TUNALIData Communication1 Chapter 4 Transmission Media.

TUNALI Data Communication 1

Chapter 4

Transmission Media


Overview• Guided - wire• Unguided – wireless

• In both media, communication is in the form of electromagnetic waves

• Characteristics and quality determined by medium and signal

• For guided, the medium is more important• For unguided, the bandwidth of the signal

produced by the antenna is more important• Key concerns are data rate and distance


Design Factors• Bandwidth

—Higher bandwidth gives higher data rate

• Transmission impairments—Attenuation—Twisted pair suffers more impairment than

coaxcial cable and optical fibre

• Interference• Number of receivers

—In guided media—More receivers (multi-point) introduce more

attenuation


Electromagnetic Spectrum


Guided Transmission Media• Twisted Pair• Coaxial cable• Optical fiber


Transmission Characteristics of Guided Media

Frequency Range

Typical Attenuatio

n

Typical Delay

Repeater Spacing

Twisted pair (with loading)

0 to 3.5 kHz 0.2 dB/km @ 1 kHz

50 µs/km 2 km

Twisted pairs (multi-pair cables)

0 to 1 MHz 0.7 dB/km @ 1 kHz

5 µs/km 2 km

Coaxial cable

0 to 500 MHz

7 dB/km @ 10 MHz

4 µs/km 1 to 9 km

Optical fiber 186 to 370 THz

0.2 to 0.5 dB/km

5 µs/km 40 km

Point to point transmission


Twisted Pair


Twisted Pair - Description• There are two insulated cupper wires.

Many pairs are bundled into a single cable.• Twisting reduces crosstalk interference

between adjacent pairs of cable.—Neighboring pairs in a bundle typically have

different twist lenghts

• On long distance links, twist length varies from 5 to 15 cm.


Twisted Pair – Applications 1• Most common medium

—For both analog and digital systems

• Telephone network—Between house and local exchange (subscriber loop)

• Within buildings—Each telephone is connected to a twisted pair, which

goes to private branch exchange (PBX)

• Designed to support voice traffic using analog signalling—Also can handle digital data traffic at modest data

rates (modem)


Twisted Pair – Applications 2• Mostly used in telephone lines.• Compared to coaxial cable and optical fiber,

twisted pair is limited in distance, bandwidth and data rate.

• Twisted pair is also used within a building for LAN supporting personal computers.

• For long distance digital point-to-point signaling a data rate around a few Mbps is possible.

• For a very short distances data rates of up to 1 Gbps have been achieved.


Twisted Pair - Pros and Cons• Cheap• Easy to work with• Low data rate• Short range


Twisted Pair - Transmission Characteristics• Analog

—Amplifiers every 5km to 6km

• Digital—Use either analog or digital signals—repeater every 2km or 3km

• Limited distance• Limited bandwidth• Limited data rate• Susceptible to interference and noise


Attenuation of Guided Media


Unshielded and Shielded TP• Unshielded Twisted Pair (UTP)

—Ordinary telephone wire—Cheapest—Easiest to install—Suffers from external EM interference

• Shielded Twisted Pair (STP)—Metal braid or sheathing that reduces

interference—More expensive—Harder to handle (thick, heavy)

UTP is cheap but subject to electromagnetic interference. STP is expensive but better.


UTP Categories• Cat 3

—up to 16MHz—Voice grade found in most offices—Twist length of 7.5 cm to 10 cm

• Cat 4—up to 20 MHz

• Cat 5—up to 100MHz—Commonly pre-installed in new office buildings—Twist length 0.6 cm to 0.85 cm

• Cat 5E (Enhanced) –see tables• Cat 6• Cat 7

Near-End Crosstalk• Coupling of signal from one pair of

conductors to another pair—Metal pins on a connector—Wire pairs in a cable

• Coupling takes place when transmit signal entering the link couples back to receiving pair at the same end of the link

• i.e. near transmitted signal is picked up by near receiving pair



Comparison of Shielded and Unshielded Twisted Pair Attenuation (dB per 100 m) Near-end Crosstalk (dB)

Frequency (MHz)

