IT-101 Section 001 Lecture #19 Introduction to Information Technology

 Overview Chapter 15 Wire and Fiber Transmission Systems Wire as a transmission medium Fiber optics as a transmission medium

 Introduction There are fundamentally two mediums for information transmission: Wire, fiber optics, etc. (guided electromagnetic (EM) waves) Air (unguided EM waves) The past two lectures have concentrated on radio communications using air as the transmission medium Now, we will learn about some important aspects of the various forms of wire and fiber optics for information transmission

Air Cable Fiber optics Guided Unuided

 Wire as a Transmission Medium Wire is currently the most common and versatile medium of transmission Wire based transmission schemes guide electromagnetic waves either between a pair of separate wires or inside a coaxial (coax) arrangement A coax cable has both a center conductor and a second shield conductor These 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 cable

In the case of non coaxial transmission, the pair of wires may be held either parallel to each other by a stiff insulating material, or individually insulated and twisted around each other A 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 arrangement is called an unshielded twisted pair (UTP)

Parallel wires UTP STP Coax

Cable characteristics A cable moves EM waves by providing a channel in which the pair of conductors act like mirrors between which the wave bounces back and forth While traversing through the cable however, the wave loses energy and the intensity of the wave diminishes due to physical effects This results in a decrease in signal amplitude at the receiving end called attenuation In other terms, the magnitude of the signal diminishes as it reaches the end of the cable Original signalAttenuated signal

The longer the cable, the larger the attenuation The 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 distances High transmission quality means that the receiver is able to detect correctly if a 1 or a 0 is transmitted If 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 figures for various cables: Cable typeSignal attenuation per 1000 MHz UTP56 dB STP37.5 dB Coax (thin ethernet)60 dB Coax (thick ethernet)20 dB Cheap Expensive

What is a decibel? In electrical engineering, the decibel (abbreviated as dB) is a logarithmic unit used to describe the ratio between two power levels (or voltage/current levels provided same resistance) Power: unit of measurement is watts (W) dB P = 10 log 10 P 1 /P 2 (power ratio) The Decibel

If 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 cable Original signal Attenuated signal 2W input power 2mW output power Example The input signal power is: P 1 =2 W The output signal power is: P 2 =2x10 -3 W The Power attenuation is: dB P = 10 log 10 P 1 /P 2 =10 log 10 2/(2x10 -3 ) = 10 log =10 x 3 =30 dB The signal power has attenuated by 30dB while passing through the cable Note: Since P 1 /P 2 = 1000, we can say that the signal has suffered a power attenuation of 1000 fold, or in other words, by 30 dB Length of cable

If the power attenuation of a length of cable is given to be 15 dB, find the ratio of the input/output power 15=10log 10 P 1 /P 2 1.5=log 10 P 1 /P 2 P 1 /P 2 = =31.62 P 1 /P 2 =31.62 The calculation above illustrates that signals passing through this length of cable suffer a power attenuation by 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= W General formula: if b=log a x, then x=a b Example 1.5=log 10 P 1 /P 2, then P 1 /P 2 =10 1.5

Note that the dB scale is a logarithmic scale, and is a convenient method to express large ratios In the first example, the ratio 1000 was expressed as 30 dB For example, if the ratio is: 800,000,000, then this expressed in dB is: 89 dB (a much smaller number) RatiodB , , ,000,00060 Note..

Exercises If the input signal power is 10mW and the output signal power is measured to be 5 μW (micro watts), calculate the power attenuation in dB of the cable If the power attenuation of a length of cable is given to be 65 dB, find the ratio of the input/output power

 Fiber Optics as a Transmission Medium Information 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 transparent) A clear cylinder of optical material called the core is surrounded by another clear wrapper of optical material called the cladding These two materials are selected to have different indices of refraction The fiber is surrounded by a plastic or teflon jacket to protect and stiffen the fiber Light is guided through the optical fiber by continual reflection from the core-cladding boundary This is made possible due to the different refractive indices of the core and cladding materials The index of refraction (n) of a material affects the angle by which a light ray is bent while passing through the material If 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 reflection

Cross section of optical fiber cable

Core-cladding boundary Core and cladding with different indices of refraction

Advantages of fiber optics Much Higher Bandwidth (Gbps) - Thousands of channels can be multiplexed together over one strand of fiber Immunity to Noise - Immune to electromagnetic interference (EMI). Safety - Doesn’t transmit electrical signals, making it safe in environments like a gas pipeline. High Security - Impossible to “tap into.” Less Loss - Repeaters can be spaced 75 miles apart (fibers can be made to have only 0.2 dB/km of attenuation) Reliability - More resilient than copper in extreme environmental conditions. Size - Lighter and more compact than copper. Flexibility - Unlike impure, brittle glass, fiber is physically very flexible. Disadvantages include the cost of interfacing equipment necessary to convert electrical signals to optical signals. (optical transmitters, receivers) Splicing fiber optic cable is also more difficult.