Wireless Communication Engineering - Data Communications

1. Wireless Communication Engineering - Data Communications Li-Der Jeng Department of Electronic Engineering Chung-Yuan Christian University Chung-Li, Taiwan, ROC TEL: (03) 265-4608 E-mail: lider@cycu.edu.tw

2. Contents • Introduction to Data Communications • Information Encoding • Analog and Digital Transmission Methods • Transmission Media

3. Introduction to Data Communications

4. Fundamental concepts • Communication can be defined as exchange of information between two humans. • Data communications can be defined as the exchange of information between two computers. • In its simplest form, the data communications can be shown as in the following figure.

5. Transmission medium Data Communication Source Destination

6. Modem Modem Transmission medium Modem Modem Modem Real-life Data Communication Systems Demultiplexer Multiplexer

7. Data communications • In the simplest form, data communications involves the exchange of data between two computers. • Computers work with a binary language consisting of zero and one. • Therefore, a computer generates a stream of zeros and ones and sends it to another computer to which it is connected in some fashion. • The connection can be either a simple wire or it can be through wireless media.

8. Data communications (I) • For enabling data communications, a combination of hardware and software is essential. In any data communications system, three characteristics are described: • Correct delivery: When a sender transmits data for an intended recipient, the data must reach only the intended recipient and not someone else. • Accurate delivery: The data sent must be received in the same form as the one in which it was sent. There must not be any sort of alternations to it in transit. • Timely delivery: The data must travel from the sender to the receiver in a finite amount of time. The term finite is quite vague, and would depend on the reasons why the data communication is taking place.

9. Data communications (II) • Two key aspects of data communication systems need a good amount of understanding. • Transmission media: the physical path over which data travels from the sender to the receiver. Ex: twisted-pair of copper wires, coaxial cable, optical fiber or wireless media such as radio waves. • Protocol: a set of rules and conventions. Ex: The sender and the receiver, the two key parties in data communication must agree on a common set of rules, i.e. protocols before they can communicate with each other.

10. Protocols • A protocol defines the following: • Syntax (What is to be communicated)– The syntax defines the structure or format of data. This means that the order in which it is to be sent is decided. For instance, a protocol could define that the first 16 bits of a data transmission must always contain the receiver’s address. • Semantics (How it is to be communicated) – The semantics define the interpretation of the data that is being sent. For example, the semantics could define that if the last two bits of the receiver’s address field contain a 00, it means that the sender and the receiver are on the same network. • Timing (When it should be communicated) – This refers to an agreement between the sender and the receiver about the data transmission rates and duration. For instance, a protocol could demand that the sender must send 1000 bytes and then wait for an acknowledgement from the receiver before sending any more data.

11. Standards • Standards are necessary in every walk of life. For instance, when you want to replace a light bulb in your home because it has been damaged, you expect the new bulb to fit in the holder straightaway and work like the old bulb did. What is the use if the bulb does not fit in the holder, or if it fits in the holder but does not illuminate because it requires a different voltage level? Consequently, everything that we use in our daily life has some common features, some standards that every manufacturer must abide by. In the absence of standards, every manufacturer can theoretically manufacture a set of goods or services that are incompatible with other manufacturers. • To avoid such anomalies, a set of standards is established, which governs the rules that manufacturers must obey. In exactly the same fashion, standards for data communications have been set. Consequently, a lot of incompatibility issues have no place in data communications, which is highly desirable.

12. Bandwidth of a signal and a medium • The term bandwidth is very commonly used in data communication. The basic idea behind bandwidth can be understood quite easily with a simple example of pipes carrying water to our homes. What is the maximum amount of water a pipe can carry at any given time? The maximum capacity of the pipe at a given instance is its bandwidth.

13. Analog and digital signals • Any signal can be classified into one of the two types: analog and digital. • An analog signal is a continuously varying signal, similar to a sinusoidal waveform. • A digital signal takes the form of pulses, where we have something or nothing.

