IT-101 Lecture #8 Introduction to Information Technology

Overview Wireless:  Chapter 16 Radio-Frequency and Satellite Systems Satellites and Orbits GPSWired:  Chapter 15 Wire 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 Sputnik-1  The first satellite to be launched into orbit was the Russian Sputnik-1 satellite in 1957  Communication satellites provided the first long-distance, wide-bandwidth communication service  Telstar 1  Telstar 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: LEO LEO MEO MEO GEO GEO

 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:

Radio communications system design There are a few basic parameters in the design of RF 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:

How does GPS work? triangulation  GPS is based on the concept of triangulation  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 attenuation  This results in a decrease in signal amplitude at the receiving end, which is termed as attenuation  In other terms, the magnitude of the signal diminishes as it reaches the end of the cable Original signalAttenuated signal Cable

 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

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? power levels 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 (P): unit of measurement is watts (W)  dB P = 10 log 10 P 2 /P 1 (power ratio) The Decibel Original signal Attenuated signal P 1 = input power P 2 =output power Cable

Fiber Optics as a Transmission Medium  Information is carried through a fiber optic cable by transmitting 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) core cladding  A clear cylinder of optical material called the core is surrounded by another clear wrapper of optical material called the cladding indices of refraction  These two materials are selected to have different indices of refraction teflon jacket  The fiber is surrounded by a plastic or teflon 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 index of refraction  The index of refraction of a material affects the angle by which a light ray is bent while passing through the material total internal reflection  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 and disadvantages of fiber optic cables 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 copperDisadvantages:  Cost: Cost of interfacing equipment necessary to convert electrical signals to optical signals  Splicing: Splicing (Linking together) fiber optic cables is also more difficult