Chapter 5 Guided and Unguided Transmission Medium

  Transmission Medium Figure 5.1

  Transmission medium and physical layer

  

Classes of transmission

medium

  Topics discussed in this section:

  Guided Media

Guided media, which are those that provide a conduit

from one device to another, include twisted-pair cable,

coaxial cable, and fiber-optic cable.

  • Twisted-Pair Cable • Coaxial Cable • Fiber-Optic Cable

  Twisted-pair cable

  A twisted pair consists of two conductors (normally copper), each with its own

  

UTP(unshielded twisted-pair) & STP(shielded

twisted-pair) cables

  UTP connector

  Coaxial cable

  Categories of coaxial cables

  Fiber construction

  Fiber-Optic Cable 

  Fiber optic cable is made of glass or plastic & transmits signals in the form of light.

  Light travels in a straight line as long as it is moving through a single uniform substance.

  If a ray of light traveling through one substance suddenly enters another (more or less dense), the ray changes

  

Bending of light ray

  Refraction – pembiasan

  Optical fiber

  

  Optical fibers use reflection to guide light through a channel.

  A glass or plastic core is surrounded by a cladding of less dense glass or plastic.

  The difference in density of the two materials must be such that a beam of light moving through the core is reflected off the cladding instead of being refracted into it. Propagation Modes

  Multimode 

  Multiple beams from a light source move through the core in different paths.

  In multimode step-index fiber, the density of the

core remains constant from the center to the edges.

  A beam of light moves through this constant density

in a straight line until it reaches the interface of the

core and the cladding.

  At the interface, there is an abrupt change to a lower

density that alters the angle of the beam’s motion.

  The term step index refers to the suddenness of this Multimode Multimode graded-index fiber

  Decrease the distortion of the signal through the cable. Index refers to the index of refraction. Index of refraction is related to density. Density is highest at the center of the core & decrease gradually to its lowest at the edge.

   Single Mode

Uses step-index fiber & a highly focused source of

light that limits beams to a small range of angles, all close to the horizontal.

  Manufactured with a much smaller diameter than

that of multimode fiber, & with substantially lower

density (index of refraction).

  

Decrease in density results in a critical angle that is

close enough to 90° to make the propagation of beams almost horizontal.

  

All the beams arrive at the destination ‘together’ &

  Fiber types

  Fiber-optic cable connectors SC connector – Subscriber channel (SC) connector

  Fiber optic cable has several advantages

over metallic cable(twisted-pair or coaxial)

  Advantages Higher bandwidth ( higher data rates) Less signal attenuation (a signal can run for 50km without regeneration. Repeater is need every 5km for coaxial or twisted- pair cable). Immunity to electromagnetic interference

   Disadvantages Installation/maintenance (need expertise) Unidirectional (propagation of lights is unidirectional. If we need bidirectional communication, two fibers are needed.

  High cost Applications 

  Often found in backbone networks because its wide bandwidth is cost- effective.

  With WDM, data can be transfer at a rate of 1600 Gbps.

  Used in SONET netwok. Waveguide 

  Waveguides are used to transfer electromagnetic power efficiently from one point in space to another.

  Some common guiding structures are shown in the figure below.

  

These include the typical coaxial cable, the two-wire and mictrostrip transmission lines, hollow conducting waveguides , and optical fibers. Waveguide

  Microstrip 

  

Microstrip is a type of electrical transmission

line which can be fabricated using printed circuit board technology, and is used to convey microwave-frequency signals.

  

It consists of a conducting strip separated from

a ground plane by a dielectric layer known as the substrate.  Microwave components such as antennas, couplers, filters, power dividers etc. can be formed from microstrip, the entire device existing Microstrip  Cross-section of microstrip geometry.

  Conductor (A) is separated from ground plane (D) by dielectric substrate (C).

Microstrip  Advantage

  Microstrip is thus much less expensive than traditional waveguide technology, as well as being far lighter and more compact. Disadvantage The disadvantages of microstrip compared with

waveguide are the generally lower power handling

capacity, and higher losses. Also, unlike waveguide, microstrip is not enclosed, and is therefore susceptible to cross-talk and unintentional radiation.

   Unguided media transport electromagnetic waves without using a physical conductor.

  This type of communication is often referred to as wireless communication.

  Propagation Methods

  Radio Wave Transmission (Ground Wave)

  There are two principal ways in which electromagnetic (radio) energy travels from a transmitting antenna to a receiving antenna.

