Communication, the process of sharing ideas, information, and messages with others in a particular time and place. Communication includes writing and talking, as well as nonverbal communication (such as facial expressions, body language, or gestures), visual communication (the use of images or pictures, such as painting , photography , video, or film), and electronic communication (telephone calls, electronic mail, cable television, or satellite broadcasts). Communication is a vital part of personal life and is also important in business, education, and any other situation where people encounter each other.
• History Of Telecommunication
The word TELECOMMUNICATION is a combination of two words...
TELE + COMMUNICATION
The word TELE in Latin means DISTANCE.
HENCE Telecommunication is DISTANCE COMMUNICATION.
The necessity of communication begun as early as the existence of mankind on earth. Communication has become the vital tool for mankind to strife prosperously in this world.
• Evolution Of The Communication System
- Primitive Sign Language
- Pounding Drum
- Smoke Signal
- Fire Signals
- Modern Telegraph
- Wireless communication
- Satellite Communication
Requirements For a Complete Communication System
• The SOURCE of the MESSAGES
• Messages are the signals you wish to send. The message itself can be voice, computer data, music, video movie, temperature reading or alarm information. All these can be considered as Messages.
• The Sources of messages can come in many forms. It can be your mouth, a cassette tape player, a video tape player, a computer, a thermometer or a security system.
• These messages are normally converted to electrical signals and are then known as intelligence, information, audio signal or base band signal, depending on the message itself.
• The MODULATOR & the TRANSMITTER
• For a message to be able to reach the required destination is has to be sent out through a transmitter. The message is converted into electrical signals and is modulator.
• The process of modulation is required as the original message or signal is generally of a low frequency waveform, and therefore has low energy. Thus modulation is the process of translating the original signal to a higher frequency signals representing the original.
• The modulation process is done inside a device called a Transmitter, whose function is to convert the source signals to signals suitable for sending out.
• The CHANNEL
The modulated signal has to be transferred from the transmitter to the receiver through a medium. The medium is also called the channel. In communication, channel means a long transmission path. The various channels available today are cables, radio waves and light waves.
• The RECEIVER & the DEMODULATOR
The Receiver has the following function:
• Receive the transmitter signal.
• Select the desired signal from many signals (mixed when passing through channel )
• Demodulate the signal to obtain a reproduction of the original signal.
• The DESTINATION of the MESSAGES
• The signal must arrive at an appropriate Destination .
• As an example, a computer signal received by your ear is not very useful. A picture received by a thermometer does not make sense.
• The destination device must also decide what to do with the signals, i.e. to just receive live, to store them or to perhaps re-transmit them to yet another destination.
• Successful communication has taken place when the original has arrived at the destination correctly.
• TRANSMISSION MEDIA
Communication networks employ a variety of transmission media ranging from copper wires to satellite channels to transport user's information. The transmission media is the physical path for the communication signal. Transmission media can be classified into two major categories: guided media, which may constrain and guide the communication signal, and unguided media, which permits signal to be transmitted but not guide them. Examples of guided transmission media are metallic cable and optical fibers. Examples of unguided transmission media are the radio signals and satellite signals. An important characteristic of these different media is the bandwidth or simply the range of frequencies each can transmit. In general, the greater the bandwidth of a given media, the more it can carry.
• Metallic Cable
• There are two types of metallic cable, paired cable and coaxial cable.
• Paired cable is best for low bit rates i.e. 1.5 to 2 Mbits/s, while coaxial cable is suited for use higher up in the multiplexing hierarchy.
• Paired cable was originally developed for analogue lines. In digital transmission for example PCM links, crosstalk between wire pairs is a limiting factor.
• Coaxial cable is used in both FDM and TDM systems. Coaxial cable is characterized by a high bandwidth, which gives the cable a very high transmission capacity i.e. up to 10800 speech channel in FDM. The coaxial cable is used in pairs, one for each direction. It is commonly used in the inter-exchange network where the traffic intensity is high.
Optical Fiber Systems
The optical fiber is a rapidly growing transmission media in the telephone network. This is because of it's remarkable characteristics: low weight, wide bandwidth, non-conductive, immunity to electromagnetic interference and low transmission loss.
The optical fiber technology is has grown to become a very cost effective way for new telecommunications installations.
Optical fiber systems are also widely used for intelligent building wiring and for Local Area Network (LAN) connections between computer systems. Extensive research and development is being carried in the area of optical switching which will make optical fibers even more useful and economical.
• Radio Link Systems
• This is a connection via a chain of transmitters and receivers.
• There are radio links for both analogue and digital transfer.
• Analogue radio systems can be used to transfer pulse modulated signals while digital systems are purely design for digital transmission.
• Each radio link connection requires two channels, one in each direction. The transmission frequency and the receiving frequency are separated by a few MHz. This is a very small difference, bearing in mind the frequency band use.
• Satellite Systems
• Satellite transmission is similar in principle to the ordinary radio link. Instead of having all the stations earthbound, we send some up into the space.
• Communications Satellite rotates at almost exactly the same rate as the earth rotate. Compared to the radio link, the satellite has a considerable large range. They are used for both in the national network and in the international network.
There are only a few problems in the transmission characteristics of the satellite link. Due to the long distance that the signals have to travel, resulted in a delay (echo) which have to be counteracted by the echo suppressors. It has to be recognized that this is a communication between two floating bodies in space thus there is always a relative movement between the earth and the satellite which can cause errors in digital transmissions. However, this may be compensated for by intermediate storage of the information in buffer memories.
The capacity of the telephone channels increase as time goes on as compared to the time when the satellite was first launch into space. The Intelsat which was first launch in 1965 have 75 duplex telephone channels but today the basic version of the new Intelsat VI satellite can handle 80 000 telephone channel.
• Telecommunication System
The first requirement is for the original information energy (such as that of the human voice, or music, or a telegraph signal) to be converted into electrical form to produce an electronic information signal. This is achieved by a suitable transducer, which is a general term given to any device that converts energy from one form to another when required.