it-101 lecture #8 introduction to information technology
TRANSCRIPT
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