Category 3 UTP

Category 5 UTP

150-ohm STP

Category 3 UTP

Category 5 UTP

150-ohm STP

1 2.6 2.0 1.1 41 62 58

4 5.6 4.1 2.2 32 53 58

16 13.1 8.2 4.4 23 44 50.4

25 — 10.4 6.2 — 41 47.5

100 — 22.0 12.3 — 32 38.5

300 — — 21.4 — — 31.3


Twisted Pair Categories and Classes

Category 3 Class C

Category 5 Class D

Category 5E

Category 6 Class E

Category 7 Class F

Bandwidth

16 MHz 100 MHz 100 MHz 200 MHz 600 MHz

Cable Type

UTP UTP/FTP UTP/FTP UTP/FTP SSTP

Link Cost (Cat 5 =1)

0.7 1 1.2 1.5 2.2


Coaxial Cable


Coaxial Cable Applications• Most versatile medium• Television distribution

—Cable TV can carry hundreds of TV channels

• Long distance telephone transmission—Can carry 10,000 voice calls simultaneously

(using FDM)—Being replaced by fiber optic

• Short distance computer systems links• Local area networks

—Using digital signaling, coax cable can be used to provide high speed channels


Coaxial Cable - Transmission Characteristics• Can be used effectively higher frequencies and

data rates• Analog

—Amplifiers every few km—Closer if higher frequency—Usable spectrum is up to 500MHz

• Digital—Repeater every 1km—Closer for higher data rates

• Performance Constraints—Attenuation—Thermal noise—Intermodulation noise


Optical Fiber


Optical Fiber - Benefits• Greater capacity

—Data rates of hundreds of Gbps

• Smaller size & weight• Lower attenuation• Electromagnetic isolation

—Are not affected by external electromagnetic fields (thus not vulnarable to interference, impulse noise, crosstalk)

—Little interference with other equipment—High degree of security from eavesdropping—inherently difficult to tap

• Greater repeater spacing—10s of km at least


Optical Fiber – Applications (1)• Long-haul trunks

—1500 km in length—20000-60000 voice channels—Telephone network, undersea

• Metropolitan trunks—12 km—100,000 voice channels—Joining telephone exchanges in a metropolitan

or city area

Optical Fiber – Applications (2)• Rural exchange trunks

—40-160 km—5000 voice channels—Links towns and villages

• Subscriber loops—Run directly central exchange to a subscriber—Handling not only voice and data, but also

image and video

• LANs—100 Mbps to 10 Gbps



Optical Fiber - Transmission Characteristics• Act as wave guide for 1014 to 1015 Hz

—Portions of infrared and visible spectrum

• Two different types of ligth of source are used:• Light Emitting Diode (LED)

—Cheaper—Wider operating temp range—Last longer

• Injection Laser Diode (ILD)—More efficient—Greater data rate

• Wavelength Division Multiplexing


Optical Fiber Transmission Modes 1


Optical Fiber – Transmission Modes 2• Step-index multimode

—Rays at shallow angles are reflected and propagated along the fiber

—Multiple propagation paths exist• Pulses spread out in time• The need to leave spacing between the pulses limits data rate

—Suitable for short distance transmission

• Single-mode—By reducing the radius of the core—No distortion (because there is a single transmission path)—Suitable for long distance transmission

• Graded-index multimode—Suitable for LANs


Frequency Utilization for Fiber Applications Wavelength (in vacuum) range (nm)

Frequency range (THz)

Band label

Fiber type Application

820 to 900 366 to 333 Multimode LAN

1280 to 1350 234 to 222 S Single mode

Various

1528 to 1561 196 to 192 C Single mode

WDM

1561 to 1620 185 to 192 L Single mode

WDM


Attenuation in Guided Media


Wireless Transmission• Unguided media• Transmission and reception via antenna• Directional

—Focused beam—Careful alignment required

• Omnidirectional—Signal spreads in all directions—Can be received by many antennae


Wireless Transmission Frequencies• 1GHz to 40GHz

—Microwave—Highly directional—Point to point—Satellite

• 30MHz to 1GHz—Omnidirectional—Broadcast radio

• 3 x 1011 to 2 x 1014 Hz—Infrared portion of the spectrum—Local point to point and multipoint applications

within confined areas


Antennas• Electrical conductor (or system of..) used to

radiate electromagnetic energy or collect electromagnetic energy

• Transmission—Radio frequency energy from transmitter—Converted to electromagnetic energy—By antenna—Radiated into surrounding environment

• Reception—Electromagnetic energy impinging on antenna—Converted to radio frequency electrical energy—Fed to receiver

