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