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Data and Computer Communications

Lecture 4Transmission Media

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OverviewGuided - wireUnguided - wirelessCharacteristics and quality determined by

medium and signalFor guided, the medium is more importantFor unguided, the bandwidth produced by

the antenna is more importantKey concerns are data rate and distance

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Design Factors Bandwidth

Higher bandwidth gives higher data rate

Transmission impairments Attenuation Order of losses: Twisted pair, coaxial then fibre

Interference Overlapping of frequencies in unguided medium Emanations from adjacent cables in guided. (Use

screening)

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

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Electromagnetic Spectrum

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Guided Transmission MediaTwisted PairCoaxial cableOptical fiber

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Twisted Pair

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Twisted Pair - ApplicationsMost common mediumTelephone 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 - Pros and ConsCheapEasy to work withLow data rateShort range

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Twisted Pair - Transmission CharacteristicsAnalog

Amplifiers every 5km to 6kmDigital

Use either analog or digital signals repeater every 2km or 3km

Limited distanceLimited bandwidth (1MHz)Limited data rate (100MHz)

1 Ghz at short distances & new encoding schemes

Susceptible to interference and noise

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Unshielded and Shielded TPUnshielded 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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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 (1 GHz using 4 pair & compression) Data grade cable Commonly pre-installed in new office buildings Twist length 0.6 cm to 0.85 cm

Cat 6 200 MHz to 250MHz 1 Ghz uncompressed: 4 x 250 Mhz http://www.rwonline.com/reference-room/wired-4-sound/rwf_lampen_2.shtml

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Attenuation & Near End CrosstalkAttenuation

Generally logarithmic & expressed as dB per unit length

Near End Cross talk 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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Coaxial Cable

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Coaxial Cable ApplicationsMost versatile mediumTelevision 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 linksLocal area networks

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Coaxial Cable - Transmission CharacteristicsAnalog

Amplifiers every few km Closer if higher frequency Usuable spectrum up to 500MHz

Digital Repeater every 1km Closer for higher data rates

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Optical Fiber

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Optical Fiber - BenefitsGreater capacity

Data rates of hundreds of Gbps

Smaller size & weightLower attenuationElectromagnetic isolationGreater repeater spacing

10s of km at least

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Optical Fiber - ApplicationsLong-haul trunks

1500km, 20 – 60k voice channelsMetropolitan trunks

12 km, 100k channelsRural exchange trunks

40 – 160Km, 5k voice channelsSubscriber loops

Voice data cables leased by corporate clientsLANs

100Mbps – 1 Ghz

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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 Form of FDM Separate coour, different channel 100 beams @ 10Gbps = 1 trillion bps in Bell labs 80 channel @ 10Gbps available

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Optical Propagation modes Step-index multimode

Good for short distance Problem with multi propagation paths

Single Mode Single path, no distortion Long distance Supports different wavelengths

Graded Index multimode Intermediate of top two Two layers with different refractive index Used commonly in LANs Supports different wavelengths

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Optical Fiber Transmission Modes

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Wireless TransmissionUnguided mediaTransmission and reception via antennaDirectional

Focused beam Careful alignment required

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

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FrequenciesIn term of data transmission, we are are interested

in 3 ranges 2GHz to 40GHz

Microwave Highly directional Point to point Satellite

30MHz to 1GHz Omnidirectional Broadcast radio

3 x 1011 to 2 x 1014

Infrared Local

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Terrestrial MicrowaveParabolic dishFocused beamLine of sightLong haul telecommunicationsHigher frequencies give higher data rates

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Satellite MicrowaveSatellite is relay stationSatellite receives on one frequency,

amplifies or repeats signal and transmits on another frequency

Requires geo-stationary orbit Height of 35,784km

TelevisionLong distance telephonePrivate business networks

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Broadcast RadioOmnidirectionalFM radioUHF and VHF televisionLine of sightSuffers from multipath interference

Reflections

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InfraredModulate noncoherent infrared lightLine of sight (or reflection)Blocked by wallse.g. TV remote control, IRD port

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Required ReadingStallings Chapter 4