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Transcript
CH. 4 Transmission Media
4.1. Guided Transmission Media
• Twisted Pair (Table 4.1, Figure 4.2)
– Two insulated copper wires arranged in a
regular spiral pattern.
– Many twisted pair can be bundled together in a
protective sheath.
– The most common medium for analog and
digital data.
– Used for telephone wiring.
– Tends to be limited in distance, bandwidth, and
data rate.
4.1 Twisted Pair (cont.)
• Categories of Twisted Pair for Data
Transmisson
• ANSI Standards
• Table 4.2—Twisted Pair Categories and
Classes
• Figure 4.5 Category 6A Channel
Requirements
4.1 Transmission Media (cont.)
• Coaxial Cable
– Made of an outer cylindrical conductor and a
single inner wire conductor, separated by an
insulator.
– The outer conductor is covered by a jacket or
shield.
– Used for long-distance communication lines,
cable TV, and LANs.
– Tends to have a higher bandwidth than twisted
pair.
4.1 Transmission Media (cont.)
• Optical Fiber
– Conducts an optical ray (ie. light).
– Can be made from glass and plastic.
– The core is surrounded by glass or plastic
cladding and a protective jacket.
– Has high bandwidth, low attenuation, small
size, is light weight, and is not susceptible to
electromagnetic interference.
– Used for trunks, local loops, and LANs.
4.2 Wireless Transmission
Media
• Antennas
– An antenna is an electrical conductor or system of
conductors used for radiating electromagnetic
energy or for collecting electromagnetic energy.
– An isotropic antenna radiates power in all
directions.
– Parabolic reflective antenna are used to focus
received signals and to transmit a parallel beam
without dispersion (Fig. 4.5).
– Then gain equation is given by equation 4.1.
4.2 Wireless Transmission Media(cont.)
• Terrestrial Microwave
– Parabolic dishes (3 meters in diameter) usually
located at substantial heights.
– Line-of-sight transmission.
– Loss = 10 log10(4pd/l)² dB.
– Rain attenuates the signal.
– Table 4.6 shows typical microwave
performance.
4.2 Wireless Transmission Media (cont.)
• Satellite Microwave (Fig. 4.8, 4.9)
–
–
–
–
Used to link two or more ground stations.
Uplinks and downlinks use different frequencies.
A single satellite might have several transponders.
Used for television, long-distance telephone
transmission, and private networks.
– Very small aperture terminals (VSATs) have reduced
the cost of satellite communications
– Optimal transmission frequencies are 1-10 G Hz but
saturation is causing higher frequencies to be used.
4.2 Wireless Transmission Media (cont.)
• Broadcast Radio
– Omnidirectional.
– Typical frequencies range from 30 MHz to
1GHz.
– Less sensitive to rain than microwave radio.
– Multipath interference is a prime source of
impairment.
4.2 Wireless Transmission Media(cont.)
• Infrared
– Transceivers modulate noncoherent infrared
light.
– Transceivers must be within line-of-sight or
each other or must be able to receive reflections
of the signal.
– Infrared does not penetrate walls, and hence has
a security advantage over radio waves.
4.3 Wireless Propagation
• Ground Wave Propagation
– Radio waves follow the contour of the earth,
and can travel beyond the horizon.
– Frequencies up to 2 M Hz.
– AM radio is an example.
4.3 Wireless Propagation (cont.)
• Sky Wave Propagation
– A signal from an earth based antenna is
reflected from the ionized layer of the upper
atmosphere
– Used by amateur radio, CB radio, and
international broadcast radio.
4.3 Wireless Propagation (cont.)
• Line-of-Sight Propagation
– For signals above 30 MHz.
– Optical line of sight: d = 3.57 x Sqrt(h) where
d is distance and h is height.
– Radio line of sight: d = 3.57 x Sqrt(K h), where
K is an adjustment factor (of about 4/3).
4.4 Line-of-Sight Transmission
• LOS Impairments
– Free Space Loss--the signal disperses with
distance.
– Atmospheric Absorption
– Multipath Reflection (Fig. 4.13)
– Refraction (bending of waves through the
atmosphere.)