channels of communication hl – option f.4 mr. jean april 22 nd, 2014
TRANSCRIPT
Channels of CommunicationHL – Option F.4
Mr. Jean
April 22nd, 2014
The plan:
• Video clip of the day
• Decibel Ratios – Further explanation of “Power”
• Methods of communication
• Practice Questions for Topic F– Solutions– Markers notes
Decibel:
• The decibel (dB) is a logarithmic unit used to express the ratio between two values of a physical quantity, often power or intensity.
• The decibel is commonly used as a measure of gain or attenuation, the ratio of input and output powers of a system.
• The number of decibels is ten times the logarithm to base 10 of the ratio of the two power quantities
The ratio of a power value P1 to another power value Po is represented by Ldb, that ratio expressed in decibels, which is calculated using the formula
Voltage and power increases:
• In electrical circuits, dissipated power is typically proportional to the square of voltage or current when the impedance is held constant.
• Taking voltage as an example, this leads to the equation:
Ways of Communicating
• Wire Pairs– This is the cheapest form of cable, they are 2
insulated wires that are run together– When the wires are twisted together, the two
signals (one in, one out) cancel out any EM interference from neighboring wires
– Because they are thin, attenuation due to resistivity is larger
• Decent for short range networking
Ways of Communicating
• Coaxial Cable– Made up of a copper wire, surrounded by an
insulator, surrounded by a conducting sheath, surrounded by an insulator
• Primarily used for TV signals and internet• Copper sheath prevents attenuation from radiation
and interference from outside signals
Ways of Communicating
• Optical Fiber– See F.3– Advantages over Coax
• Great for transmitting digital data• Higher bit rate• Smaller Diameter• Low Noise
– Disadvantages• Can be difficult to access and repair• Amplifiers and reshapers have a finite life
Fiber Optics:
• Clear Curve Fibers: – https://www.youtube.com/watch?v=mUBRjiVh
JTs
Ways of Communicating
• Radio Waves– See F.1– With radio signals, keep in mind that the
signal is always analogue but it can transmit digital information
– Advantage is that no wires are necessary to transmit data
Geostationary Satellites
• Orbit the Earth at the equator and has an orbital period of one sidereal day– Time it takes the Earth to make one complete
rotation around its axis– This means that the satellite is always above
the same geographic position on Earth– Altitude is about 3.6 x 107 m up– Currently there are 5 or 6 at different positions
Geostationary Satellites
• Used for more direct transmission of signals– One satellite can get a signal to where it
would take many grounded antennas to get to– The frequencies range from
.1 – 10 GHz• Uplink and down link
frequencies must be different to avoid interference and unwanted noise
Polar-Orbiting Satellites
• A satellite that is in orbit around the Earth in a N/S or S/N path.– The satellite passes over both poles in its
orbit– Path is nearly normal to
the equator– Not geosynchronous and
so has a much lower altitude
– Passes along a different longitude with each trip
Comparing Satellites• Benefits of GS
– always above the same position on Earth so there is no need for tracking
– That location will always have access to the satellite
• Benefits of PS– Lower altitude so the signal doesn’t need as
much power to transmit• Also cheaper to put into orbit due to altitude
– GS can’t transmit more than about 60-70 deg. N or S of the equator
Issues with Communication
• Moral, ethical, economic, environmental