Wednesday, 11 June 2014

Transmission Media

Transmission Media

  • Physical path between transmitter and receiver.
  • May be guided (wired) or unguided (wireless).
  • Communication achieved by using EM waves.
  • Characteristics and quality of data transmission.
  • Dependent on characteristics of medium and signal.


Guided media, which are those that provide a conduit from one device to another, include twisted-pair cable, coaxial cable, and fiber-optic cable.
Guided Transmission Media uses a "cabling" system that guides the data signals along a specific path. The data signals are bound by the "cabling" system. Guided Media is also known as Bound Media. Cabling is meant in a generic sense in the previous sentences and is not meant to be interpreted as copper wire cabling only. Cable is the medium through which information usually moves from one network device to another.
Twisted pair cable and coaxial cable use metallic (copper) conductors that accept and transport signals in the form of electric current. Optical fiber is a glass or plastic cable that accepts and transports signals in the form of light.


Unguided media, or wireless communication, transport electromagnetic waves without using a physical conductor. Unguided Transmission Media consists of a means for the data signals to travel but nothing to guide them along a specific path. The data signals are not bound to a cabling media and as such are often called Unbound Media.
Signals are broadcast through air and thus are available to anyone who has a device capable of receiving them. In wireless communication, transmission and reception are achieved using an antenna. Transmitter sends out the electromagnetic signal into the medium. Receiver picks up the signal from the surrounding medium.
Wireless transmission can be divided into three groups: radio waves, microwave, and infrared waves. The section of the electromagnetic spectrum defined as radio communication is divided into eight ranges, called bands.
These bands are rated form very low frequency (VLF) to extremely high frequency (EHF).

Communicatin Technology

Early forms of communication included runners, homing pigeons and smoke signals. The rise of communication technology came to make communication faster, more accessible and more efficient.
As communication technology evolved the need for regulation grew. Founded in 1934, the FCC regulates communication technology, including radio and television.

Communication Network

Computer network or data network is a telecommunications network that allows computers to exchange data. In computer networks, networked computing devices pass data to each other along data connections. The connections (network links) between nodes are established using either cable media or wireless media. The best-known computer network is the Internet.
¨  types of network communication technology:
                                i)            Intranet
                              ii)            Extranet
                              iii)            Internet

Network topology is the arrangement of the various elements (links, nodes, etc.) of a computer network. Essentially, it is the topological structure of a network, and may be depicted physically or logically. Physical topology refers to the placement of the network's various components, including device location and cable installation, while logical topology shows how data flows within a network, regardless of its physical design. Distances between nodes, physical interconnections, transmission rates, and/or signal types may differ between two networks, yet their topologies may be identical.

There are two basic categories of network topologies:
1.     Physical topologies
2.     Logical topologies

The study of network topology recognizes eight basic topologies:
  • ·         Point-to-point
  • ·         Bus
  • ·         Star
  • ·         Ring or circular
  • ·         Mesh
  • ·         Tree
  • ·         Hybrid
  • ·        Daisy Chain

Wired (guided media) Transmission:
Twisted pair:
  • Twisted Pair
*      Consists of two insulated copper wires arranged in a regular spiral pattern to minimize the electromagnetic interference between adjacent pairs
*      Often used at customer facilities and also over distances to carry voice as well as data communications
*      Low frequency transmission medium

There are two types:
Ø  unshielded Twisted Pair (UTP)
l  ordinary telephone wire
l  cheapest
l  easiest to install
l  suffers from external EM interference
Ø  shielded Twisted Pair (STP)
l  metal braid or sheathing that reduces interference
l  more expensive
l  harder to handle (thick, heavy)

Coaxial Cable:

  • Coaxial Cable
Ø  superior frequency characteristics to TP
Ø  performance limited by attenuation & noise
Ø  analog signals
l  amplifiers every few km
l  closer if higher frequency
l  up to 500MHz
Ø  digital signals
l  repeater every 1km closer for higher data rates
Ø  Used for cable television, LANs, telephony
Ø  Has an inner conductor surrounded by a braided mesh
Ø  Both conductors share a common center axial, hence the term “co-axial”

Fiber Optic
Ø  uses total internal reflection to transmit light
l  effectively acts as wave guide for 1014 to 1015 Hz
Ø  can use several different light sources
l  Light Emitting Diode (LED)
         cheaper, wider operating temp range, lasts longer
l  Injection Laser Diode (ILD)
         more efficient, has greater data rate relation of wavelength, type & data rate
Ø  Relatively new transmission medium used by telephone companies in place of long-distance trunk lines
Ø  Also used by private companies in implementing local data communications networks
Ø  Require a light source with injection laser diode (ILD) or light-emitting diodes (LED)
  • optic fiber
*      multimode step-index fiber
n  the reflective walls of the fiber move the light pulses to the receiver
*      multimode graded-index fiber
n  acts to refract the light toward the center of the fiber by variations in the density
*      single mode fiber
n  the light is guided down the center of an extremely narrow core

Wireless (Unguided Media) Transmission

     transmission and reception are achieved by means of an antenna directional
 n  transmitting antenna puts out focused beam
 n  transmitter and receiver must be aligned omnidirectional
 n  signal spreads out in all directions
 n  can be received by many antennas

*      earth stations communicate by sending signals to the satellite on an uplink
*      the satellite then repeats those signals on a downlink
*      the broadcast nature of the downlink makes it attractive for services such as the distribution of television programming

This is for:
*      television distribution
n  a network provides programming from a central location
n  direct broadcast satellite (DBS)
*      long-distance telephone transmission
n  high-usage international trunks
*      private business networks

Infra Red

*      Uses transmitters/receivers (transceivers) that modulate noncoherent infrared light.
*      Transceivers must be within line of sight of each other (directly or via reflection ).

   *      Unlike microwaves, infrared does not penetrate walls.
   *      modulate noncoherent infrared light
   *      end line of sight (or reflection)
   *      are blocked by walls
   *      no licenses required 
   *      typical uses
   *      TV remote control
   *      IRD port