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Transmission Media..

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Transmission medium is the physical path between the transmitter and receiver.

Factors to Select Transmission Media
Ø  Data Rate and Bandwidth
Ø  Distance and Attenuation
Ø  Interference Characteristics
Ø  Number of receivers
Ø  Cost - Remember cabling is a long term investment!

Guided Media Sub-types
Ø  Unshielded Twisted Pair (UTP) Cable
Ø  Shielded Twisted Pair (STP) Cable
Ø  Coaxial Cable
Ø  Fiber Optic Cable
Un-guided Media Sub-types
Ø  Terrestrial microwave transmission
Ø  Satellite transmission
Ø  Broadcast radio
Ø  Infrared
Twisted Pair Wires
Ø  Consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs
Ø  Often used at customer facilities and also over distances to carry voice as well as data communications
Ø  Low frequency transmission medium
Twisted Pair Types
Two varieties
Ø  STP (shielded twisted pair)
The pair is wrapped with metallic foil or braid to insulate the pair from electromagnetic interference
Ø  UTP (unshielded twisted pair)
Each wire is insulated with plastic wrap, but the pair is encased in an outer covering
Shielded Twisted Pair (STP) Cable
Ø  A disadvantage of UTP is that it may be susceptible to radio and electrical frequency interference (RFI, EFI).
Ø  Shielded twisted pair (STP) is suitable for environments with electrical interference; however, the extra shielding can make the cables quite bulky.
Ø  Shielded twisted pair is often used on networks using Token Ring topology.
Ø  More expensive and harder to work with it.
Coaxial Cable
Coaxial cabling has a single copper conductor at its center. A plastic layer provides insulation between the center conductor and a braided metal shield. The metal shield helps to block any outside interference from fluorescent lights, motors, and other computers.
Coax Advantages
Ø  Higher bandwidth
-       400 to 600MHz
-       up to 10,800 voice conversations
Ø  Can be tapped easily (pros and cons)
Ø  Much less susceptible to interference than twisted pair
Ø  Greater cable lengths between network devices than twisted pair cable.
Coax Disadvantages
Ø  High attenuation rate makes it expensive over long distance
Ø  Bulky - coaxial cabling is difficult to install

Fiber Optic Layers
Consists of three concentric sections
Facts about Fiber Optic Cables
Ø  Outer insulating jacket is made of Teflon or PVC.
Ø  Kevlar fiber helps to strengthen the cable and prevent breakage.
Ø  A plastic coating is used to cushion the fiber center.
Ø  Center (core) is made of glass or plastic fibers

Optical Fiber - Transmission Characteristics
Act as wave guide for 1014 to 1015 Hz
-       Portions of infrared and visible spectrum
Light Emitting Diode (LED)
-       Cheaper
-       Wider operating temp range
-       Last longer
Injection Laser Diode (ILD)
-       More efficient
-       Greater data rate
Wavelength Division Multiplexing
Fiber Optic Types
Ø  multimode step-index fiber
-       the reflective walls of the fiber move the light pulses to the receiver
Ø  multimode graded-index fiber
-       acts to refract the light toward the center of the fiber by variations in the density
Ø  single mode fiber
-       the light is guided down the center of an extremely narrow core
Wireless (Unguided Media)
Ø  Transmission
-       Transmission and reception are achieved by means of an antenna
Ø  Directional
-       transmitting antenna puts out focused beam
-       transmitter and receiver must be aligned
Ø  Omni directional
-       signal spreads out in all directions
-       can be received by many antennas
Wireless (Unguided Media) Frequencies
Ø  Three general ranges of frequencies
Ø  2GHz to 40GHz microwave frequencies
-       Microwave
-       Highly directional
-       Point to point
-       Satellite
Ø  30MHz to 1GHz
-       Omni directional
-       Broadcast radio
Ø  3 x 1011 to 2 x 1014
-       Infrared
Terrestrial Microwave Transmission
-       Uses the radio frequency spectrum, commonly from 2 to 40 GHz
-       Transmitter is a parabolic dish, mounted as high as possible
-       Used by common carriers as well as by private networks
-       Requires unobstructed line of sight between source and receiver
-       Curvature of the earth requires stations (called repeaters) to be ~30 miles apart
-       Distance between antennas:d = 7.14 (Kh)1/2 , d = distance in km, h is antenna height in meters, K = constant = 4/3
Satellite Microwave Transmission
Ø  A microwave relay station in space
Ø  Can relay signals over long distances
Ø  Geostationary satellites
-       remain above the equator at a height of 22,300 miles (geosynchronous orbit)
-       travel around the earth in exactly the time the earth takes to rotate
Principal Satellite Transmission Bands
Ø  C band: 4(downlink) - 6(uplink) GHz   (the first to be designated)
Ø  Ku band: 12(downlink) -14(uplink) GHz (rain interference is the major problem)
Ø  Ka band: 19(downlink) - 29(uplink) GHz (equipment needed to use the band is still very expensive)

Satellite Advantages
Ø  Can reach a large geographical area
Ø  High bandwidth
Ø  Cheaper over long distances
Satellite Disadvantages
Ø  High initial cost
Ø  Susceptible to noise and interference
Ø  Propagation delay (0.25 sec) - requires sophisticated flow control
Infrared
Ø  Transceivers operate with line of sight or reflection from light-colored surface
Ø  Modulate noncoherent infrared light
Ø  e.g. TV remote control, IRD port
Ø  Advantages
-       Does not penetrate walls - enhanced security
-       No licensing of frequencies
Ø  Disadvantages
-       Operate on limited distances



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