PART I: DIGITAL COMMUNICATION SYSTEMS Chapter 3: Transmission Media.

PART I: DIGITAL COMMUNICATION SYSTEMS

Chapter 3: Transmission Media

Contents

Twisted Pair Coaxial Cable Optical Fiber Terrestrial Radio Satellite Radio Radio Spectrum Management

Objectives

To study the various transmission media used for electrical communication.

To study the characteristics and the advantages of each for practical communication systems.

To study the issue of radio spectrum management.

Twisted Pair

Twisted Pair

Twisted pair gets its name because a pair of copper wires is twisted to form the transmission medium.

Least expensive transmission medium and hence the most widely used.

Used extensively in the local underground telephone network, in Private Branch Exchanges (PBX’s), and also in local area network.

Repeater – amplifies the signal to the required level and retransmits on the medium.

Data rate supported by the twisted pair depends on the distance to be covered and the quality of the copper.

Category 5 twisted pair supports data rates in the range of 10Mbps to 100Mbps up to a distance of 100 meters.

Coaxial Cable

Coaxial Cable

Used extensively for cable TV distribution, long distance telephone trunks and LANs.

Supports a maximum data rate of 500Mbps for a distance of about 500 meters

Repeaters are required every 1 to 10 kilometeres.

Optical Fiber

Optical Fiber

Deployed extensively and is most preferred medium for all types of networks because of the high data rates that can be supported.

Light in a glass medium can carry more information over large distances, as compared to electrical signals in a copper cable or coaxial cable.

The speed of transmission is 2 x 108 meters/second

The two types of optical fiber are single mode and multimode fiber. Single-mode fiber allows only one ray (or mode) of light to propagate at a time whereas multimode fiber allows multiple rays (or modes).

Terrestrial Radio

Terrestrial Radio

Free space as the medium Free space is called an unguided medium

because the electromagnetic waves can travel freely in all directions.

A radio channel consists of a pair of frequencies – one frequency is used for uplink and one frequency is used for downlink.

In some radio systems, a single frequency is used in both directions.

Special Problems

Radio Spectrum

Electromagnetic Spectrum

Radio waves 300GHz and lower (frequency)

Sub-millimeter waves

100 micrometers to 1 millimeter (wavelenght)

Infrared 780 nanometeres to 100 micrometers

Visible light 380 nanometers to 780 nanometers

Ultraviolet 10 nanometers to 380 nanometers

X-ray 120eV to 120keV

Gamma rays 120keV and up

Radio Frequency

Frequency Band Frequency Range Application Areas

Very Low Frequency (VLF)

3kHz to 30kHz Radio navigation, maritime mobile

Low Frequency 30kHz to 300kHz Radio navigation, maritime mobile

Medium Frequency 300kHz to 3MHz AM radio broadcast, aeronautical mobile

High Frequency 3MHz to 30MHz Maritime mobile and aeronautical mobile

Very High Frequency 30MHz to 300MHz Land mobile, FM broadcast, TV broadcast, aeronautical mobile, radio paging, trunked radio

Frequency Band

Frequency Range

Application Areas

Ultra-High Frequency

300MHz to GHz

TV broadcast, mobile satellite, land mobile, radio astronomy

L Band 1GHz to 2GHz

Aeronautical radio navigation, radio astronomy, earth exploration satellite

S Band 2GHz to 4GHz

Space research, fixed satellite communication

C Band 4GHz to 8GHz

Fixed satellite communication, meteorological satellite communication

X Band 8GHz to 12GHz

Fixed satellite broadcast, space research

Ku Band 12GHz to 18GHz

Mobile and fixed satellite communication, satellite broadcast

Frequency Band Frequency Range Application Areas

K Band 18GHz to 27GHz Mobile and fixed satellite communication

Ka Band 27GHz to 40GHz Inter-satellite communication, mobile satellite communication

Millimeter 40GHz to 300GHz Space research, Inter-satellite communications

International Tellecommunications Union (ITU) Frequency BandsAM Radio 535 to 1605 MHz

Citizen band radio 27MHz

Cordless telephone devices 43.69 to 50 MHz

VHF TV 54 to 72 MHz, 76 to 88 MHz, 174 to 216 MHz

Aviation 118 to 137 MHz

Ham radio 144 to 148 MHz, 420 to 450MHz

UHF TV 470 to 608 MHz, 614 to 806 MHz

Cellular Phones 824 to 849 MHz, 869 to 894 MHz

Personal Communication services 901-902 MHz, 930 – 931 MHz, 940 – 941 MHz

Search for extra-terrestrial intelligence

1420 to 1660 MHz

Inmarsat satellite phones 1525 to 1559 MHz, 1626.5 to 1660.5

Some frequency bands as ham radio and the industrial, scientific and medical band are free bands – no prior government approvals are required to operate radio systems in those bands.

Satellite Radio

Satellite Radio

Arthur C. Clarke proposed the concept of communication satellites.

Star and Mesh

Radio Spectrum Management

Allotted Spectrum

Radio spectrum management ensures that the allotted spectrum is being used efficiently, to ensure that there is no interference between different radio systems and to allocate new frequency bands for new services.

Several Factors in RSM

There are some frequency bands for the exclusive use of govermental agencies and some for nogovermental agencies.

When frequency band is allocated for a particular application.

Higher user demands. New application areas emerge. New technologies. Agencies will be allocated fixed frequencies

for use.

Spectrum Management Activities Spectrum assignment and selection involves

recommending a specific frequency band operation for use in a given location.

Spectrum engineering and analysis involves computations for installations of radio equipment at specific locations and for predicting the system performance in the radio environment.

Spectrum planning involved long-term/ emergency planning, keeping in view, among other things, the demands for new services and technological changes.

Cost of Spectrum

Operators that obtain specific frequency bands for radio services need to pay for the cost of the spectrum.

End of Chapter 3