Transmission medium
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We can say that transmission media belong to layer zero
Transmission medium
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Transmission media can be divided into two broad categories:
– Guided media include twisted-pair cable, coaxial cable, and fiber-optic
– Unguided media is usually air
Guided media
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Guided media, which are those that provide a conduit from one device
to another, include twisted-pair cable, coaxial cable, and fiber-optic
cable.
A signal traveling along guided media is directed and contained by the
physical limits of the medium
Twisted-pair and coaxial cable use metallic (copper) conductors that
accept an transport signals in the form of electric current
Optical fiber is a glass cable that accepts and transports signals in the
form of light
Twisted-Pair Cable
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Consists of
– two conductors (normally copper)
– each with its own plastic insulation
– twisted together
Twisted-Pair Cable
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One of the wires is used to carry signals to the receiver
And the other is used as a ground reference
Receiver uses the difference between the two level
Interference (noise) and crosstalk may affect both wires and create
unwanted signals
Receiver operates only on the difference between these unwanted
signals
– If the two wires are affected by noise or crosstalk equally
 Receiver is immune (the difference is zero)
– If the two wires are parallel
 The effect of these unwanted signals is not the same in both wires (one close
and one farther)
– By twisting the pairs, a balance is maintained
Twisted-Pair Cable
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The number of twists per unit of the length (e.g. inch) determines the
quality of the cable
– More twists mean better quality
Unshielded versus Shielded
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Two common twisted-pair cable used in communications
– Unshielded twisted-pair (UTP)
– Shielded twisted-pair (STP)
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STP has a metal foil or braided-mesh covering
– Preventing the penetration of noise or crosstalk
– It is bulkier and more expensive
Categories
EIA: the Electronic Industries association classifies unshielded twistedpair cable into seven categories which determined by quality (1 lowest
and 7 highest)
Connectors
Most common UTP connector is RJ45
–
R J stands for Registered Jack
Performance
One way measure performance is to compare
–
attenuation vs. frequency and distance.
– A twisted-pair can pass a wide range of frequencies.
– Figure 7.6 shows with increasing frequency, attenuation in decibels per kilometer
(dB/km), sharply increase with frequency above 100 kHz.
– Gauge is a measure of the thickness of the wire.
Application
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Used in telephone Lines (UTP)
DSL lines (UTP)
Local area networks
Coaxial Cable
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Carries signals of higher frequency ranges than twisted-pair cable
Coax has a central core conductor of solid or stranded wire (usually
copper)
Encased in an outer conductor of metal foil, braid, or a combination of
the two
The outer metallic wrapping
Outer conductor is also enclosed in an insulating sheath
Whole cable is protected by a plastic cover
Coaxial Cable Standards
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Coaxial cables are categorized by their radio government (RG) ratings
Each RG number denotes a unique set of physical specifications
– Wire gauge of the inner conductor
–
Thickness and type of the inner insulator
– Construction of the shield
– Size and type of the outer casing
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Each RG ratings is adapted for a specialized function
Coaxial Cable Connectors
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Coaxial cable connector is BNC (Bayone-Neill-Concelman)
Figure shows the BNC connector, the BNC T connector, and the BNC
terminator
Applications
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It was used in analog and digital telephone networks (replaced by
fiber-optic)
Cable TV networks (replaced by fiber-optic)
Traditional Ethernet Lans
Fiber-Optic Cable
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It is made of glass or plastic and transmits signals in the form of light
If a ray pf light traveling through one substance and enters another
(more or less dense), the ray changes directions
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Optical fibers use reflection to guide light through a channel
Fiber-Optic Cable
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Glass or plastic core is surrounded by a cladding of less dense glass or
plastic
Propagation Modes
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Current technology supports two modes (multimode and single mode)
for propagating light along optical channels
Multimode: multiple beams from a light source move through the core
in different paths
Multimode can be implemented in two forms:
– Step-index
– Graded-index
Propagation Modes
Fiber Sizes
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By the ratio of the diameter of their core to the diameter of their
clading [micrometers]
Cable compostion
Fiber-Optic Connections
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Fiber-optic use three different type of connectors
Subscriber Channel (SC) connector
1.
