guided & unguided

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Electromagnetic Spectrum

The electromagnetic (EM) spectrum is the rangeof all types of EM radiation . Radiation isenergy that travels and spreads out as it goes the visible light that comes from a lamp in yourhouse and the radio waves that come from aradio station are two types of electromagnetic

radiation. The other types of EM radiation thatmake up the electromagnetic spectrum aremicrowaves , infrared light , ultraviolet light , X-

rays and gamma-rays .
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Figure 7-1

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998


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Propagation characteristics are different in each frequency band Licensed and Unlicensed bands

UV

1 MHz1 kHz 1 GHz 1 THz 1 PHz 1 EHz

infrared visible

X raysGamma rays

LF HF VHF UHF SHF EHF MF

902 928 Mhz2.4 2.4835 Ghz

5.725 5.785 Ghz

30kHz 300kHz 3MHz 30MHz 300MHz 30GHz 300GHz

10km 1km 100m 10m 1m 10cm 1cm 100mm

3GHz


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Transmission Impairment

With any communications system, thesignal that is received may differ from thesignal that is transmitted, due to varioustransmission impairments.


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Transmission Impairment

Attenuation Delay distortion Noise


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Attenuation

Attenuation means loss of energy.


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Attenuation

Bel deciBel

Pin = 100mW, P out = 10mW, attenuation =10 dB


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Delay Distortion

Delay distortion is due to velocity of propagation that varies withfrequency. Thus, various frequency components of a signal arrive atthe receiver at different times.

Critical in particular for digital data, because signal components of bit positions spill into other bit positions, and so limiting the allowed rateof transmission.


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Noise

Noise

Signal

signal + noise

Several types of noise such as thermal noise,

induced noise, crosstalk and impulse noisemay corrupt the signal.


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Noise

Thermal noise is the random motion of electrons in a wire that createsan extra signal not originally sent by the transmitter.

Induced noise comes from sources such as motors and appliances.

These devices acts as sending antenna and the transmission mediumacts as receiving antenna. Crosstalk is a noise that is caused by the inductive coupling between

two wires that are closed to each other. Sometime when talking on thetelephone, you can hear another conversation in the background. That

is cross talk. Impulse noise is irregular disturbances, such as lightning, and flawed

communication elements. It is a primary source of error in digital data.


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Transmission Media

Means by which a communication signal iscarried from one system to another (for

example twisted-pair wire, co axial cable,fiber-optic cable) and wireless links (forexample satellite, microwave, radio and

infrared systems.


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Figure 7-2



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Figure 7-3



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Guided Transmission Media

Waves are guided along solid medium.


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Twisted-Pair CableFigure 7-4 and 7-5



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Twisted-Pair Cable

Pair of twisted conductors Twisting reduces interference (two parallel wires constitute a simple

antenna; a twisted pair does not.)

Cheap medium Commonly used for communications within buildings and in telephone

networks Produced in unshielded (UTP) and shielded (STP) forms, and in

different performance categories.

Cables may hold hundreds of pairs. Neighbor pairs typically havedifferent twist lengths to reduce crosstalk.
http://wagner.princeton.edu/cgi-bin/foldoc?twisted-pairhttp://wagner.princeton.edu/cgi-bin/foldoc?twisted-pair


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Effect of Noise on Parallel LinesFigure 7-6



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Noise on Twisted-Pair LinesFigure 7-7



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Unshielded Twisted-Pair Cable

Figure 7-8



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UTP ConnectorsFigure 7-9



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Shielded Twisted-Pair Cable

Figure 7-10



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Shielded Twisted-Pair Cable

STP has a metal foil or braided-mesh covering that encases each pairof insulated conductors.

The metal casing prevents the penetration of electromagnetic noise.

Shielding each pair of a twisted- pair cable can eliminate mostcrosstalk.

Shield must be connected to the ground. Connectors used in STP & UTP are RJ 45, RJ 11 etc. STP is more expensive than UTP but less susceptible to noise.


