During the past few decades, the field of communications and computer networks has dealed with efficient and reliable transfer of information from one point to another. The need to exchange information is not new but the techniques employed to achieve information exchange have been steadily improving. Those techniques have experienced an unprecedented and innovative growth and several factors have been and continue to be responsible for this growth. The Internet is the most visible product of this growth and it has impacted the life of each and every one of us.
A typical communication system consists of the following components:
• Source that generates or has the information to be transported
• Transmitter that prepares the information for transportation
• Transmission medium that carries the information from one end to the other
• Receiver that receives the information and prepares it for delivering to the receiver
• Destination that takes the information from receiver and utilizes it as necessary
The information can be generated in analog or digital form. Analog information is represented as a continuous signal that varies smoothly in time. As one speaks in a microphone, an analog voice signal is generated. Digital information is represented by a signal that stays at some fixed level for some duration of time followed by a change to another fixed level. A computer works with digital information that has two levels (binary digital signals). We have the following four possibilities in a communication system:
• Analog information transmitted as an analog signal
• Analog information transmitted as a digital signal
• Digital information transmitted as an analog signal
• Digital information transmitted as a digital signal
Each of these possibilities has its pros and cons. When a signal carrying information is transmitted, it loses its energy and strength and gathers some interference (noise) as it propagates away from the transmitter. If the energy of the signal is not boosted at some intermediate point, it may attenuate beyond recognition before it reaches its intended destination. That will certainly be a wasted effort. In order to boost energy and strength of a signal, it must be amplified (in case of analog signals) and rebuilt (in case of digital signals). When an analog signal is amplified, the noise also becomes amplified and that certainly lowers expectations about receiving the signal at its destination in its original (or close to it) form. On the other hand, digital signals can be processed and reconstructed at any intermediate point and, therefore, the noise can essentially be filtered out. Moreover, transmission of information in digital form has many other advantages including processing of information for error detection and correction, applying encryption and decryption techniques to sensitive information, and many more. Thus, digital information transmission technology has become the dominant technology in the field of communications.
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