TELECOMMUNICATION

1. INTRODUCTION TO TELECOMMUNICATIONS

1.1 TELECOMMUNICATIONS: The word Telecommunication is a combination of two words TELE+COMMUNICATION. The word Tele in Latin means Distance. Hence Telecommunication is also known as Distance Communication. The French word telecommunication was coined in 1904 by the French engineer and novelist Edouard Estaunie.

It is defined as the transmission of signals over a distance for the purpose of communication. In earlier times this may have involved the use of smoke signals, drums, semaphore, flags fire or heliograph and other optical communications.
In modern times it typically involves the use of electronic devices such as telephones, televisions, radio or computers with the help of electromagnetic waves by transmitters and receivers. Early inventors in the field of telecommunication include Alexander Graham Bell, Guglielmo Marconi and John L.Baird.

BASIC ELEMENTS
A basic telecommunication system consists of three elements:
· A transmitter that takes information and converts into a signal
· A transmission medium that carries the signal
· A receiver that receives the signal and converts back into usable information.

For example, in a radio broadcast the broadcast tower is the transmitter, free space is the transmission medium and the radio is the receiver. Often telecommunication systems are two-way with a single device acting as both transmitter and receiver or Transceiver. For example, Mobile phone is a transceiver.
Telecommunication over a telephone line is called point-to-point communication because it is between one transmitter and one receiver but communication through radio broadcast is called broadcast communication as it is between one powerful transmitter and numerous receivers.

1.2 PRINCIPLES OF TELECOMMUNICATION SERVICES

The original information energy (human voice, or music, or a telegraph signal) is converted into electrical form to produce an electronic information signal. This is achieved by a suitable transducer, device that converts energy from one form to another whenever required.
When a radio frequency current flows into a transmitting antenna (aerial), power is radiated in a number of directions which is called an Electromagnetic wave.
The radiated energy will reach the receiving station by one or more of the five different propagation modes:
· Surface wave propagation
· Sky wave propagation
· Space wave propagation
· Satellite propagation and
· Scatter wave propagation

MODULATION OF SIGNALS:
Modulation is the process of translating the audio frequencies to higher level frequencies that is suitable for transmission.
In general, it is the process by which some characteristic (amplitude, frequency, and phase) of a carrier is varied in accordance with the modulating wave signal (input).

INPUT OUTPUT
(Modulating / baseband /audio / → MODULATION → (modulated signal)
Low frequency signal) ↓
Carrier (RF-high frequency signal)

NEED FOR MODULATION:
· It simplifies power amplifier design
· Frequency band shifting is made possible
· For efficient radiation and reception antenna height should be comparable to λ/4 of the frequency used.
· Low frequency message is shifted to radio frequency range and can be communicated to a long distance.

TYPES OF MODULATION:

1. Continuous wave modulation-Amplitude modulation (AM)
-Angle modulation-frequency modulation (FM)
-phase modulation (PM)
2. Pulse modulation -analog modulation
-digital modulation

1.3 TELECOMMUNICATION STANDARDS
Standardization is vital in telecommunication as it allows world wide communication. There are international, regional and national standardization agencies.

INTERNATIONAL STANDARDS:
The most encompassing international standard is the ITU (International Telecommunication Union) based in Geneva, Switzerland and has literally over 2000 standards. It has two sectors namely ITU-T (telecommunication sector) produced by CCITT (International Consultative Committee for Telephone & Telegraph) and the other sector is the ITU-R (radio communication sector) produced by CCIR (International Consultative Committee for Radio).

REGIONAL STANDARDS:
Regional standard bodies exist such as CEN (European Committee for Standardization), CEN ELEC (European Committee for Electro Technical Standardization), ETSI (European Telecommunications Standards Institute), IRMM (Institute for Reference Materials and Measurements) in Europe. ETSI is one of the important regional standards which produced the European version digital network PCM.

NATIONAL STANDARDS:
American national standards institute based in New York produces a wide range of standards. EIA (Electronics Industries Association) and TIA (Telecommunication Industries Association) are the standards that are responsible for the preparation and dissemination of telecommunication standards.
The IEEE (Institute of Electrical and Electronics Engineers) standard produces 802 series specification used for enterprise networks.
The ATSC (Advanced television System Committee) standard used for video compression is used to produce CATV (Cable Television standards) standard.