Transmission Media..

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