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Categories of Twisted-Pair Cables

Category 1: The basic cabling used for voice. Category 2: Suitable for voice and data up to 4 Mbps. Category 3: Required to have at least three twists per foot and can

be used for data transmission of up to 10 Mbps. It is now thestandard cable for most telephone systems.

Category 4: Must also have at least three twists per foot and canbe used for data transmission of up to 16 Mbps.

Category 5: Suitable for data up to 100 Mbps


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Coaxial CableFigure 7-11 and 7-12



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Coaxial Cable Standards

Different coaxial cable designs are categorized by theirradio government(RG) ratings.

RG-8 used in thick Ethernet. RG-9 used in thick Ethernet. RG-11 used in thick Ethernet. RG-58 used in thin Ethernet.

RG-59 used for TV.


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Coaxial Cable Connectors

To connect co-axial cable to devices, we need coaxial connectors. The mostcommon type of connector used today is the Bayone-NeiIl-Concelman, orBNC, connectors.

Fi 7 13


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Refraction

Figure 7-13


Fig 7 14


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Critical Angle

Figure 7-14


Figure 7 15


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Reflection

Figure 7-15



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Figure 7 20


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Fiber Construction

Figure 7-20



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Light propagation Meaning - Light travels along a fiber cable by a

process called 'Total Internal Reflection' (TIR), this is made possible byusing two types of glass which have different refractive indexes. Theinner core has a high refractive index and the outer cladding has a lowindex.

Fiber-optic cable has two propagation modes-multimode and single mode. They performdifferently with respect to both attenuationand time dispersion . The single-modefiber-optic cable provides much better

performance with lower attenuation.

Propagation Modes in Fiber optic cable


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Multimode Fiber

Multi-mode fiber is used for communication over shortdistances, such as within a building or on a campus or a LAN.Typical multimode links have data rates of 10 Mbit/s to 10 Gbit/sover link lengths of up to 600 meters (300 m for 10 Gbit/s) more than sufficient for the majority of premises applications.

Categories of Multimode fiber

- Step Index Fiber

- Graded Index Fiber


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Multimode Step-Index

Multimode fiber is designed to carry multiple light rays or

modes concurrently, each at a slightly different reflection anglewithin the optical fiber core.Multimode fiber transmission

is used for relatively short distances because the modes tend to disperseover longer lengths ( this is calledmodal dispersion ).

Multimode fiber has a larger core than

single mode.


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Multimode graded index fiber has a higher refractive index in

the core that gradually reduces as it extends from the cylindricalaxis outward.The core and cladding are essentially a single graded unit (fig).This corresponds to multimode propagation with a refractiveindex profile that is called graded index .Here the variation of the index of refraction is gradual as itextends out from the axis of the core through the core to thecladding. There is no sharp discontinuity in the indices ofrefraction between core and cladding .

Graded -Index


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Single Mode FiberA single-mode optical fiber (SMF) is an optical fiberdesigned to carry only a single ray of light (mode).

The diagram corresponds to single-mode propagationwith a refractive index profile that is called step index .

The diameter of the core is fairly small relative to the

cladding. Because of this, when light enters the fiber-Optic cable on the left, it propagates down toward theright in just a single ray, a single mode, which is thelowest-order mode.


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Unguided Transmission Media

Unguided media transport electromagnetic waves without using a physicalconductor. [type of communication is often referred to as wirelesscommunication.

There are three types of Unguided Media(i) Radio waves(ii) Micro waves(iii) Infrared.


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U id d M di B d d h i


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Unguided Media Bands and their ranges

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 GHz Line-of-sight UHF 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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Radio Waves

Radio (3kHz and 1GHz) waves are omnidirectional when an antenna transmitsradio waves they are propagated in all directions. This means that sending andreceiving antenna do not have to he aligned. A sending antenna can send waves

that can be received by any receiving antenna. Radio waves particularly those waves that propagate in sky mode, can travel long

distances. This makes radio waves a good candidate for long-distance broadcasting such as AM radio.

Radio waves particularly those of low and medium frequencies can penetrate

walls. It is an advantage because; an AM radio can receive signals inside a building. It is the disadvantage because we cannot isolate a communication tofirst inside or outside a building. The radio waves band is relatively narrow justunder I GHz, compared to the microwave band.