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The Straight-Tip (ST) connector
2.
Used for connecting cable to networking devices
–
3.
Used in cable TV and it uses a push/pull locking system
MT-RJ in new connector with the same size as RJ45
Fiber-Optic Connections
Applications
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It used in backbone networks
For cable TV with coaxial cable (a hybrid network)
Advantages and Disadvantages
of Optical
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Advantages over twisted-pair and coaxial
– Higher bandwidth
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Less signal attenuation
– Signal with fiber-optic can run for 50 km with requiring regeneration
– 5 km for coaxial or twisted-pair cable
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Immunity to electromagnetic interference
– Electromagnetic noise cannot affect fiber-optic cables
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Resistance to corrosive materials
– Glass is more resistant to corrosive materials than copper
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Light weight
More immune to tapping
– Fiber-optic cables definitely more immune to tapping than copper cables
Advantages and Disadvantages
of Optical
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Disadvantages
– Installation/maintenance
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Because it is new technology, need expertise
– Unidirectional
 If we need bidirectional we need two fibers
–
Cost
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The cables and interfaces are expensive
UNGUIDED MEDIA: WIRELESS
Unguided media: transport electromagnetic waves without using a
physical conductor (wireless communication)
– Signal broadcast through air
UNGUIDED MEDIA: WIRELESS
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Unguided signals can travel from the source to destination in several
ways:
– Ground propagation
– Sky propagation
– Line-of-sight propagation
UNGUIDED MEDIA: WIRELESS
Ground propagation (below 2 MHz)
– Radio waves travel through the lowest portion of atmosphere
– Waves emanate in all directions from the transmitting antenna
– Distance depends on the amount of power in the signal
Sky propagation (2 - 30 MHz)
– Higher frequency radio waves radiate upward into ionosphere where they
reflected back to earth
– This type of transmission allows for greater distance with lower power
output
UNGUIDED MEDIA: WIRELESS
Line-of-sight propagation (above 30 MHz)
– Very high-frequency signals are transmitted in straight lines
directly from antenna to antenna
– Antennas must be directional
– Facing each other
– Either tall enough or close enough
UNGUIDED MEDIA: WIRELESS
Electromagnetic spectrum defined as radio waves and microwaves is
divided into eight ranges, called bands:
UNGUIDED MEDIA: WIRELESS
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Wireless transmission can be divided into:
– Radio waves
– Microwaves
– Infrared waves
UNGUIDED MEDIA: WIRELESS
Radio waves
– Radio waves: waves range in frequencies between 3 KHz and 1 GHz
– Microwaves: waves ranging in frequencies between 1 and 300 GHz
– Radio waves are omnidirectional (propagated in all directions)
– The sending and receiving antennas do not have to be aligned
– Disadvantage
 One antenna interferences another antenna when they using same frequency
– Radio waves that propagate in the sky mode, can travel long distance [AM radio]
– Radio waves [low and medium frequencies] can penetrate walls inside a building
 Advantage: an AM radio can be received inside a building
 Disadvantage: we cannot isolate a communication to just inside or outside a building
Applications
–
Useful for multicasting: Radio, television. Cordless phones and paging system
UNGUIDED MEDIA: WIRELESS
Microwaves
– Microwaves are unidirectional
– Antenna need to be aligned
– Advantage: pair of antenna can be aligned without interfering with
another pair
– Microwave propagation is line-of-sight
– For long distance communication
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Very tall towers
Repeater
– Very high-frequency microwaves cannot penetrate walls
 Disadvantage if receiver inside a building
UNGUIDED MEDIA: WIRELESS
Applications
– Due to unidirectional properties, microwaves useful when
unicasting (one-to-one) communication
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Cellular phones
Satellite networks
Wireles LANs
UNGUIDED MEDIA: WIRELESS
Infrared
– From 300 GHz to 400 THz (wavelengths from 1 mm to 770 nm)
– Use for short-range communication
– It has high frequency, cannot penetrate walls
 Prevents interference between one system and another
– Remote control not interfere with our neighbors
– Can not be used outside because sun’s rays contain infrared waves
(interference)
– (IrDA) Infrared Data Association established standards for communicating
between devices:
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Keyboards, mice, PCs and printers