Transmission MediaTransmission Media

• Physical layer– Transportation of a raw bit stream.

• Choosing the physical media– Bandwidth, delay, cost, …

• Two flavors– Guided media

• e.g. Copper wire, fiber optics, …

– Unguided media• e.g. Radio, lasers, ...

Magnetic mediaMagnetic media

• Physical transportation of magnetic or optical disk.– e.g. Exabyte tape:

• 50x50x50 cm box can hold 1000 7 GB tapes.

• Fed Ex anywhere is US in 24 hrs.

• Effective bandwidth: 648 Gbps.

• 1000 tapes = $5000, reuse > 10 times, $200 shipping.

• $700 for 7000 GB, about 10 cents per GB.

• Large propagation delay.

Two-wire open linesTwo-wire open lines

• Each wire is insulated from the other and are open to free space.

• Connected equipment < 50m apart, < 19.2 kbps.• Signal encoded using voltage or current level relative to some

ground reference.• Used to connect data terminal equipment (DTE) to data

communication equipment (DCE) (e.g. modem).• Wires enclosed in

– single protected multicore cable, or– flat ribbon cable.

Problems with Problems with two wire open linestwo wire open lines

• Crosstalk– Cross coupling of electrical signals between wires

caused be capacitive coupling.

• Susceptible to noise due to open structure.– Noise primarily from electromagnetic radiation

from other electrical signal sources.– May amplify signal on only one wire creating an

additional difference signal.• Solution: twist the two lines together.

Twisted pairTwisted pair• Oldest and most common transmission medium.

– Connects most telephones to the telephone equipment.

– Usually many pairs are bundled together.

• Improved immunity to spurious noise.– i.e. Noise picked up by both wires.

• Reduced crosstalk.• Used for both analog and digital transmission.• Bit rates depend on thickness and distance of wire.

– ~1Mbps, < 100m

• Shielded and unshielded twisted pair.

Problems with twisted pairProblems with twisted pair

• Skin effect– At high bit rates the current tends to flow on the

outer surface of the wire.• Uses less of the available cross section.

• Increases electrical resistance of wires for higher frequency signals increasing attenuation.

• More signal power is lost due to radiation effects.

• Solution: – More sophisticated driver and receiver electronics.– Another transmission medium.

Coaxial cableCoaxial cable• Signal and ground wire

– Solid center conductor running coaxially inside a solid (usually braided) outer circular conductor.

– Center conductor is shielded from external interference signals.

Properties of coaxial cableProperties of coaxial cable

• Better shielding allows for longer cables and higher transfer rates.

• 100 m cables– 1 to 2 Gbps feasible (modulation used)– 10 Mbps typical

• Used for long haul routes by Phone Co.– Mostly replaced now by optical fiber.

• Still widely used for Cable TV and LANs.

• Baseband vs. broadband coax.

Fiber OpticsFiber Optics

• Optical fiber cable carries the transmitted information in the form of a fluctuating beam of light in a glass fiber.

• Light has much wider bandwidth than electrical waves.• Light waves are immune to EM interference and

crosstalk.– Good to use in electrically noisy environment.

• Achievable bandwidth > 50 Tbps.– Current limitation: electrical/light interface (1 Gbps).

Components of an Components of an optical transmission systemoptical transmission system

• 3 components1. Light source

2. Transmission medium

3. The detector

• Light means a 1 bit, no light means a 0 bit.• Transmitter LED or injection laser diode.• Detector (photodiode or photo transistor) generates an electrical

pulse when light falls on it.• Unidirectional data transmission system.

– Electrical signal to light signal and back again.

Trapping the light inside the fiberTrapping the light inside the fiber• Light is reflected and refracted at the silica/air boundary.

– Amount of refraction depends on the silica and air indices of refraction.

– If the angle of incidence is above critical value then total internal reflection is achieved.

Modes of fiberModes of fiber• Fiber consists of two parts: the glass core and glass cladding with a lower

refractive index.• Light propagates in 1 of 3 ways depending on the type and width of the core

material.– Multimode stepped index fiber

• Both core and cladding have different but uniform refractive index.• Relies on total internal reflection; Wide pulse width.

– Multimode graded index fiber• Core has variable refractive index (light bends as it moves away from core). • Narrow pulse width resulting in higher bit rate.

– Singlemode fiber (> 100 Mbs)• Width of core diameter equal to a single wavelenth.

Fiber cablesFiber cables

• Multimode: diameter of core is ~50 microns.– About the same as a human hair.

• Single mode: diameter of core 8-10 microns.

• They can be connnected by connectors, or by splicing, or by fusion.

Fiber vs. copperFiber vs. copper• Fiber (pros)

– Higher bandwidth, – Lower attenuation, – Immune to electromagnetic noise and corrosive chemicals,– Thin and lightweight,– Security (does not leak light, difficult to tap).

• Fiber (cons)– Not many skilled “fiber engineers,”– Inherently unidirectional,– Fiber interfaces are expensive.

Wireless transmissionWireless transmission

• Electromagnetic waves are produced by moving electrons and can propagate through free space.– Frequency: oscillations per second of EM wave.– Wavelength: distance between consecutive maxima.– Speed of light: 3 x 108 m/s in a vacuum.

• EM waves can be broadcast/received via antenna attached to an electrical circuit.

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Electromagnetic spectrumElectromagnetic spectrum

Frequency bands and data rateFrequency bands and data rate

• Amount of info an EM wave can carry is related to its bandwidth.– Can encode 3-8 bits per cycle with EM waves.– Data rate deduced from width of frequency

band.– FCC: who gets to use what frequency band.

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Radio TransmissionRadio Transmission• Radio waves

– Easy to generate, travel long distances, and penetrate buildings easily.

– Omnidirectional.– Low frequencies

• Pass through obstacles well,• Quick power drop off (e.g. 1/r2 in air).

– High frequencies• Travel in straight lines and bounce off obstacles.• Absorbed by rain.

– Subject to electrical interference

Microwave transmissionMicrowave transmission

• Microwave waves– Travel in straight lines and thus can be narrowly

focused. • Easy to avoid interference with other microwaves.

– Parabolic antenna is used to concentrate the energy (improves SNR).

– More popular before fiber.– Waves do not pass through buildings.– Multiple towers used as repeaters.

Infrared WavesInfrared Waves

• Short range communication.– e.g. Remotes on VCR’s and TV’s.

• Directional.• Do not pass through walls.

– Behaves more like visible light.

• Can be used for LANs– indoors only.

• Can just use visible unguided light (lasers).