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Omni directional Antenna


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Microwaves

Electromagnetic waves having frequencies between I and 300 GHz arecalled microwaves.

Microwaves are unidirectional, when an antenna transmits microwaves they

can be narrowly focused. This means that the sending and receiving antennasneed to be aligned. On the other hand microwaves propagation is line-of-sight. Since the towers

with the mounted antennas needs to be in direct sight of each other, towersthat are for apart need to he very tall, the curvature of the earth as well asother blocking obstacles do not allow two short towers to communicate usingmicrowaves, Repeaters are often needed for long distance communicationvery high frequency microwaves cannot penetrate waIls.

Parabolic dish antenna and horn antenna are used for this means oftransmission.


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Unidirectional Antenna

Figure 7-7


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Terrestrial microwave


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Applications

Cellular phones Satellite networks Wireless LANs


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Infrared

Infrared signals with frequencies ranges from 300GHz to 400 THz can be used for short rangecommunication. Infrared signals, having highfrequencies, cannot penetrate walls. This helps to

prevent interference between one system andanother. A short range communication system in

one room cannot be affected by the infrared wavesin another room.


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Applications

There are no. of computer devices which areused to send data through infrared medium

e.g. keyboard mice, PCs and printers. Thereare some manufacturers provide a special

port called the (Infrared Data Association)

IrDA port that allows a wireless keyboardto communicate with a PC.


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Satellite Systems

Cover very large areas Different orbit heights

GEOs (39000 Km) versus LEOs(2000 Km)

Optimized for one-way transmission Radio and movie (SatTV) broadcasting

Most two-way systems Expensive alternative to terrestrial system


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Geostationary Earth Orbit (GEO)

These satellites are in orbit 35,863 km above the earths surface alongthe equator.

Objects in Geostationary orbit revolve around the earth at the samespeed as the earth rotates. This means GEO satellites remain in thesame position relative to the surface of earth.

AdvantagesA GEO satellites distance from earth gives it a large coveragearea, almost a fourth of the earths surface. GEO satellites have a 24 hour view of a particular area.These factors make it ideal for satellite broadcast and othermultipoint applications.


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GEO (cont.)

DisadvantagesA GEO satellites distance also cause it to have

both a comparatively weak signal and a timedelay in the signal, which is bad for point to

point communication.GEO satellites, centered above the equator,have difficulty broadcasting signals to near

polar regions

h ll


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Geosynchronous Satellite

Satellite communication


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Satellite communication


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Types of Telecommunications Media

SatelliteTransmission

Communications satellites arerelay stationsthat receivesignals fromone earth

station andrebroadcastthem to another.


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Point-to-Point Link via Microwave


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Latency of Satellite Systems GEO satellite systems have a high latency. Consider a transmission between two remote nodes.

The sending station transmits a message to the satellite(uplink). The satellite transmits this message to the

destination node (the downlink). The destination nodesends an acknowledgement (ACK) to the satellite. Thesatellite then transmits the ACK to the sender.

The total distance involved is 88,000 miles.

Dividing by the speed of light, we get a total propagation delay of 470 milliseconds.


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Latency of Satellite Systems

The longer the latency, the less bandwidth thesystem can support. The bandwidth capability of satellite systems

is a function of the frequency at which thesatellites transmit. Four common frequencies are:

C-band 6GHz uplink and 4 Ghz downlink Ka-band 28 Ghz uplink, 18 Ghz downlink Ku-band 14 Ghz uplink, 12 Ghz downlink V-band above 30 Ghz


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Disadvantages

expensive large propagation delay (high latency) not very secure; signals can be easily intercepted affected by atmospheric conditions


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Advantages of Satellite

The advantages of satellite communication over terrestrialcommunication are:

The coverage area of a satellite greatly exceeds that of a terrestrialsystem.

Transmission cost of a satellite is independent of the distance fromthe center of the coverage area.Satellite to Satellite communication is very precise. Higher Bandwidths are available for use. Covers very large areas.

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