmade devices. Above 10 GHz, the signals will be attenuated by atmospheric absorption
and precipitation.
There are three bands:
C band: C band has 4GHz downlink and 6 GHz uplink. C band is the first band to be
designated.
Ku band: has 12 GHz downlink and 14 GHz uplink. Rain interference is the major
problem with Ku band.
Ka band: has 19 GHz downlink and 29 GHz uplink. The problem with Ka band is that
equipment needed to use the band is still very expensive.
Question #5: Describe the three data encoding techniques, highling their practical
uses. (p 183 – 184 and p.188 – 191)
1. Amplitude Shift Keying (ASK)
 In radio transmission, known as amplitude modulation (AM)
 The amplitude (or height) of the sine wave varies to transmit the ones and zeros
 Major disadvantage is that telephone lines are very susceptible to variations in
transmission quality that can affect amplitude.
2. Frequency Shift Keying (FSK)
 In radio transmission, known as frequency modulation (FM)
 Frequency of the carrier wave varies in accordance with the signal to be sent
 Signal transmitted at constant amplitude
 More resistant to noise than ASK
 Less attractive because it requires more analog bandwidth than ASK
3. Phase Shift Keying (PSK)
 Also known as phase modulation (PM)
 Frequency and amplitude of the carrier signal are kept constant
 The carrier signal is shifted in phase according to the input data stream
 Each phase can have a constant value, or value can be based on whether or not
phase changes (differential keying)
Remember to draw the diagram on p.184. It is very very important