Principles Digital Communication System & Computer Networks (Charles River Media Computer Engineering)

We begin the journey into the exciting field of telecommunications by studying the basic building blocks of a telecommunication system. We will study the various types of communication and how the electrical signal is impaired as it travels through the transmission medium. With the advances in digital electronics, digital communication systems slowly are replacing analog systems. We will discuss the differences between analog communication and digital communication.

1.1 BASIC TELECOMMUNICATION SYSTEM

A very simple telecom system is shown in Figure 1.1. At the transmitting end, there will be a source that generates the data and a transducer that converts the data into an electrical signal. The signal is sent over a transmission medium and, at the receiving end, the transducer again converts the electrical signal into data and is given to the destination (sink). For example, if two people want to talk to each other using this system, the transducer is the microphone that converts the sound waves into equivalent electrical signals. At the receiving end, the speakers convert the electrical signal into acoustic waves. Similarly, if video is to be transmitted, the transducers required are a video camera at the transmitting side and a monitor at the receiving side. The medium can be copper wire. The public address system used in an auditorium is an example of such a simple communication system.

Figure 1.1: Basic telecommunication system.

What is the problem with this system? As the electrical signal passes through the medium, the signal gets attenuated. The attenuated signal may not be able to drive the transducer at the receiving end at all if the distance between the sender and the receiver is large. We can, to some extent, overcome this problem by using amplifiers between. The amplifier will ensure that the electrical signals are of sufficient strength to drive the transducer.

In an electrical communication system, at the transmitting side, a transducer converts the real-life information into an electrical signal. At the receiving side, a transducer converts the electrical signal back into real-life information.

But we still have a problem. The transmission medium introduces noise. The noise cannot be eliminated at all. So, in the above case, we amplify the signal, but at the same time, we also amplify the noise that is added to the actual signal containing the information. Amplification alone does not solve the problem, particularly when the system has to cover large distances.

The objective of designing a communication system is for the electrical signal at the transmitting end to be reproduced at the receiving end with minimal distortion. To achieve this, different techniques are used, depending on issues such as type of data, type of communication medium, distance to be covered, and so forth.

The objective of designing a communication system is to reproduce the electrical signal at the receiving end with minimal distortion.

Figure 1.2 shows a communication system used to interconnect two computers. The computers output electrical signals directly (through the serial port, for example), and hence there is no need for a transducer. The data can be passed directly through the communication medium to the other computer if the distance is small (less than 100 meters).

Figure 1.2: PC-to-PC communication.

Figure 1.3 shows a communication system in which two PCs communicate with each other over a telephone network. In this system, we introduced a new device called a modem (modulator-demodulator) at both ends. The PCs send digital signals, which the modem converts into analog signals and transmits through the medium (copper wires). At the receiving end, the modem converts the incoming analog signal into digital form and passes it on to the PC.

Figure 1.3: PC-to-PC communication over telephone network.

Two computers can communicate with each other through the telephone network, using a modem at each end. The modem converts the digital signals generated by the computer into analog form for transmission over the medium at the transmitting end and the reverse at the receiving end.

Figure 1.4 shows a generic communication system. In this figure, a block "medium access processing" is introduced. This block has various functions, depending on the requirement. In some communication systems, the transmission medium needs to be shared by a number of users. Sometimes the user is allowed to transmit only during certain time periods. Sometimes the user may need to send the same data to multiple users. Additional processing needs to be done to cater to all these requirements. At the transmitting side, the source generates information that is converted into an electrical signal. This signal, called the baseband signal, is processed and transmitted only when it is allowed. The signal is sent on to the transmission medium through a transmitter. At the receiving end, the receiver amplifies the signal and does the necessary operations to present the baseband signal to the user. Any telecommunication system is a special form of this system. Consider the following examples:

Figure 1.4: Generic communication system.

In the case of a radio communication system for broadcasting audio programs, the electrical signal is transformed into a high-frequency signal and sent through the air (free space). A radio transmitter is used to do this. A reverse of this transformation— converting the high-frequency signal into an audio signal—is performed at the receiving station. Since it is a broadcasting system, many receivers can receive the information.

In a radio communication system, the electrical signal is transformed into a high-frequency signal and sent over the air.

In a communication system on which two persons communicate with two other persons located somewhere else, but only on one communication link, the voice signals need to be combined. We cannot mix the two voice signals directly because it will not be possible to separate them at the receiving end. We need to "multiplex" the two signals, using special techniques.

In a mobile communication system, a radio channel has to be shared by a number of users. Each user has to use the radio channel for a short time during which he has to transmit his data and then wait for his next turn. This mechanism of sharing the channel is known as multiple access.

When multiple users have to share the same transmission medium, multiple access techniques are used. These techniques are required in both radio communication and cable communication.

Hence, depending on the type of communication, the distance to be covered, etc., a communication system will consist of a number of elements, each element carrying out a specific function. Some important elements are:

• Multiplexer: Combines the signals from different sources to transmit on the channel. At the receiving end, a demultiplexer is used to separate the signals.

• Multiple access: ess: When two or more users share the same channel, each user has to transmit his signal only at a specified time or using a specific frequency band.

• Error detection and correction: If the channel is noisy, the received data will have errors. Detection, and if possible correction, of the errors has to be done at the receiving end. This is done through a mechanism called channel coding.

• Source coding: If the channel has a lower bandwidth than the input signal bandwidth, the input signal has to be processed to reduce its bandwidth so that it can be accommodated on the channel.

• Switching: If a large number of users has to be provided with communication facilities, as in a telephone network, the users are to be connected based on the numbers dialed. This is done through a mechanism called switching.

• Signaling: In a telephone network, when you dial a particular telephone number, you are telling the network whom you want to call. This is called signaling information. The telephone switch (or exchange) will process the signaling information to carry out the necessary operations for connecting to the called party.

The various functions to be carried out in a communication system are: multiplexing, multiple access, error detection and correction, source coding, switching and signaling.