Data Transmission Media


 a)Twisted-Pair Wire

  • Twisted pair is further classified as :
  •                    l  Unshielded Twisted Pair (UTP)
  •                    l  Shielded Twisted Pair (STP)


  • UTP:

Ordinary telephone wire


Easiest to install

Suffers from external interference

  • STP:

Metal braid or sheath that reduces interference

More expensive

Easiest to install

Harder to handle (thick and heavy)

Twisted Pair Cable

¢ This is the most widely used medium for telecommunication.

¢ Twisted-pair wires are ordinary telephone wires which consist of two insulated copper wires twisted into pairs and are used for both voice and data transmission.

¢ The use of two wires twisted together helps to reduce crosstalk and electromagnetic induction. The transmission speed range from 2 million bits per second to 100 million bits per second.


b)Coaxial Cable

  • Coaxial cable carries signals of higher frequency ranges than twisted pair cable.
  • It has a central core conductor of solid or stranded  wire enclosed in an insulating sheath. This in turn encased in an outer conductor of metal foil ,braid or a combination of  the two.
  • The metallic wrapping serves both as a shield against noise and as the second conductor completes the circuit.

The outer conductor is also enclosed in an insulating sheath and the whole cable is protected by a plastic cover.

Coaxial cable Standards

Coaxial cables are  categorized by their radio government (RG) ratings  .Each RG number denotes a set of physical specifications such as,

  • wire gauge of the inner conductor
  • thickness and type of the inner insulator
  • the construction of the shield
  • the size and type of outer casing

Categories of coaxial cables

Category Impedance Use
RG-59 75 Cable TV
RG-58 50 Thin Ethernet
RG-11 50 Thick Ethernet

Also known as coax.

It has better shielding (protecting) than twisted pair, so that it can span longer distances at higher speeds.

Widely used within telephone system for long-distances but now have largely been replaced by fiber optics.

Two types of coax used:

50 ohm (Resistance Power)(Baseband coax): used for digital transmission

75 ohm (Broadband coax): used for analog transmission and cable television.

Co-axial Cable

¢ These cables are widely used for cable television systems, office buildings, and other worksites for local area networks.

¢ The cables consist of copper or aluminum wire wrapped with insulating layer typically of a flexible material with a high dielectric constant, all of which are surrounded by a conductive layer.

¢ The layers of insulation help minimize interference and distortion.

¢ Transmission speed range from 200 million to more than 500 million bits per second

Coaxial Cable Connectors

Coaxial Cable Connectors are used to connect coaxial cable to devices. The most common type of connector is the Bayone Neill-concelman or BNC connectors. There  are three popular types of connectors

  • BNC connector
  • BNC T connector &
  • BNC terminator

BNC connector

It is used to connect the end of the cable to a device such as a TV set.

BNC T connector

It is used in Ethernet networks to branch out a cable for connection to a computer or other devices.

BNC terminator

It is used at the end of the cable to prevent the reflection of the signal.


  • Attenuation is  much higher in coaxial cables than in twisted pair cable.
  • Coaxial cable has a much higher bandwidth the signal weakens rapidly and needs the frequent use of repeaters.




c)Fiber Optics

A fiber optic cable is made of glass or plastic and transmit signals in the form of light.

An optical fiber is a glass or plastic fiber that carries light along its length. 

Fiber optics is the overlap (part) of applied science and engineering concerned with the design and application of optical fibers.

Optical fibers are widely used in fiber-optic communications, which permits transmission over longer distances and at higher bandwidths (data rates) than other forms of communications.

 Fibers are used instead of metal wires because signals travel along them with less loss, and they are also immune to electromagnetic interference.

Fibers are also used for illumination (lighting), and are wrapped in bundles so they can be used to carry images, thus allowing viewing in tight spaces.

Properties of light

  • Light travels in a straight line as long as it moves through a single uniform substance. If array traveling through one substance suddenly enters another the ray changes direction.


If the angle of incidence (the angle the ray makes with the line perpendicular to the interface between the two substances) is less than the critical angle the ray refracts and moves closer to the surface.


If the angle of incidence is greater  than the critical angle the ray reflects and travels again in the denser substance.

Optical fibers use reflection to  guide light through a channel.

Fiber Optics

An optical transmission system has three components:

the light source

the transmission medium and

the detector

Light source: LED (Light Emitting Diode) or Laser (Light Amplification by Simulated Emission of Radiation)

Transmission Media: ultra-thin fiber of glass

Detector: using light-electricity effect, generate an electrical pulse when light falls on it 

Fiber Optics 1

Fiber Optics 2


(a) Three examples of a light ray from inside a silica fiber impinging on the air/silica boundary at different angles.

(b) Light trapped by total internal reflection.

¢ These cables consist of one or more thin filaments of glass fiber wrapped in a protective layer.

¢  It transmits light which can travel over long distance and higher bandwidths.

¢  Fiber-optic cables are not affected by electromagnetic radiation.

¢ Transmission speed could go up to as high as trillions of bits per second.

¢ The speed of fiber optics is hundreds of times faster than coaxial cables and thousands of times faster than twisted-pair wire.

Fiber Optics 3


Microwave Transmission

Above 100 MHz, the waves travel in straight lines and can therefore be narrow focused.

Since the microwave travel in a straight line, if the towers are too far apart, the earth’s surface will get in way.

The higher the towers are, the further apart they can be.

They are also relatively inexpensive

There are four major types of frequencies. They are:

l  Terrestrial microwave

l  Satellite microwave

l  Broadcast microwave

l  Infrared


Terrestrial Microwave

Physical description-

l  The most common type of microwave antenna is called “dish”. A typical size is about 3 meters in diameter.

l  The antenna is fixed rigidly and focuses a narrow beam to achieve line-of-sight transmission to the receiving antenna.

l  Microwave antennas are usually located at a substantial heights above ground level to extend the range between antennas and to be able to transmit over the obstacles.

l  To achieve long distance transmission, a series of microwave relay towers is used, and the point-to-point microwave links are stung together over the desired distance.

Line-of-sight = transmission of very high frequency signals in straight lines directly from antenna to antenna.


  1. The primary use of terrestrial microwave system is telecommunications, as an alternative to coaxial cable op optical fiber.
  2. The microwave requires far fewer amplifiers and repeaters than coaxial cable over the same distance but requires line-of-sight transmission.

- Microwave is commonly used for both voice and television transmission.

  1. Microwave is also used for short point-to-point links between buildings. This can be used for closed-circuit TV or as a data link between LANS.
  2. Cellular systems and fixed wireless access are also the important uses of microwave transmission.


Satellite microwave

Physical Description-

l  A communication satellite is a microwave relay station. It is used to link two or more ground based microwave transmitters/receivers, known as ground stations.

l  The satellite receives transmissions one frequency band (uplink), amplifies it on another frequency (downlink).

l  A single orbiting satellite will operate on a number of frequency bands called transponders.



l  Following are the most important applications for satellites.

l  Television distribution

l  Long distance telephone system

l  Private business networks.

Because of their broadcast nature, satellites are well suited to television distribution


Communication Satellite


Communication satellites and some of their properties, including altitude above the earth, round-trip delay time and number of satellites needed for global coverage

Satellite 1

Satellite 2

Satellite 3










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