IT-101

Lecture #8

Introduction to Information Technology

OverviewWireless:Wireless: Chapter 16Radio-Frequency and Satellite Systems

Satellites and Orbits GPS

Wired:Wired: Chapter 15Wire and Fiber Transmission Systems

Wire as a transmission medium Fiber-optic cable

Satellites and orbits

Satellites are wireless transmitters/receivers that are launched into orbit by special rockets

The first satellite to be launched into orbit was the Russian Sputnik-1Sputnik-1 satellite in 1957

Communication satellites provided the first long-distance, wide-bandwidth communication service

Telstar 1Telstar 1 was the first commercial communications satellite to be launched into orbit in 1962

Satellites can be distinguished into 3 primary categories, depending on the type of orbit they reside on:

LEOLEO MEO MEO GEOGEO

LEO: Low Earth Orbit satellites have a small area of coverage. They are positioned in an orbit approximately 3000km from the surface of the earth They complete one orbit in approximately 90 minutes to

a few hours The large majority of satellites are in low earth orbit The Iridium system utilizes LEO satellites The satellite in LEO orbit is visible to a point on the

earth for a very short time MEO: Medium Earth Orbit satellites have orbital altitudes

between 3,000 and 30,000 km. They are commonly used in navigation systems such as

GPS They complete one orbit in approximately 2 to 12 hours

GEO: Geosynchronous (Geostationary) Earth Orbit satellites are positioned over the equator. The orbital altitude is around 30,000-40,000 km They complete one orbit every 24 hours. This causes

the satellite to appear stationary with respect to a point on the earth, allowing one satellite to provide continual coverage to a given area on the earth's surface

One GEO satellite can cover approximately 1/3 of the world’s surface

They are very commonly used in communication systems

Let’s look at a GEO animation: http://www.jpl.nasa.gov/basics/bsf5-1.htm

Radio communications system design

There are a few basic parameters in the design of RF There are a few basic parameters in the design of RF systems:systems:

Transmitter power Transmitter frequency Receiver sensitivity Desired bit rate Limitations on antenna size, location, etc. Desired transmission distance

The Global Positioning System (GPS) GPS is funded and controlled by the Department of Defense (DoD) Some applications of GPS include:

Aircraft navigation Marine navigation Driving Surveying Farming

Although there are many thousands of civil users of GPS worldwide, the system was originally designed for the U. S. military

GPS provides special satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position and velocity

Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver

For around $100, you can get a hand held gadget that will tell you exactly what point you are on the earth

Source:http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html

How does GPS work?

GPS is based on the concept of triangulationtriangulation

On the surface of the earth, if you know your distance from a given point, you know that you are somewhere on a circle with that point in the center

If, at the same time, you know your distance from a different point, you know that you are on another different circle

These two circles intersect in at most 2 points

With a known distance from a third point, you can identify your location, since there is only one point where the 3 circles intersect

With GPS, you can identify your location in 3 dimensions, so the circles are replaced by spheres

The orbital period of the GPS satellites, which reside on MEO is around 12 hours

The satellites move with respect to the receivers on the earth, but at a slower speed compared to LEO’s

Wire and Fiber Transmission Systems

There are fundamentally two different ways to transmit information:

Wireless systems using free space as the transmission medium

Wired systems using copper wire, coax cable, fiber-optic cable, etc.

Air

Coax cable

Fiber optic cable

Wired

Wireless

Examples of wireless and wired transmission systems

Wire as a Transmission Medium

Wired transmission systems are currently the most common and versatile transmission systems

Wire based transmission schemes carry signals within 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

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

When a signal is traversing through a cable, the signal loses energy, and its intensity is diminished

This results in a decrease in signal amplitude at the receiving end, which is termed as attenuationattenuation

In other terms, the magnitude of the signal diminishes as it reaches the end of the cable

Original signal Attenuated signalCable

The longer the cable, the larger the attenuation!The longer the cable, the larger the attenuation! The larger (radius) the conductor in the cable, the The larger (radius) the conductor in the cable, the

lower the attenuation (up to some extent)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

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

What is a decibel? In electrical engineering, the decibel (abbreviated as dB) is a

logarithmic unit used to describe the ratio between two power power levelslevels (or voltage/current levels provided same resistance)

Power (P): unit of measurement is watts (W)

dBP = 10 log10 P2/P1 (power ratio)

The Decibel

Original signal Attenuated signalP1= input power

P2=output power

Cable

Fiber Optics as a Transmission Medium

Information is carried through a fiber optic cable by transmitting Information is carried through a fiber optic cable by transmitting pulses of light pulses of light

An electrical signal arriving at one end of the fiber optic transmission system can be converted to light by a laser or LED, and sent through the fiber optic cable

A receiver, such as a photodetector at the other end of the fiber optic cable can be used to convert the light back into an electrical signal

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 corecore is surrounded by another clear wrapper of optical material called the claddingcladding

These two materials are selected to have different indices of indices of refraction refraction

The fiber is surrounded by a plastic or teflon jacketteflon jacket to protect and stiffen the fiber

Principle of fiber optic cables

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 index of refraction 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 total internal reflectionreflection

Cross section of optical fiber cable

Core-cladding boundary

Core and cladding with different indices of refraction

Advantages and disadvantages of fiber optic cables

Advantages:Advantages: High Data Rate (Gbps) – Fiber optic cables can support very high data rates 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 – Very difficult to “tap into” Low attenuation- Fibers can be made to have only 0.2 dB/km of

attenuation, so repeaters can be spaced very far apart Reliability - More resilient than copper in extreme environmental conditions Size - Lighter and more compact than copper

Disadvantages:Disadvantages: Cost: Cost of interfacing equipment necessary to convert electrical signals

to optical signals Splicing: Splicing (Linking together) fiber optic cables is also more difficult