14. 0 1 1 0 1 0 VO L T A G E Tempe r a t ur e Time Time Analog Signal Digital Signal

15. Amplitude, period, frequency, phase • Amplitude: the signal has maximum value • Period: the time taken for the completion of one cycle • Frequency: the number of cycles or revolutions that our particle would make in one second • Phase: the phase of a signal is related to the position of a waveform relative to time zero

16. Fourier analysis and the concept of bandwidth of a signal • Ex: A periodic signal has been decomposed using Fourier analysis to yield four sine waves of frequencies 100, 400, 600 and 800 Hz. What is the bandwidth of the resulting periodic signal? (800-100=700) • Ex: A signal has a bandwidth of 20Hz and its highest frequency is 60Hz. What is the lowest frequency? (60-20=40)

17. A Digital Signal With Infinite Bandwidth 0 1 0 0 0 1 0 Pulses before transmission :Bit rate: 2000 bits per second (a) (b) Bandwidth 500 Hz (c) Bandwidth 1500 Hz (d) Bandwidth 2000 Hz (e) Bandwidth 2500 Hz (f) Bandwidth 3000 Hz (g) Bandwidth 5000 Hz (h) ¥ Bandwidth Hz

18. Period (T) = 1 second 1 second A Digital Signal of 1Hz 0 1 0 1 0 1 0 1 0

19. 1 0 1 0 V X 0 T = 1/10 sec T = 1/10 sec Time A Sinusoidal Wave with Frequency = 10 Hz

20. 40 Hz: 20 bps (Channel 0) 40 Hz: 20 bps (Channel 1) A Medium and Channels

21. Information Encoding

22. Introduction • How computers store data? • Can a computer understand English? • Does it store data in some other language? • If a computer cannot understand English, how can we codify the data to be stored in a fashion that the computer will be able to understand? • Why a computer uses binary language?

23. The BCD Equivalent of Decimal Digits

24. The BCD code • Binary Coded Decimal (BCD) code Decimal 2 5 Binary 0010 0101 • The BCD number for a decimal number 25 is 00100101 Decimal 1 0 Binary 0001 0000 • The BCD number for a decimal number 10 is 00010000

25. Character ASCII (Binary) ASCII (Decimal) Character ASCII (Binary) ASCII (Decimal) A 1000001 65 [ 1011011 91 B 1000010 66 \ 1011100 92 C 1000011 67 ] 1011101 93 Z 1011010 90 ^ 1011110 94 1 0110001 49 _ 1011111 95 2 0110010 50 ‘ 1100000 96 3 0110011 51 * 0101010 42 9 0111001 57 + 0101011 43 a 1100001 97 , 0101100 44 b 1100010 98 - 0101101 45 c 1100011 99 . 0101110 46 z 1111010 122 / 0101111 47 Portion of the ASCII Table

26. Portion of the EBCDIC Table

27. Multimedia • These days, computers can also be used for the following: • Drawing, storing and viewing pictures • Storing sounds and playing them back • Storing videos and playing them back • However, pictures, videos and sounds are not made up of alphabets and numbers. How can a computer recognize and store them? How can we codify information about these so that a computer can store them? • To solve this problem of codification of pictures, videos and sounds, the concept of multimedia came into being. As the name says, multimedia means multiple media.

28. Picture/Images • Now let us imagine that we divide the picture by a number of horizontal and vertical lines to form a grid. Each rectangle is called picture element or pixel. • As the number of pixels increases (also called higher resolution), the pixel size decreases. • Ex: We choose the resolution such that each pixel has either a dot or a blank.

29. Pixels Being Mapped to Zeroes and Ones Computer’s Memory Screen Output 00000000 01100000 00000000 01100000 00001111 11110000 00010000 00001000 00100000 00000100 01000000 00000010 01000000 00000010 01111111 11111110 00100111 01110100 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Graphics Hardware / Software

30. Picture/Image • In practice, it is common to have 8, 16 or 24 bits to represent one pixel on the screen. • It is well known that any color can be derived by adding or mixing various intensities of three basic colors: Red, Green, Blue – thus the name RGB. • Many systems that use 24 bits to describe the color of one pixel use 8 bits each to describe the intensities of Red, Green, Blue.

31. Video • The basic principle behind video is the technology animation. The idea behind animation is very simple and is used in cartoons and films. • If a set of pictures is shown rapidly, the human eye can be fooled into believing that the pictures are in motion. • It has been proved that if 24 pictures are shown in succession in one second, our eyes sense it as a continuous motion.

32. Sound • A sound wave (i.e. an audio signal) is continuous in nature. • The continuity is in two respects: the strength of wave/signal (called amplitude) and time. • In contrast, a computer works only with binary values: 0 and 1. • If we want to translate an audio signal so that it can be mapped to the computer-recognizable data of 0 and 1, it should be clear that we must somehow map an analog signal as a digital signal.

33. Audio Signal in the Analog Form Amplitude Time

34. Signal Representing Only Binary Values (0 and 1) Amplitude Time 1 0

35. Sound • Sampling: measuring the audio signal at fixed intervals of time is called sampling. Thus, if we decide that the audio signal would be measured say 60 times a second, the sampling rate would be 60. • Quantizing: having determined how many times the signal should be measured, the next step is to assess the range of amplitudes.

36. Sampling and Quantizing Original signal Sampling Measuring the signal Stored on the disk as numbers Quantizing 6 5 2 1 2 3 6 9 11 10 7 5 4 3 2 1 0

37. Analog and Digital Transmission Methods

38. Introduction • We have studied that the two major types of signals are analog and digital. However, the manner in which these two types of signals can be transmitted are also of the same types, that is analog and digital. • We have four possible combinations: • Analog Signal, Analog Transmission • Digital Signal, Digital Transmission • Digital Signal, Analog Transmission • Analog Signal, Digital Transmission

39. (a) Telephone (b) Analog signal Analog Signal, Analog Transmission

40. Analog signal, analog transmission • The term analog is very common and used for decades in the field of telephony. • The human voice generates an analog(i.e. continuously varying) signal, which is transmitted as an analog signal over the medium. • On the way, the signal suffers attenuation. • Amplifiers are used to overcome this problem, but then amplifiers amplify noise along with the original signal, too. • The problem with this type of combination is that if the signal gets distorted, it cannot be reconstructed at all! • This is the reason why this type is not used where a high level of accuracy is desired.

41. Digital signal, digital transmission • We know that the information coming out of a computer is the form of digital signals. • We also know that a digital signal has an infinite bandwidth, whereas any medium has only a limited bandwidth. • Therefore, as the signal is generated and enters the medium, the signal is distorted. • The hardware equipment called regenerative repeater or repeater is used to regenerate the signal.

42. 0 1 0 0 1 0 1 From Data Processing Machine 0 1 0 0 1 0 1 0 1 0 0 1 0 1 Regenerative Repeater Regenerative Repeaters A C B

43. Digital signal, digital transmission • The input to the regenerative repeater is a signal, which looks like a digital signal. Therefore, the repeater measures the signal values at regular intervals to recognize the 0s and 1s in the signal and regenerate them. Therefore, there is no loss of information. • However, only one repeater will not do. You will require many such repeaters. The distance between the repeaters is very crucial. We may like to increase that distance as much as possible to reduce the cost but then there is also a disadvantage to this. (it may be difficult to differentiate 0 and 1.) • Any line with repeaters placed at the appropriate distance is called a digital line. • AT&T put such repeaters on the wire pairs used for telephonic conversations, separated by a distance of only 6000 feet. This digital line is called a T1 line, which can carry a data rate of 1,54,400 bits per second (1.544 Mbps).

44. Source Destination A T1 Line Contains Many Repeaters Destination Repeater Repeater Repeater Digital line

45. Digital signal, analog transmission • The designers had two choices for data communications between two computers.One was to create a new digital network with repeaters etc, or use the existing telephone network. • When computers were invented, the telephone network was already in existence. However, telephones use analog signals and analog circuits. The problem: how to send a digital signals over an analog network? • We use a modem for this purpose. The modem is derived from two components: a modulator and a demodulator.

46. Digital signal Analog signal Network modem Digital signal Analog signal modem Use of Modem for Sending Digital Data Over Analog Lines

47. Digital signal, analog transmission • As the above figure shown, the digital signals originating from the computer go through the modem where they are converted (i.e., codified or modulated) into analog signals whose bandwidth is < 4000 Hz. This is because the channel for telephone conversation requires a bandwidth of 4000 Hz.

48. Modulation techniques • Amplitude Shift Keying: ASK • Frequency Shift Keying: FSK • Phase Shift Keying: PSK • Quadrate Amplitude Modulation: QAM • The main limitation of PSK is the inability of the hardware equipment to distinguish small differences in terms of phase changes. This puts a limitation on its data rate. • Combine ASK and PSK, makes higher data rates possible (since the bandwidth of the transmission medium is a major limitation, we cannot combine FSK with anything else)

49. Amplitude Shift Keying (ASK) The frequency is between 0 and 4000 Hz

50. 1 0 0 1 1 0 1 0 0 1 1 Frequency Shift Keying (FSK) f1 f2 f2 f1 f2 f 1 f2 f1 f1 and f2 are between 0 and 4000 Hz