Fundamentals of Telecommunication

ICT – TE - BVF- 4.1

Hassan Mesfer ICT-TE-7

Fundamentals of Telecommunication

ICT – TE - BVF- 4.1

Hassan Mesfer ICT-TE-7

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Lecture 3

Transmission Media

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Overview

• Physical path between transmitter and receiver• Guided - wire• Unguided - wireless• Characteristics and quality of a data

transmission determined by medium and signal• For guided, the medium is more important• For unguided, the bandwidth produced by the

antenna is more important (directionality)• In designing of data transmission systems

— Key concerns are data rate and distance (greater better)

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Design Factors

• Bandwidth—Higher bandwidth gives higher data rate

• Transmission impairments—Attenuation, limits the distance

• Interference— From competing signals in overlapping

frequency bands

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

attenuation

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Electromagnetic Spectrum

Guided Transmission Media

• Twisted Pair• Coaxial cable• Optical fiber• The transmission capacity depends on:

— Distance — Point-to-point or multipoint

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Transmission Characteristics of Guided Media

 

  Frequency Range

Typical Attenuation

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

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W. Stalling, 7th Edition

Point-to-point applications

• Unshielded twisted-pair cable (UTP) is a four-pair wire medium

• Each of the 8 individual copper wires in the UTP cable is covered by insulating material.

• In addition, each pair of wires is twisted around each other.

• This type of cable relies solely on the cancellation effect produced by the twisted wire pairs, to limit signal degradation caused by EMI and RFI.

• CAT 5e is the one most frequently recommended and implemented in installations today.

Unshielded Twisted-Pair Cable

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Twisted Pair

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• Unshielded twisted-pair cable has many advantages. • It is easy to install• Less expensive than other types of networking media.

• However, the real advantage is the size.• Small external diameter, UTP does not fill up wiring ducts as rapidly as other types of cable.

• Disandvantages:• Low data rate• Short range

UTP Pros and Cons

Unshielded Twisted Pair

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Twisted Pair - Applications• Most common• Telephone network

—Between house and local exchange (subscriber loop)

• Within buildings—To private branch exchange (PBX)

• For local area networks (LAN)—10Mbps or 100Mbps

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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 (1MHz)• Limited data rate (100MHz)• Susceptible to interference and noise

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Near End Crosstalk• Coupling of signal from one pair to

another• Coupling takes place when transmit signal

entering the link couples back to receiving pair

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

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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)• Cat 6 - up to 200MHz

• Cat 7 - up to 600MHz

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• Shielded twisted-pair cable (STP) combines the techniques of shielding, cancellation, and twisting of wires.

• Each pair of wires is wrapped in metallic foil. • The four pairs of wires are wrapped in an overall

metallic braid or foil.• STP affords greater protection from all types of

external interference, but is more expensive and difficult to install than UTP.

• The metallic shielding materials in STP need to be grounded at both ends.

Shielded Twisted-Pair Cable

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Shielded Twisted-Pair Cable

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)

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Coaxial Cable• Coaxial Cable consists of a hollow outer cylindrical

conductor that surrounds a single inner wire made of two conducting elements.

• One of these elements, located in the center of the cable, is a copper conductor.

• Surrounding the copper conductor is a layer of flexible insulation.

• Over this insulating material is a copper braid or metallic foil that acts as the second wire in the circuit and as a shield for the inner conductor.

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Coaxial Cable

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Speed and Throughput : 10-100 MbpsAverage cost per node : inexpensiveMedia and connector type : MediumMaximum cable length : 500m

Coaxial Cable

Coaxial Cable Applications• Most versatile medium• Television distribution

—Ariel to TV—Cable TV

• Long distance telephone transmission—Can carry 10,000 voice calls simultaneously—Being replaced by fiber optic

• Short distance computer systems links• Local area networks

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• For LANs, Coaxial Cable offers several

advantages.

—It can be run longer distances than shielded

twisted pair, STP, and unshielded twisted pair,

UTP, cable without the need for repeaters.

—Coaxial Cable is less expensive than fiber-optic

cable, and the technology is well known.

Coaxial Cable Applications

Coaxial Cable - Transmission Characteristics

• Analog—Amplifiers every few km—Closer if higher frequency—Up to 500MHz

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

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• Fiber Optics are cables that are made of optical fibers that can transmit large amounts of information at the speed of light.

Optical Fiber

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Optical Fiber

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Optical Fiber

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Optical Fiber

Optical Fiber - Benefits

• Greater capacity—Data rates of hundreds of Gbps

• Smaller size and weight• Lower attenuation• Electromagnetic isolation• Greater repeater spacing

—10s of km at least

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• Telephones• Internet• LANs - local area networks• CATV - for video, voice and Internet

connections• Long-haul trunks• Metropolitan trunks• Rural exchange trunks• Subscriber loops• Security - closed-circuit TV• Military - everywhere!

Optical Fiber - Applications

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

—Portions of infrared and visible spectrum

• Light Emitting Diode (LED)—Cheaper—Wider operating temp range—Last longer

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

• Wavelength Division Multiplexing

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Wireless Transmission Frequencies• 2G Hz to 40 GHz

— Microwave— Highly directional— Point to point— Satellite

• 30M Hz to 1 GHz— Omnidirectional— Broadcast radio

• 3 x 1011 to 2 x 1014

— Infrared— Local

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Broadcast Radio• Omnidirectional• FM radio• UHF and VHF television• Line of sight• Suffers from multipath interference

—Reflections

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Infrared• Modulate noncoherent infrared light• Line of sight (or reflection)• Blocked by walls• e.g. TV remote control, IRD port

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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• Signal reflected from ionosphere layer of upper

atmosphere• (Actually refracted)

—Line of sight• Above 30Mhz• May be further than optical line of sight due to refraction• More later…

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Ground Wave Propagation

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Sky Wave Propagation

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b) Sky-wave propagation 2 -30 MHz

Line of Sight Propagation

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Line of Sight Transmission• Free space loss

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

• Atmospheric Absorption—Water vapour 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

FreeSpaceLoss

Multipath Interference

Microwave line of sight

Mobile radio

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