• Same antenna often used for both


Radiation Pattern• Power radiated in all directions• Not same performance in all directions• A way to characterize the performance of an

antenna• Graphical representation of the radiation

propoerties of an antenna as a function of space coordinates

• Isotropic antenna is (theoretical) point in space—Radiates in all directions equally—Gives spherical radiation pattern


Parabolic Reflective Antenna• Used for terrestrial and satellite microwave• Parabola is locus of all points equidistant from a

fixed line and a fixed point not on that line—Fixed point is focus—Line is directrix

• Revolve parabola about axis to get paraboloid—Cross section parallel to axis gives parabola—Cross section perpendicular to axis gives circle

• Source placed at focus will produce waves reflected from parabola in parallel to axis—Creates (theoretical) parallel beam of light/sound/radio

• On reception, signal is concentrated at focus, where detector is placed


Parabolic Reflective Antenna


Antenna Gain 1• Measure of directionality of antenna• Power output in particular direction

compared with that produced by isotropic antenna

• Measured in decibels (dB)• Results in loss in power in another

direction• Effective area relates to size and shape

—Related to gain


Antenna Gain 2• Relationship between antenna gain and effective

area

• G= 4Ae /2 = 4f 2Ae / c2 —G = antenna gain

—Ae effective area

—f carrier frequency—c speed of light carrier wavelength

• The effective area of —an isotropic antenna : 2 / 4 (power gain of 1)—a parabolic antenna with face area of A: 0.56A (power

gain of 7A/2)


Antenna Gain 3• For a parabolic reflective antenna with a

diameter of 2m, operating at 12 GHz, what is the effective area and antenna gain?—A = r 2 = —Effective area for parabolic antenna is known

as Ae = 0.56 A yielding Ae = 0.56

—The wavelength is = c / f = (3108)/(12109)=0.025m

—The power gain of parabolic antenna is known as 7A/2 yielding (7 ) / (0.025)2 = 35,186 which is 45.46dB


Terrestrial Microwave – Physical Description• Parabolic dish sizing 3 m in diameter• Focuses narrow beam• Line of sight transmission to the receiving

antenna• To achieve long distance transmissions, a

series of microwave relat towers is used


Terrestrial Microwave - Applications • Long haul communications systems

—Alternative to coax cable or optical fiber—Requires far fewer amplifiers or repeaters than

coax but requires line of sight transmission—Used for both voice and television

transmission

• Short point to point links between buildings

• Cellular systems


Terrestrial Microwave – Transmission Characteristics • Common frequencies: 1 to 40 GHz

—The higher the frequency used, the higher the potential bandwidth

• The loss in communications is computed by—L= 10 log10 (4d/)2 dB where

—d is the distance is the wave length

• Repeaters and amplifiers every 10 to 100 km


Terrestrial Microwave - Transmission Characteristics • Attenuation increases with rainfall• Interference might be a problem

—Most common bands for long haul communication are 4 GHz to 6 GHz

—11 GHz band is coming to use

• Higher microwave frequencies—Increased attenuation—Used in shorter distances —Antennas are smaller and cheaper


Satellite Microwave – Physical Description• Satellite is relay station• Satellite receives on one frequency,

amplifies or repeats signal and transmits on another frequency—Operate on a number of frequency bands:

transponder channels

• Requires geo-stationary orbit to remain stationary—Height of 35,784km


Satellite Point to Point Link


Satellite Broadcast Link

Satellite Microwave - Applications• Television

—Public Broadcasting Service (PBS)—Programs are transmitted to the satellite, broadcast down

to the stations, individual viewers—Direct Broadcast Satellite (DBS)—Direct to the home user

• Long distance telephone• Private business networks

—Very Small Aperture Terminal (VSAT) System—A powerful hub station acts as a relay for communication

of subscribers

• Global Positioning Systems



Satellite Microwave – Transmission Characteristics• To avoid interference, between satellites 4

spacing required—This limits the number of satellites

• For continuous operation without interference, satellite cannot transmit and receive on the same frequency

• Inherently a broadcast medium• There is 270 msec propagation delay• Operation is in the range of 1 -10 GHz


Broadcast Radio• Omnidirectional• 30 MHz to 1 GHz

—FM radio—UHF and VHF television

• Line of sight• Less sensitive to attenuation from rainfall• Suffer relatively less attenuation

