Chapter 4 Transmission Media
Overview
• Guided – wire (twisted pair, coaxial cable, optical fiber)
• Unguided – wireless (broadcast radio, terrestrial microwave, satellite)
• Characteristics & quality of data transmission is determined by
medium & signal
• For guided medium, the medium itself is more important in determining
the transmission limitations
• For unguided medium, the bandwidth produced by the antenna is more
important
• Key concerns are data rate and distance in the design of trans.
sys.
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Design Factors
• Bandwidth:

Higher bandwidth (HZ) gives higher data rate (bps)
• Transmission impairments: e.g. Attenuation
 Limits the transmission distance
 Twisted pair -> coaxial cable -> optical fiber
• Interference: from competing signals in overlapping
frequency bands can distort or wipe out a signal.
• Number of receivers: In guided media
 Point-to-point link or share link with multiple attachment:
each attachment introduce some attenuation & distortion
on the line, limiting distance and/or data rate
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Electromagnetic Spectrum for Telecom
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EE 4272
Guided Transmission Media
• Twisted Pair
• Coaxial cable
• Optical fiber
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Twisted Pair & Applications
• Most common medium: least expensive
• Telephone network

Between house and local exchange (subscriber loop)
• Within buildings

To private branch exchange (PBX)
• For local area networks (LAN)

10Mbps or 100Mbps
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Twisted Pair - Transmission Characteristics
• Pros and Cons


•
•
•
•
•
•
Cheap; Easy to work with
Low data rate; Short distance <- bigger attenuation
Analog Signal: Amplifiers every 5km to 6km
Digital Transmission: repeater every 2km or 3km
Limited distance
Limited bandwidth (1MHz)
Limited data rate (100Mbp)
Susceptible to interference and noise
Spring, 2003
EE 4272
Unshielded and Shielded TP
• Unshielded Twisted Pair (UTP)




Ordinary telephone wire
Cheapest
Easiest to install
Suffers from external EM interference
• Shielded Twisted Pair (STP)

Metal braid or sheathing that reduces interference
 More expensive
 Harder to handle (thick, heavy)
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EE 4272
UTP Categories
• Cat 3



up to 16MHz
Voice-grade cable found in most offices
Twist length of 7.5 cm to 10 cm
• Cat 4

up to 20 MHz
• Cat 5



up to 100MHz
Commonly pre-installed in new office buildings
Twist length 0.6 cm to 0.85 cm
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Coaxial Cable
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EE 4272
Applications & Transmission Characteristics
Applications:
• Television distribution : Cable TV
• Long distance telephone transmission


Can carry 10,000 voice calls simultaneously (via FDM)
Being replaced by fiber optic
• Short distance links between computer devices
Transmission Characteristics:
• Analog

Amplifiers every few km; Closer if higher frequency
 Up to 500MHz
• Digital: Repeater every 1km; Closer for higher data rates
Spring, 2003
EE 4272
Optical Fiber - Transmission Characteristics
• Optical Fiber: transmit a signal-encoded beam of light by means
of total internal reflection
• Act as waveguide for freq. of 1014 to 1015 Hz

Portions of infrared and visible spectrum
• Three Transmission Modes:

Step-index multimode; Graded-index multimode; Single-mode
• Two Type of Light Sources:

Light Emitting Diode (LED)
 Cheaper; Wider operating temp

range; Last longer
Injection Laser Diode (ILD): More efficient; Greater data rate
• WDM: multiple beams of light at different freq. are transmitted
on the same fiber. This is a form of FDM.
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EE 4272
Optical Fiber Transmission Modes
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Optical Fiber - Properties & Applications
Properties:
• Greater capacity: in the unit of Gbps, Tbps
• Smaller size & lighter weight
• Lower Attenuation
• EM Isolation: not affected by external EM interference
• Greater Repeater Spacing: every hundreds of km
Applications:
• Long-haul trunks ; Metropolitan trunks; LANs
• Rural exchange trunks
• Subscriber loops
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EE 4272
Wireless Transmission
• Unguided media
• Transmission & reception via antenna
• Directional

Focused beam
 Careful alignment required
• Omnidirectional


Signal spreads in all directions
Can be received by many antennae
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EE 4272
3 Frequency Ranges
• 30MHz to 1GHz:


Suitable to Omnidirectional app.
Refer to as broadcast radio range
• 2GHz to 40GHz: Microwave frequency



Highly directional beams are possible
Suitable to point- to-point transmission
Can also be used for satellite comm.
• 3 x 1011 to 2 x 1014


Infrared portion of the spectrum
Local apps. Within confined areas, e.g. a single room
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EE 4272
Terrestrial Microwave
• Physical Description: the most common type of antenna is the
parabolic “dish”
• Focuses a narrow beam: to achieve line-of-sight trans. to the
receiving antenna
• Apps.: Long haul telecommunications
• Higher frequencies give higher data rates

common freq : 2 – 40 GHZ
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EE 4272
Satellite Microwave
• Satellite is a microwave relay station
• Satellite receives on one frequency, amplifies or
repeats signal and transmits on another frequency
• Requires geo-stationary orbit

Height of 35,784km
• Applications:



Television
Long distance telephone
Private business networks
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Broadcast Radio
• Omnidirectional
• Freq: 30 MHZ to 1GHZ


FM radio
UHF and VHF television
• Trans. is limited to the line of sight
• Suffers from multipath interference
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Infrared
• Transceivers modulate noncoherent infrared light
• Two parties must be within line of sight
(or reflection)
• Blocked by walls
• e.g. TV remote control
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EE 4272