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L ectur e 3
Transmission
Media
M odul e 1
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ASETTransmission Media
a) The transmission medium is the physical path by whicha message travels from sender to receiver.
b) Computers and telecommunication devices use signals torepresent data. These signals are transmitted from a
device to another in the form of electromagnetic energy.
c) Examples of Electromagnetic energy include power,radio waves, infrared light, visible light, ultraviolet light,and X and gamma rays.
d) All these electromagnetic signals constitute theelectromagnetic spectrum
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ASETElectromagnetic Spectrum forTransmission Media
Not all portion of the spectrum are currently usable fortelecommunications Each portion of the spectrum requires a particular
transmission medium
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ASETClasses of transmission media
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ASETGuided Media
Transmission capacity depends on the distance andon whether the medium is point-to-point or multipointExamples
twisted pair wires coaxial cables optical fiber
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ASETTwisted-pair cable
Twisted pair consists of two conductors (normally copper),each with its own plastic insulation, twisted together.Often used at customer facilities and also over distances tocarry voice as well as data communicationsThe twisting helps to reduce the interference (noise) andcrosstalk.
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ASETTypes of Twisted Pair
STP (shielded twisted pair) the pair is wrapped with metallic foil or braid to insulate
the pair from electromagnetic interference
UTP (unshielded twisted pair) each wire is insulated with plastic wrap, but the pair is
encased in an outer covering
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ASET
UTP and STP cable
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ASETUnshielded Twisted-pair UTP cable is the most common type of telecommunication
medium in use today. The range is suitable for transmitting both data and video. Advantages of UTP are its cost and ease of use. UTP is
cheap, flexible, and easy to install.
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The Electronic Industries Association (EIA) has developedstandards to grade UTP.
1. Category 1. The basic twisted-pair cabling used in telephonesystems. This level of quality is fine for voice but inadequatefor data transmission.
2. Category 2. This category is suitable for voice and datatransmission of up to 2Mbps.3. Category 3.This category is suitable for data transmission of
up to 10 Mbps. It is now the standard cable for mosttelephone systems.
4. Category 4. This category is suitable for data transmissionof up to 20 Mbps.
5. Category 5. This category is suitable for data transmissionof up to 100 Mbps.
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Categories of unshielded twisted-pair cablesCategory Data Rate Digital/Analog Use
1 < 100 kbps Analog Telephone
2 2 Mbps Analog/digital T-1 lines
3 10 Mbps Digital LANs
4 20 Mbps Digital LANs
5 100 Mbps Digital LANs
6 200 Mbps Digital LANs
7 600 Mbps Digital LANs
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UTP connector
The most common UTP connector is RJ45 (RJ stands forRegistered Jack).
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ASETTwisted Pair Advantages
Inexpensive and readily available Flexible and light weight
Easy to work with and install
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ASETTwisted Pair Disadvantages
Interference and noise Attenuation problem
For analog, repeaters needed every 5-6km
For digital, repeaters needed every 2-3km
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ASETShielded Twisted (STP) Cable STP cable has a metal foil
or braided-mesh coveringthat enhances each pairof insulated conductors.
The metal casing preventsthe penetration ofelectromagnetic noise.
Materials andmanufacturing
requirements make STPmore expensive than UTPbut less susceptible tonoise.
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ASETCoaxial Cable (or Coax)
Used for cable television, LANs, telephony Has an inner conductor surrounded by a
braided mesh Both conductors share a common center axial,
hence the term co -axial
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Coaxial cable
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ASETCoax Layers
copper or aluminumconductor
insulating material
shield(braided wire)
outer jacket(polyethylene)
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Categories of coaxial cables
Category Use
RG-59 Cable TV
RG-58 Thin Ethernet
RG-11 Thick Ethernet
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ASETCoax Advantages
Higher bandwidth 400 to 600Mhz up to 10,800 voice conversations
Much less susceptible to interference thantwisted pair
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ASETCoax Disadvantages
High attenuation rate makes it expensiveover long distance
Bulky
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ASETThe Nature of the Light
Light travels in a straight line as long as it is moving through asingle uniform substance.