—Because of the longer wavelength

• Suffers from multipath interference—Reflection from land, water or objects


Infrared• Modulate noncoherent infrared light• Tranceivers must be within the line of

sight (directly or via reflection)• Blocked by walls

—No security problems—No frequency allocation

• e.g. TV remote control


Wireless Propagation• Signal travels along three routes

—Ground wave• Follows contour of earth• Up to 2MHz• AM radio

—Sky wave• Amateur radio, BBC world service, Voice of America• 2 MHz to 30 MHz• Signal reflected from ionosphere layer of upper

atmosphere• (Actually refracted)

—Line of sight• Above 30Mhz because it is not reflected by the ionosphere• May be further than optical line of sight due to refraction


Ground Wave Propagation


Sky Wave Propagation


Line of Sight Propagation

55

Refraction• Velocity of electromagnetic wave is a function of density

of material—~3 x 108 m/s in vacuum, less in anything else

• As wave moves from one medium to another, its speed changes—Causes bending of direction of wave at boundary—Towards more dense medium

• Index of refraction (refractive index) is—Sin(angle of incidence)/sin(angle of refraction)—Varies with wavelength

• May cause sudden change of direction at transition between medium

• May cause gradual bending if ref. idx. gradually changes—Density of atmosphere decreases with height—Results in bending towards earth of radio waves


Optical and Radio Line of Sight• Optical line of sight

d = 3.57 ( h )1/2 • Radio line of sight

d = 3.57 (K h )1/2 where d is the distance between an antenna and

the horizon in kilometers h is the antenna height in meters K is the adjustment factor (microwaves are

bent with the curvature of the earth) K is approximately 4/3


Optical and Radio Horizons


Example• The maximum distance between two antennas for LOS

transmission if one antenna is 100m high and the other is at ground level is d = 3.57 (Kh) ½ = 3.57 (133) ½ = 41 km

Now suppose that the receiving antenna is 10m high. To achieve the same distance, how high must the transmitting antenna be? The result is 41 = 3.57 ((Kh1) ½ + (13.3) ½ )

h1 = 46.2 m

• Savings over 50 m in the height of transmission antenna—Raising receiving antennas reduces the necessary height of

the transmitter

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Line of Sight Transmission• Free space loss

—Signal disperses with distance—Greater for lower frequencies (longer wavelengths)

• Atmospheric Absorption—Water vapor and oxygen absorb radio signals—Water greatest at 22GHz, less below 15GHz—Oxygen greater at 60GHz, less below 30GHz—Rain and fog scatter radio waves

• Multipath—Better to get line of sight if possible—Signal can be reflected causing multiple copies to be received—May be no direct signal at all—May reinforce or cancel direct signal

• Refraction—May result in partial or total loss of signal at receiver


Free Space Loss for Isotropic Antennas

Pt / Pr = (4 d ) 2 / 2 = ( 4 f d ) 2 / c 2

Pt = signal power at the transmittin antenna

Pr = signal power at the receiving antenna

= carrier wavelength (m)d = propagation distance between

antennas (m)c = speed of light (3108 m/s)


Free Space Loss for Other Antennas

Pt / Pr = (4 )2 (d)2 / GrGt 2

= (d )2/Ar At

= (cd )2 / f 2 ArAt

Gt = gain of the transmitting antenna

Gr = gain of the receiving antenna

At = effective area of the transmitting antenna

Ar = effective area of the receiving antenna


FreeSpaceLoss


Example• Determine the isotropic free space loss at 4 GHz for the

shortest path to a synchronous satellite from earth (35,863 km).— At 4 GHz, the wavelength is (3108)/(4109)=0.075m

— LdB=195.6 dB (computed by loss in isotropic antenna)

• Now consider the antenna gain of both satellite-and ground-based antennas. Typical values are 44 dB and 48 dB, respectively. The free space loss is — LdB = 195.6 - 44 - 48 = 103.6 dB

• Now assume a transmit power of 250 W at the earth station. What is the power received at the satellite antenna?— A power of 250 W translates into 24 dBW, so the power at the

receiving antenna is 24 – 103.6 = -79.6 dBW.


Multipath Interference


Required Reading• Stallings Chapter 4

TUNALIData Communication1 Chapter 4 Transmission Media.

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Transcript of TUNALIData Communication1 Chapter 4 Transmission Media.