Multimedia topics

8/7/2019 Multimedia topics

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Transmission Media

EECE 542


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Classes of Transmission Media Guided (uses a physical conduit)

Twisted-pair cable Unshielded Shielded

Coaxial cable

Fiber-optic cable Unguided (no conduit)


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Propagation Speed

Speed of light in a vacuum = 3x10 8 m/s = c

Other media:

I

= dielectric constant of the medium, >= 1 Copper: v = 2.3x10 8 m/s Fiber-optic cable: v = 2x10 8 m/s

I

cv !


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Unshielded Twisted- P air (UT P )Cable

A pair of coated wires is twisted together Why twist the wires together?

Reduces the undesirable effects of noise andcrosstalk (watch out for NEXT)

Inexpensive and easy to use/install EIA Categories:

Category 1: telephone/low data Category 2: voice/data up to 4 Mbps Category 3: 3 twists/ft; 10 Mbps; new phone std. Category 4: 3 twists/ft; 16 Mbps Category 5: 100 Mbps


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Shielded Twisted- P air (ST P )Cable

Each cable is encased in a metal foil or mesh. Less noise susceptibility Less susceptibility to crosstalk from adjacent

cables

More expensive than UT P More difficult to install than UT P


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Coaxial Cable


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Coax cont. Very low susceptibility to noise Cable dimensions and dielectric materialcan be configured for different applications

Radio Government (RG) ratings RG-8, RG-9, and RG-11: Thick Ethernet RG-58: Thin Ethernet and RF Comm. (50 ohm) RG-59: CATV (75 ohms)


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Optical Fiber Cable made of glass or plastic to transmit light Transmission depends on refraction andreflection


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Propagation Modes of Fiber

The physical construction of the fiber canlead to three modes or types of propagation

Different material densities can be used toaffect the reflective and refractive propertiesof the cable


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Multimode, step-index

Multiple beams take different paths Core density remains uniform Reflections occur at the core/claddinginterface due to a step change in density

Considerable distortion and loss


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Multimode, graded-index

Multiple beams, multiple paths Density of the core decreases as itapproaches the cladding interface

No reflections Distortion occurs, but less signal loss


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Single Mode

Light source is highly focused Core density: uniform and much lower

Critical angle is close to 90 degrees (noreflections)

Smaller diameter than multimode

Low distortion and low loss


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Unguided Media Uses free space as the medium Always modulated signals (sinusoidalcarriers)

Ex: WLANs, Bluetooth, cell phones,satellite phones, broadcast TV, wirelessP DAs


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Propagation Time

P ropagation time = Distance / P ropagationSpeed

Determines the amount of data in thechannel (pipe) during a transmission

A major factor in network performance!!!


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An Example A SONET fiber-optic system using STS-48signaling carries data at 2488 Mbps over a

distance of 2000 km. How much data is in thecable at any given time?P rop. Speed = 2x10 8 m/s

so the propagation time =

2000x10 3/2x10 8 = .01 sec.# of bits in the fiber at any time =

2488x10 6 bps x .01 sec. = 24.88 x 10 6 bits

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