IT-101Section001

1、IT-101Section 001Lecture #19Introduction to Information TechnologyOverviewChapter 15Wire and Fiber Transmission SystemsWire as a transmission mediumFiber optics as a transmission mediumIntroductionThere are fundamentally two mediums for information transmission:Wire, fiber optics, etc. (guided elect

2、romagnetic (EM) waves)Air (unguided EM waves)The past two lectures have concentrated on radio communications using air as the transmission mediumNow, we will learn about some important aspects of the various forms of wire and fiber optics for information transmissionAirCableFiber opticsGuidedUnuided

3、Wire as a Transmission MediumWire is currently the most common and versatile medium of transmissionWire based transmission schemes guide electromagnetic waves either between a pair of separate wires or inside a coaxial (coax) arrangementA coax cable has both a center conductor and a second shield co

4、nductorThese conductors are separated by an insulating material, such that the shield conductor entirely surrounds the center conductor All of these wire-based transmission media are called cables, not just the coaxial cableIn the case of non coaxial transmission, the pair of wires may be held eithe

5、r parallel to each other by a stiff insulating material, or individually insulated and twisted around each otherA surrounding shield may be placed around the resulting twisted pair to form a shielded twisted pair (STP)If a surrounding shield is not placed around the twisted pair, then this arrangeme

6、nt is called an unshielded twisted pair (UTP)Parallel wiresUTPSTPCoaxCable characteristicsA cable moves EM waves by providing a channel in which the pair of conductors act like mirrors between which the wave bounces back and forthWhile traversing through the cable however, the wave loses energy and

7、the intensity of the wave diminishes due to physical effectsThis results in a decrease in signal amplitude at the receiving end called attenuationIn other terms, the magnitude of the signal diminishes as it reaches the end of the cable Original signalAttenuated signal1010100110.1010100110.The longer

8、 the cable, the larger the attenuationThe larger (radius) the conductor in the cable, the lower the attenuation (up to some extent)It is desirable to use larger, more expensive cables in situations that require high transmission quality over long distancesHigh transmission quality means that the rec

9、eiver is able to detect correctly if a 1 or a 0 is transmittedIf a signal is highly attenuated at the receiving end, the receiver will not be able to distinguish between the levels of 1 and 0, and this will lead to erroneous transmission of information (remember threshold?) Typical attenuation figur

10、es for various cables:Cable typeSignal attenuation per 1000 ft 100 MHzUTP56 dBSTP37.5 dBCoax (thin ethernet)60 dBCoax (thick ethernet)20 dBCheapExpensiveWhat is a decibel?In electrical engineering, the decibel (abbreviated as dB) is a logarithmic unit used to describe the ratio between two power lev

11、els (or voltage/current levels provided same resistance)Power: unit of measurement is watts (W)dBP = 10 log10 P1/P2 (power ratio)The DecibelIf the input signal power is 2 W and the output signal power is measured to be 2 milliWatts, calculate the power attenuation in dB of the cableOriginal signalAt

12、tenuated signal2W input power2mW output powerExampleThe input signal power is: P1=2 WThe output signal power is: P2=2x10-3 WThe Power attenuation is: dBP = 10 log10 P1/P2 =10 log10 2/(2x10-3)= 10 log10 1000=10 x 3 =30 dBThe signal power has attenuated by 30dB while passing through the cableNote: Sin

13、ce P1/P2 = 1000, we can say that the signal has suffered a power attenuation of 1000 fold, or in other words, by 30 dBLength of cableIf the power attenuation of a length of cable is given to be 15 dB, find the ratio of the input/output power 15=10log10 P1/P2 1.5=log10 P1/P2 P1/P2 =101.5 =31.62 P1/P2

14、 =31.62The calculation above illustrates that signals passing through this length of cable suffer a power attenuation by 31.62 times. Note that this is a ratio! There are no units. If, for example, the input power is 1W, the power at the output of the cable would be 1/31.62=0.0316 W General formula:

15、 if b=logax, then x=abExample 1.5=log10 P1/P2, then P1/P2 =101.5 Note that the dB scale is a logarithmic scale, and is a convenient method to express large ratiosIn the first example, the ratio 1000 was expressed as 30 dBFor example, if the ratio is: 800,000,000, then this expressed in dB is: 89 dB

16、(a much smaller number)RatiodB1010101002010003010,00040100,000501,000,00060Note.ExercisesIf the input signal power is 10mW and the output signal power is measured to be 5 W (micro watts), calculate the power attenuation in dB of the cableIf the power attenuation of a length of cable is given to be 6

17、5 dB, find the ratio of the input/output powerFiber Optics as a Transmission MediumInformation is carried through a fiber optic cable by transmitting pulses of light (which is also an EM wave)!A fiber optic cable is a coaxial arrangement of glass or plastic material of immense clarity (i.e., highly

18、transparent)A clear cylinder of optical material called the core is surrounded by another clear wrapper of optical material called the claddingThese two materials are selected to have different indices of refractionThe fiber is surrounded by a plastic or teflon jacket to protect and stiffen the fibe

19、rLight is guided through the optical fiber by continual reflection from the core-cladding boundaryThis is made possible due to the different refractive indices of the core and cladding materialsThe index of refraction (n) of a material affects the angle by which a light ray is bent while passing thr

20、ough the materialIf the light incident on the core-cladding boundary is at a suitable angle, then the light will be totally reflected from the boundary. This is called total internal reflectionCross section of optical fiber cableCore-cladding boundaryCore and cladding with different indices of refractionAdvantages of fiber optics Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiberImmunity to Noise - Immune