If a ray of light traveling through one substance suddenlyenters another (less or more dense) substance, its speedchanges abruptly, causing the ray to change direction. Thischange is called refraction .
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ASETRefraction
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ASETCritical angle If the angle of incidence increases, so does the angle of
refraction.The critical angle is defined to be an angle of incidence forwhich the angle of refraction is 90 degrees.
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ASETReflection
When the angle ofincidence becomes greaterthan the critical angle, anew phenomenon occurscalled reflection .
Light no longer passesinto the less densemedium at all.
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Optical fibers use reflection to guide light through achannel.A glass or core is surrounded by a cladding of less denseglass or plastic. The difference in density of the two
materials must be such that a beam of light movingthrough the core is reflected off the cladding instead of being into it.Information is encoded onto a beam of light as a series ofon-off flashes that represent 1 and 0 bits.
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Optical fiber
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ASETTypes of Optical Fiber
There are two basic types of fiber: multimode fiber andsingle-mode fiber.
Multimode fiber is best designed for short transmissiondistances, and is suited for use in LAN systems and videosurveillance.
Single-mode fiber is best designed for longer transmissiondistances, making it suitable for long-distance telephony andmultichannel television broadcast systems.
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Propagation modes
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Multimode : In this case multiple beams from a light sourcemove through the core in different paths.
In multimode step-index fiber , the density of the coreremains constant from the center to the edges. A beam oflight moves through this constant density in a straight lineuntil it reaches the interface of the core and cladding.
In a multimode graded-index fiber the density is highest atthe center of the core and decreases gradually to its lowest atthe edge.
P t i M d
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ASETPropagat ion Modes
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Single mode uses step-index fiber and a highly focusedsource of light that limits beams to a small range of angles,all close to the horizontal.
Fiber Sizes
Optical fibers are defined by the ratio of the diameter oftheir core to the diameter of their cladding, both expressedin microns (micrometers)
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ASETLight sources for optical fibers
The purpose of fiber-optic cable is to contain and directa beam of light from source to target.The sending device must be equipped with a light sourceand the receiving device with photosensitive cell (calleda photodiode) capable of translating the received lightinto an electrical signal.The light source can be either a light-emitting diode(LED) or an injection laser diode.
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Fiber types
TypeCore(micro
meter)Cladding(micro
meter)Mode
50/125 50 125 Multimode, graded-index
62.5/125 62.5 125 Multimode, graded-index
100/125 100 125 Multimode, graded-index
7/125 7 125 Single-mode
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ASETFiber-optic cable connectors
The subscriber channel (SC) connector is used in cable TV.The straight-tip (ST) connector is used for connecting cable to
networking devices.MT-RJ is a new connector with the same size as RJ45.
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ASETFiber Optic Advantages
greater capacity (bandwidth of up to 2 Gbps)
smaller size and lighter weight
lower attenuation
immunity to environmental interference
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ASETFiber Optic Disadvantages
Expensive over short distance
Requires highly skilled installers
Adding additional nodes is difficult
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Radio Waves
Microwaves
Infrared
Unguided Media: Wireless
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Electromagnetic spectrum for wirelesscommunication
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Propagation methods
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ASETBands
Band Range Propagation Application
VLF 3 30 KHz Ground Long-range radio navigation
LF 30 300 KHz Ground Radio beacons andnavigational locators
MF 300 KHz 3 MHz Sky AM radio
HF 3 30 MHz Sky Citizens band (CB),ship/aircraft communication
VHF 30 300 MHz Sky andline-of-sight
VHF TV,FM radio
UHF 300 MHz 3 GHzLine-of-
sightUHF TV, cellular phones,
paging, satellite
SHF 3 30 GHz Line-of-sight
Satellite communication
EHF 30 300 GHz Line-of-sight
Long-range radio navigation
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ASETWireless transmission waves
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ASETOmni directional Antennas
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Radio waves are used for multicast
communications, such as radio andtelevision.
Note:
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Unidirectional antennas
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Microwaves are used for unicast
communication such as cellulartelephones, satellite networks, andwireless LANs.
Note:
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Infrared signals can be used for short-
range communication in a closed areausing line-of-sight propagation.
Note: