1. PART I: DIGITAL COMMUNICATION SYSTEMS Chapter 1: Basics of Communication Systems

2. Contents  Basic Telecommunication System  Types of Communication  Transmission Impairment  Analog Versus Digital Transmission

3. Objectives  To study the basic building block of a telecommunication system.  To study the various types of communication and how the electrical signal is impaired as it travels through the transmission medium.  To understand the difference between the analog communication and digital communication.

4. Basic Telecommunication System

5.  Figure 1.1 Basic telecommunication system Source Transducer Sink Transmission Medium

6. What is the problem of 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.

7. The solution...  We can use an amplifiers, the amplifier will ensure that the electrical signals are 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.

8. But we still have a problem...  The transmission medium introduces noise. The noise cannot be eliminated at all.  Amplification alone does not solve the problem, particularly when the system has to cover large distance. As the electrical signal passes through the transmission medium, the signal gets attenuated. In addition, the transmission medium introduces noise and as a result, the signal gets distorted.

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

10. Interconnecting two computers RS232 Port  Figure 1.2: PC-to-PC communication.  The serial ports of two computers can be connected directly using a copper cable. However, due to the signal attenuation, the distance cannot be more than 100 meters. Channel

11.  Figure 1.3: PC-to-PC communication over telephone network Telephone Network Telephone Network  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.

12. Generic Communication system  Figure 1.4: Generic communication system Source Baseband Signal Processing Medium Access Processing TransmitterReceiver Decoding of Data Baseband Signal Processing Sink

13. Radio Communication System  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.

14. One communication link  In a communication system on which two persons communicate with other persons located somewhere else, but only one communication link, the voice signals need to be combined.  It is impossible to mix the two voice signals directly because it will not be possible to separate them at the receiving end.  The two signals need to “multiplex”, using special technique.

15. Mobile Communication System  A radio channel has to be shared by a number of users.  Multiple Access - Each user has to use radio channel for a short time period during which he has to transmit his data and then wait for his next turn.  These techniques are required in both radio communication and cable communication.

16. Elements of Communication System Multiplexer Combines the signals from different sources to transmit on the channel. At the receiving end, a demultiplexer is used to separate the signal. Multiple access 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 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 the large number of users has to be provided with communication facilities, as in a telephone network, the users are to be connected on the numbers dialed. This is done through 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 information to carry out the necessary operations for connecting to the called party.

17. Types of Communication

18. Different types of communication Communication Point-to-Point Point-to- Multipoint BroadcastingSimplexHalf-duplexFull-duplex

19. Point-to-Point Communication  In this type, communication takes place between two end points. Party 2 Party 1

20. Point-to-Multipoint Communication  In this type of communication, there is one sender and multiple recipients. Party 1Party 2Party 3Party n

21. Broadcasting  In broadcasting system, there is a central location from which information is sent to many recipients, as in case of audio or video broadcasting. In a broadcasting system, the listener are passive, and there is no reverse communications. Sender Receiver 1 Receiver 2 Receiver n

22. Simplex Communication  In simplex communication, communication is possible only in one direction. There is one sender and one receiver; the sender and receiver cannot change roles. Party 1Party 2

23. Half-Duplex Communication  Half-duplex communication is possible in both directions between two entities (computers or persons), but one at a time. These types of system is required limited channel bandwidth, so they are low cost systems. Party 1Party 2 Switch

24. Full-Duplex Communication  In a full-duplex communication system, the two parties - the caller and the called – can communicate simultaneously, as in telephone system. Party 1Party 2

25. Convergence Technologies  Multimedia Communication – Various types of information (voice communication, data communication, fax communication, and video communications) are clubbed together.  With the advent of technology, this distinction is slowly disappearing, and multimedia communication is becoming the order of the day.

26. Transmission Impairments

27. 3 Classification of Transmission Impairment Amplitude of the signal wave decreases as the signal travels through the medium. Attenuation distortion Occurs as a result of different frequency components arriving at different times in the guide media such as copper wire or coaxial cable. Delay distortion Divided into four categories: Thermal noise Intermodulation Crosstalk Impulse noise Noise

28. When two signals of different frequencies are sent through the medium, due to nonlinearity of the transmitter, frequency components such as f1 + f2 and f1 – f2 are produced, which are unwanted components and need to be filtered out. Intermodulation Noise Unwanted coupling between signal paths is known as crosstalk. In the telephone network, this coupling is quite common. As a result of this, we hear other conversations. Crosstalk needs to be eliminated by using appropriate design techniques Crosstalk This is cause by external electromagnetic disturbance such as lightning. This noise is unpredictable. When the signal is traversing the medium, impulse noise may cause sudden burst of errors. This may cause a temporary disturbance in voice communication. For data communication, appropriate methods need to be devised whereby the lost data is retransmitted. Impulse Noise

29. Noise occur due to the thermal agitation of electrons in a conductor. This is distributed uniformly across the spectrum and hence called white noise. This noise cannot be eliminated and hence, when designing telecom systems, we need to introduce some method to overcome the ill effects of thermal noise. Thermal Noise

30. No = kT Thermal Noise for a bandwidth of 1 Hz is obtained from the formula: k is Boltzmann’s constant 1.3803 * 10 -23 J/K T is temperature, K where No is noise power density, watts per Hz N = kTB (watts) Thermal noise for a bandwidth of B Hz is given by N = 10 log k + 10 log T + 10 logB db watts = -228.6 + 10 log T + log B If N is expressed in dB (decibels) Thermal noise for a bandwidth B Hz is given by N = kTB (watts), where k is Boltzmann’s constant and T is temperature. N is generally expressed in decibels.

31. Analog Versus Digital transmission

32. What is Analog Transmission?  In analog communication, the signal, whose amplitude varies continuously, is transmitted over the medium.  Reproducing the analog signal at the receiving end if very difficult due to transmission impairments.  Analog communication systems are badly affected by noise.

33. What is Digital Transmission  In a digital communication system, 1s and 0s are transmitted as voltage pulses.  If the pulse is distorted due to noise, it is not very difficult to detect the pulses at the receiving end.  Digital communication is much more immune to noise as compared to analog communication.

34. Advantages of Digital Systems  More reliable transmission because only discrimination between ones and zeroes is required.  Less costly implementation because of the advances in digital logic chips.  Ease of combining various types of signals (voice, video, etc.)  Ease of developing secure communication systems.

35. Convergence Technologies  Though a large number of analog communication systems are still in use, digital communication systems are now being deployed.  The old analog system are being replaced by digital systems.  All newly developed communication systems are digital systems. Only in broadcasting applications, is analog communication used extensively.

36. End of Chapter 1