SIMS-201

Wire and Fiber Transmission Systems

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Overview

Chapter 15Wire and Fiber Transmission

Systems Wire as a transmission medium Fiber optics as a transmission

medium

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Introduction There are fundamentally two mediums for

information transmission: Guided electromagnetic (EM) waves - wire, fiber

optics, etc. Unguided EM waves - air

The past two lectures have concentrated on radio communications using air as the transmission medium

Next, we will learn about some important aspects of the forms of wire and fiber optics used for information transmission

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Air

Cable

Fiber optics

Guided

Unguided

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Wire as a Transmission Medium

Wire is currently the most common and versatile medium of transmission

All wire-based transmission media are called cables

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

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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)

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Parallel wires

UTP

STP

Coax

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Cable characteristics A cable moves EM waves by providing a channel. The EM

waves traverse the cable moving through the conductors. The EM waves are confined in this way, as they interact with the free electrons in the conductor, which are responsible for guiding the waves.

While traversing through the cable however, due to physical effects, the wave loses energy and the intensity of the wave diminishes, the farther it goes.

This results in a decrease of the signal amplitude at the receiving end – called attenuation

In other words, the magnitude of the signal diminishes as it reaches the end of the cable Original signal Attenuated signal

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The longer the cable, the larger the attenuation

The larger the conductor in the cable (radius), 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

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Typical attenuation figures for various cables:

Cable type Signal attenuation per 1000 ft @100

MHz

UTP 56 dB

STP 37.5 dB

Coax (thin ethernet)

60 dB

Coax (thick ethernet)

20 dB

Cheap

Expensive

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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

Power: unit of measurement is watts (W) dB = 10 log10 P1/P2 (power ratio)

The Decibel

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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 signal2W input power

2mW output power

Example

The input signal power is: P1=2 W

The output signal power is: P2=2x10-3 W

The 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: Since P1/P2 = 1000, we can say that the signal has suffered a power

attenuation of 1000 times, or in other words, by 30 dB

Length of cable

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Logarithm (log)

How do we know that 10 log10 1000 = 10 x 3 ?

Logarithmic and Exponential Functions Logarithmic and exponential functions are inverses of each other:

If y = logbx then x = by

In words, logb x is the exponent you put on base b to get x. lol So,

If x = log10 1000 then 1000 = 10x and how much is x here? 10 to the power of what is

equal to 1000? It would be 10 to the power of 3 (10x10x10) = 1000.

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If 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 = 31.62P1/P2 = 31.62

The 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

Logarithm: if b = logax then x = ab

Example

1.5 = log10 P1/P2 then P1/P2 = 101.5

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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)

Ratio dB

1 0

10 10

100 20

1000 30

10,000 40

100,000 50

1,000,000 60

Note..

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Exercises

If the input signal power is 10mW (milliWatts) 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

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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

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Cross section of optical fiber cable

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Core-cladding boundary

Core and cladding with different indices of refraction

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Let us look at some useful fiber optics demos:

http://www.eas.asu.edu/~palais/demos/demos.htm

http://electronics.howstuffworks.com/fiber-optic2.htm

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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.