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Part II: Data Transmission
The basics of media, signals, bits, carriers, and modems
Fall 2005
Qutaibah MalluhiComputer Science and Engineering
Qatar University
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Fall 2005
Transmission Media
Overview of transmission media used in network systems
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Sending Data
At sender: encode data as energy and transmit energy
At receiver: decode energy at destination back into data
Energy can be electrical, light, radio, sound, etc.– Each form of energy has different properties and
requirements for transmission
– Transmitted energy is carried through some sort of medium: copper, glass, air, ...
Sender(encodes data)
Receiver(decodes data)
data10110 … 110
data10110 … 110
Medium
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Physical Layer and Data Transmission
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Copper Wire
Primary medium to connect computers– Inexpensive– easy to install– low resistance to electric current => signal travels
further Interference: How to minimize? Twisted pair Coaxial cable (Coax) Shielded twisted pair
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Twisted Pair
One wire for signal and other as ground reference
Receiver uses the difference between signal– Receiver is immune to interference if both wires are
affected equally Twisting
– limits electromagnetic energy these wires emit– protects the wires from external interference– Two wires has same distance to noise source. One is
closer in one twist, the other is closer in the second twist.
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UTP and STP
Unshielded Twisted Pair (UTP)– Classified into 7 categories based on cable quality– Category 1 is the lowest quality (phone) and 5 is the
most common today (100 Mbps Ethernet) Shielded Twisted Pair (STP)
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Coaxial Cable
Can carry higher frequencies that TP Improved performance through shield Particularly useful for wires that pass next to
equipment generating EM fields
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Glass Fibers
Thin glass fiber carries light with encoded data
Plastic jacket allows fiber to bend (some!) without breaking
Fiber is very clear and designed to reflect light internally for efficient transmission
Light emitting diode (LED) injects light into fiber. Light sensitive receiver at other end translates light back into data
Applications:
– Backbone networks (cost-effective wide-bandwidth)
– Backbone of TV cable networks (coax for connection to premise)
– Some LANS (100baseFX and 1000base-X Ethernets)
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Glass Fibers: More Details
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Glass Fiber Adv. and Disadv.
Advantages– Much higher bandwidth
» Signal generation and not the medium is the bottleneck today
– No electromagnetic interference– Further signal distances (less signal attenuation)
» 50 KM for fiber vs. 5 KM for copper– Better encoding of bits (store more bits in signal)– More immune to tapping
Disadvantages– Special installation equipment
» difficulty of joining fibers– Unidirectional (but requires single wire – no ground)– difficulty to locate fiber problems
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Connectors
Twisted Pair Connectors
Coax Connectors
Coax Connectors
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Radio
Data transmitted using radio waves. Conceptually similar to radio, TV, pagers
Energy travels through the air rather than copper or glass – Physical connection is not required
– Antennas are needed
Omni-directional (travels in all directions) Can travel through walls and through an entire
building Can be long distance or short distance
Long distance with satellite relay
Short distance: wireless computer network
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Wireless Network
Wireless bridge and net interface
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Microwave
High frequency radio waves – 2-300 GHz
Higher speed than lower frequency RF signals Unidirectional, for point-to-point communication Can not penetrate structures – clear path is
required– Line of sight transmission
Antennas mounted on towers relay transmitted data
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Infrared
Infrared light transmits data through the air Similar to technology used in TV remote control Can propagate throughout a room (bouncing off
surfaces), but will not penetrate walls Becoming common in personal digital assistants
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Laser
Unidirectional, like microwave Higher speed than microwave Uses laser transmitter and photo-sensitive
receiver at each end Point-to-point, typically between buildings
Can be adversely affected by weather
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Satellites
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Summary
Copper wire is mature technology, rugged and inexpensive; maximum transmission speed is limited
Glass fiber: Higher speed
More resistant to electro-magnetic interference
Spans longer distances
Requires only single fiber
More expensive; less rugged
Radio and microwave don't require physical connection
Radio and infrared can be used for mobile connections Laser also does not need physical connection and
supports higher speeds