McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Data Transmission Techniques• Data to be transmitted is of two types

1. Analog data

2. Digital data

Therefore, the data transmission is categorized into two types : 1. Analog Transmission

2. Digital Transmission

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Analog Transmission• Analog (or analogue) transmission is a transmission method of

conveying voice, data, image, signal or video information using a continuous signal which varies in amplitude, phase, or some other property in proportion to that of a variable. It could be the transfer of an analog source signal, using an analog modulation method such as frequency modulation (FM) or amplitude modulation (AM), or no modulation at all.

• Another method is of analog transmission is known as passband data transmission using a digital modulation methods such as ASK, PSK and QAM, i.e. a sine wave modulated by a digital bit-stream, as analog transmission and as an analog signal.

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Digital Transmission• Data transmission, digital transmission, or

digital communications is the physical transfer of data (a digital bit stream or a digitized analog signal[1]) over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optical fibers, wireless communication channels, storage media and computer buses. The data are represented as an electromagnetic signal, such as an electrical voltage, radiowave, microwave, or infrared signal.

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

• While analog transmission is the transfer of a continuously varying analog signal over an analog channel, digital communications is the transfer of discrete messages over a digital or an analog channel. The messages are either represented by a sequence of pulses by means of a line code (baseband transmission), or by a limited set of continuously varying wave forms (passband transmission), using a digital modulation method.

• Data transmitted may be digital messages originating from a data source, for example a computer or a keyboard. It may also be an analog signal such as a phone call or a video signal, digitized into a bit-stream for example using pulse-code modulation (PCM) or more advanced source coding (analog-to-digital conversion and data compression) schemes. This source coding and decoding is carried out by codec equipment.

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-1

Data Transmission Conversion/Modulation Schemes

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-2

Digital to Digital Encoding

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

A baseband signal ("digital-over-digital" transmission): A sequence of electrical pulses or light pulses produced by means of a line coding scheme such as Manchester coding. This is typically used in serial cables, wired local area networks such as Ethernet, and in optical fiber communication. It results in a pulse amplitude modulated(PAM) signal, also known as a pulse train.

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-3

Types of Digital to Digital Encoding

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Analog to Digital Encoding

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

• Pulse Amplitude Modulation (PAM)

• Pulse Code Modulation (PCM)– Sampling (means measuring the amplitude of the

signal at an equal interval)– Sampling rate

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-16

WCB/McGraw-Hill

PAM

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-17

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Quantized PAM Signal

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-18

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Quantizing Using Sign and Magnitude

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Steps in PCM

There are four steps used in PCM

1. PAM

2. Quantization

3. Binary Encoding

4. Digital to Digital encoding

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-19

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

PCM

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-20

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

From Analog to PCM

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-20-continued

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

From Analog to PCM

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-20-continued

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

From Analog to PCM

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-20-continued

From Analog to PCM

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Animation of PAM

• http://www.mhhe.com/engcs/compsci/forouzan/dcn/graphics/animations/05_20.swf

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Nyquist Theorem

• The Nyquist Theorem, also known as the sampling theorem, is a principle that is used in the digitization of analog signals. It is done by dividing the signals into the slices called as samples

• These samples are taken frequently.

• The number of samples taken per second is called the sampling rate or sampling frequency.

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

• Suppose the highest frequency component, in hertz, for a given analog signal is fmax.

• According to the Nyquist Theorem, the sampling rate must be at least 2fmax, or twice the highest analog frequency component.

• The sampling in an analog-to-digital converter is actuated by a pulse generator (clock). If the sampling rate is less than 2fmax, some of the highest frequency components in the analog input signal will not be correctly represented in the digitized output.

• When such a digital signal is converted back to analog form by a digital-to-analog converter, false frequency components appear that were not in the original analog signal. This undesirable condition is a form of distortion called as aliasing.

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-21

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Nyquist Theorem

• Example 5.3 : What sampling rate is needed for the signal with a bandwidth of 10,000 Hz ( 1,000 to 11,000 hz) ?

• SOLUTION : The sampling rate must be twice the highest frequency in the signal:

Sampling rate = 2(11,000)

= 22,000 samples/ seconds.

EXAMPLE 5.4 : A signal is sampled. Each sample requires at least 12 levels of precision ( +0 to +5 and -0 to -5). How many bits should be sent

for each sample?

SOLUTION : We need four bits ;one bit for the sign and three bits for the value. A three-bit value can represent 23 = 8 levels ( 000 to 111), which is more than what we need. A two – bit value is not enough since 22=4. A four –bit value is too much

because 24 = 16.

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

EXAMPLE : We want to digitize the human voice. What is the bit rate assuming eight bits per

sample?

SOLUTION: The humans voice normally contains frequencies from 0 to 4000 Hz. So the sampling rate

is

Sampling rate = 4000 x 2 = 8000 samples/ seconds

The bit rate can be calculated as :

Bit rate= sampling rate x Number of bits per sample = 8000 x 8 = 64,000 bits/s = 64 kbps

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Digital To Analog Conversion• A passband signal ("digital-over-analog" transmission):

A modulated sine wave signal representing a digital bit-stream. Note that this is in some textbooks considered as analog transmission, but in most books as digital transmission. The signal is produced by means of a digital modulation method such as PSK, QAM or FSK. The modulation and demodulation is carried out by modem equipment. This is used in wireless communication, and over telephone network local-loop and cable-TV networks.

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-22

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Digital to Analog Encoding

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-23

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-24

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

ASK

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-25

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Bandwidth for ASK

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

FSKFigure 5-27

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Bandwidth for FSKFigure 5-28

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

PSKFigure 5-29

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

PSKConstellation

Figure 5-30

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

4-PSKFigure 5-31

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

4-PSKCharacteristics

Figure 5-32

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

8-PSKCharacteristics

Figure 5-33

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

PSKBandwidth

Figure 5-34

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

4-QAM and 8-QAMConstellations

Figure 5-35

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

8-QAM SignalFigure 5-36

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

16-QAMConstellation

Figure 5-37

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Bit Rate and Baud Rate

Figure 5-38

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-38-continued

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Bit Rate and Baud Rate

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Analog To Analog Transmission

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-39

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Analog to Analog Modulation

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-40

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-41

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Amplitude Modulation

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-42

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

AM Bandwidth

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-43

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

AM Band Allocation

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-44

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

Frequency Modulation

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-45

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

FM Bandwidth

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Figure 5-46

WCB/McGraw-Hill The McGraw-Hill Companies, Inc., 1998

FM Band Allocation

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Digital To Analog

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

McGraw-Hill ©The McGraw-Hill Companies, Inc., 2001

Analog To Analog