transmission media updated

8/11/2019 Transmission Media Updated

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ASET

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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ASET

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






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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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ASET

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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ASET

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:

transmission media updated

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