  One way is by GROUND WAVES and the other is by SKY WAVES.

  Ground waves are radio waves that travel near the surface of the Earth (surface and space waves). Ground Waves 

  

The ground wave is actually composed of two

separate component waves.

  These are known as the SURFACE WAVE and the SPACE WAVE. The determining factor in whether a ground wave component is classified as a space wave or a surface wave is simple.

  A surface wave travels along the surface of the Earth.

  A space wave travels over the surface. Ground Wave & Sky Wave

  Surface Wave 

  The surface wave reaches the receiving site by traveling along the surface of the ground.

  A surface wave can follow the contours of the Earth because of the process of diffraction. When a surface wave meets an object and the dimensions of the object do not exceed its wavelength, the wave tends to curve or bend around the Surface Wave Propagation

  Space Wave 

  The space wave follows two distinct paths from the transmitting antenna to the receiving antenna - one through the air directly to the receiving antenna, the other reflected from the ground to the receiving antenna.

  The primary path of the space wave is directly from the

transmitting antenna to the receiving antenna. So, the receiving antenna must be located within the radio horizon of the transmitting antenna. Because space waves are refracted slightly, even when propagated

through the troposphere, the radio horizon is actually about one-third farther than the line-of-sight or natural Space wave propagation

  Sky Wave Propagation 

  Sky-wave propagation allows communication over great distances with simple equipment and reasonable power levels : 100 W to a few kW.

  The sky wave, often called the ionospheric wave, is radiated in an upward direction and returned to Earth Satellite Communications 

  A communications

  satellite (sometimes abbreviated

  to COMSAT) is an artificial satellite stationed in space for the purpose of telecommunications. Modern communications satellites use a variety of orbits. Satellite Orbit

  HISTORICAL -The First in Malaysia Satellite First

  Launched Measat-1 First Malaysian communication satellite Jan 1996 Measat-2 Malaysian communication satellite Nov 1996 Tiungsat-1 First Malaysian microsatellite in Low Earth Orbit (LEO)

  26 Sept.

  2000 Razaksat First world remote sensing satellite launched into Near Equatorial Orbit (NEqO)

  14 July 2009 Overview 

  Satellite is a microwave repeater in the space.

  There are about 750 satellite in the space, most of them are used for communication.

  They are:

Wide area coverage of the earth’s surface.

Transmission delay is about 0.3 sec.

   Satellite up links and down links can operate in different frequency bands:

  Band Up-Link Down-link

  ISSUES (Ghz) (Ghz) C

  4

  6 Interference with ground links.

  Ku

  11

  Ka

  20

  30 High Equipment cost The down-link can have a footprint providing coverage for a The up-link is a highly directional, point to point link Satellite Advantages Satellite versus terrestrial link (Mitra, Communication, 2005)

Capable of transmitting signal long distances without

   using relay with higher capacity Point to multipoint Satellite circuit can be installed rapidly Flexibility in interconnecting mobile vehicle Cost independent of terrain

Digital Satellite (Keesee, Satellite Communication, 2009)

Less distortion and interference, easy to regenerate, Satellite Limitation  Long signal delay High propagation loss

  Repairing and maintenance after launching is difficult High cost and high risks on launching Congestion of frequencies and orbit

Active communications satellite systems are

limited by two things

  • –Satellite transmitter power on the downlinks
Antenna 

  An antenna (or aerial) is an electrical device which converts electric currents into radio waves, and vice versa.

  An antenna can be used for both transmitting and receiving. Antenna 

  An antenna is an electrical conductor or system of conductors o Transmission - radiates electromagnetic energy into space o Reception - collects electromagnetic energy from space In two-way communication, the same antenna can be used for transmission and Types of Antennas 

  Isotropic antenna (idealized) o Radiates power equally in all directions Dipole antennas o Half-wave dipole antenna (or Hertz antenna) o Quarter-wave vertical antenna (or Marconi antenna) Parabolic Reflective Antenna o Used for terrestrial microwave and satellite applications o Larger the diameter, the more tightly directional is the

  Radio waves are used for multicast communications, such as radio and television, and paging systems.

  They can penetrate through walls.

  Highly regulated. Use omni directional antennas

  Unidirectional antennas

  Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

  Higher frequency ranges cannot penetrate walls.

  Use directional antennas - point to point line of sight communications.