tranmission media.pdf

8/18/2019 tranmission media.pdf

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Transmission medium

• A transmission medium is any material substance which

can propagate waves or energy.

•Electromagnetic radiation can be transmitted through

media such as optical fiber, twisted pair wires, coaxialcable, dielectric-slab waveguides. They may also pass

through any physical material which is transparent to the

specific wavelength, such as water, air, glass, or concrete.

Electromagnetic waves do not require a transmissionmedium unlike mechanical waves, and so can travel

through vacuum of free space.


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•Guided (or bounded)

Waves are guided along a solid medium

such as a transmission line.

• Wireless (or unguided)

Transmissions and receptions are achievedby means of an antenna.

For telecommunications purposes in the United States,

Federal Standard 1037C, transmission media are classified

as one of the following:


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Guided or Bounded

Parallel two-wire line

•consists of two wires that are generally spaced from 2 to 6 inches apart

by insulating spacers.

•most often used for power lines, rural telephone lines and telegraph

lines.

•sometimes used as a transmission line between a transmitter and an

antenna or between and antenna and a receiver.

•Has a simple construction.

•High radiation losses and electrical noise pick-up because of lack of

shielding.


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Two-wire

Ribbon Type

Line

•also known as “Twin Lead”. •commonly used to connect a televisionreceiving antenna to a home television set.•Same as two-wire open line except uniform

spacing is assured.•the wires are embedded in a low-loss dielectric.(polyethylene).


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Twisted Pairs

conductorsInsulating jackets

•Consists of two insulated wires twisted together to form a

flexible line without the use of spacers.

•Not used for transmitting high frequency because of high

dielectric loss.

•types of twisted pair including STP, UTP, and plenum. STP

means shielded twisted pair, UTP means unshielded twisted

pair and plenum wiring is non toxic when burning.

•Twisted pair wiring carries the data in electrons. The reason

behind twisting the pairs is to cancel out interference. These

are the most common cables used in networks and carry the

signal up to 100 meters.


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Shielded

Cable Pair

• consists of parallel conductors separated from each other and

surrounded by a solid dielectric. The conductors are containedwithin a braided copper tubing that acts as an electrical shield.

Rubber cover protects the line from moisture and mechanical

damage.

• Advantage- the conductors are balanced to ground; that is,

the capacitance between the wires is uniform throughout thelength of the line.

• The uniform spacing braided copper shield isolates the

conductors from stray magnetic fields.


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Coaxial Cable

• Two types: rigid (air) fill coaxial line and flexible (solid) coaxial line.•Used extensively for high frequency application to reduce losses and to

isolate transmission paths.

•it has two conductors, one wire in center and a conductive sheath around it,

that share a "common axis".•example of RG-58 with a BNC connector used in cable television.

• The wiring standards used for network coax are different from those used

for cable TV.

•50 ohm cable, available as RG-8 and RG-11. Used in Thick Ethernet, alsocalled "Ether Hose".

•50 ohm cable, available as RG-58. Used in Thin Ethernet .

•75 ohm cable, available as RG-59. This one is for TV, not networks.

•93 ohm cable, available as RG-62. Used in ARCnet.


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Rigid (Air) Fill

Coaxial Line

•

The center conductor is surrounded coaxially by a tubularouter conductor and the insulating material is air. The outer

conductor is physically separated from the center conductor

by a spacer (Pyrex, polystyrene, etc. non conductive material).

•

Advantages- ability to minimize radiation losses, interferenceof from other lines are reduced.

•Disadvantages- expensive to construct, must be kept dry to

prevent leakage between conductors, still excessive to limit

the practical length of the wire.


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Solid

Flexible

Coax Cable

•The outer conductor is braided, flexible, and coaxial to the center

conductor. The inner conductor is flexible copper wire that can be

either solid or hallow.

•solid nonconductive polyethylene material that provides both

support and electrical isolation between the inner and outer

conductors.

•Polyethylene plastic is a solid substance that remains flexibleover a wide range of temperatures.

•Coaxial cable is good for transmitting data over long distances

and for reliably supporting higher data rates when using LESS

sophisticated equipment.


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Optical Fiber

•can be glass or plastic, and is meant to conduct light instead of

electricity.

•

The conductor is called a waveguide, and is covered withcladding, a material to reflect the signal back into the center of

the conductor.

•two modes: single mode conducts a single signal, while multi-

mode conducts many signals simultaneously.

•harder to install and splice and expensive.

•carries the signal in photons, there is no interference and fiber

can not be tapped into and data stolen.

•Fiber is good for high speed, high capacity transmission

because signal is transmitted quickly.


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Wireless or Unguided

Terrestrial Microwave

•Frequency range - 4 to 6 or 21 to 23 GHz

•Cost - moderate to high•Installation - difficult

•Capacity -1 Mbps to 10 Mbps

• Attenuation - relatively high, varies with weather

•

Immunity from EMI - low

•Requires line of sight

•Requires fewer repeaters or amplifiers

•Long haul telecommunication services

• Voice and TV transmission

•Point-to-point links between buildings.

•microwave:


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Terrestrial and Satellite Links


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Satellite systems•Relay used to link ground stations

•Functions as an amplifier or a repeater

•Can provide point-to-point to multi-point connectivity

• Television distribution

•Long distance telephone transmission

•Private business networks

•Frequency range - 11 to 14 GHz

•Cost - high

•

Installation - very difficult•Capacity -1 Mbps to 10 Mbps

• Attenuation - relatively high, varies with weather

•Immunity from EMI - low


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Broadcast Radio

• Omni-directional

• Does not require complex antennas

•

Antennas need not to be precise aligned.• FM radio

• VHF and UHF television

• Data networks


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Infra-red

Line of sight is needed

No frequency allocation is needed.

Provides point-to-point connectivity

•Frequency range - 100 GHz to 1000 THz

•Cost - low to moderate

•

Installation - moderate to difficult•Capacity - 1 to 16 Mbps

• Attenuation - varies with weather and light purity

•Immunity from EMI - moderate

Infrared systems come in two types: point-to-point and

broadcast. Point-to-point systems are like the remote

controls we use for televisions.


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Broadcast infraredused in single room settings, as these waves willbounce off walls, but not penetrate them.

Factors for Broadcast infrared:

Frequency range - 100 GHz to 1000 THz

Cost - low

Installation - simple

Capacity - up to 1 Mbps Attenuation - high

Immunity from EMI - low

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