Analog and Digital Transmission

Achyut S.Godbole, “ Data Communications and Networks”, Tata McGraw-Hill Publishing Company Limited, Ninth reprint, 2007

1. Data Communications and Networks Analog and Digital Transmission R.Anushiya Assstiant Professor, P.K.R Arts College for Women, Gobi Author: Achyut S.Godbole , “ Data Communications and Networks”, Tata McGraw-Hill Publishing Company Limited, Ninth reprint, 2007

2. Analog and Digital Transmission Modes

3. A  signal  is an electrical or electromagnetic current that is used for carrying data from one device or  network  to another … Transmission  mode refers to the mechanism of transferring of data between two devices connected over a  network . It is also called Communication Mode. Two forms of transmission: Digital Transmission: data transmission using square waves Analog Transmission: data transmission using all sine waves.

4. Four possibilities to consider: Analog Signal, Analog transmission → “as is” (e.g., Telephone, radio) Analog Signal, Digital transmission → sampling (e.g., voice, audio, video) Digital Signal, Analog transmission → broadband & wireless Digital Signal, Digital transmission → baseband (e.g., Ethernet)

5. Analog Signal, Analog Transmission Telephone is the best example Human voice generates an analog signal which is transmitted as analog signal over the medium. When the signal suffers attenuation , amplifiers are used to overcome the problem

6. Digital Signal, Digital Transmission Digital-to-digital encoding is the representation of digital information by a digital signa l. When binary 1s and 0s generated by the computer are translated into a sequence of voltage pulses that can be propagated over a wire, this process is known as digital-to-digital encoding Binary signals have two basic parameters: amplitude and duration . As the number of bits sent per unit of time increases, the bit duration decreases

7. Digital Signal, Analog Transmission Converts digital signal to analog signal, such that it could be over by telephone network at the other end it again converted to digital signal To make his process easy we use modem for conversion Modulation Demodulation

8. Modulation Techniques

9. Amplitude Shift Keying (ASK) To represent binary signals, the amplitude is varied by1 or 0. Keying means turning a transmitter on and off. Highly susceptible to noise interference. Noise -- random electrical signals (voltages) that tend to generate errors in transmission; introduced into a line by heat from circuit components, or natural disturbances .

10. Frequency Shift Keying (FSK) Frequency is varied to represent binary 1 or 0. Noise interference not a problem because it's looking for frequency changes and doesn't care about voltage spikes.

11. Phase Shift Keying (PSK) Phase is varied to represent binary 1 or 0. Limited by the ability of the equipment to detect small differences in phase. This limits its potential bit rate.

12. Bit Rate and Baud Rate Used in transmitting data from one computer to another across a public access phone line Bit Rate and Baud Rate •Bit rate is the number of bits transmitted per second. •Bit rate is always >/= to the baud rate •Baud rate is the number of signal units per second required to send those bits. Carrier signal •A high-frequency signal that acts as a basis for the information signal - by sender •Digital information is encoded by modulating the signal's: amplitude, frequency, or phase . Bit rate=Baud rate X No. of bits per signal element

13. Analog to Digital Transmission

14. Nyquist Theorem The Nyquist Theorem, also known as the sampling theorem, is a principle that engineers follow in the digitization of analog signals Nyquist sampling (f) = d/2, where d=the smallest object, or highest frequency, you wish to record.

15. The Nyquist Theorem states that in order to adequately reproduce a signal it should be periodically sampled at a rate that is 2X the highest frequency you wish to record. With images, frequency is related to structure size. Small structures are said to have a high frequency. Thus, the imaging sample rate (or pixel) size should be 1/2 the size of the smallest object you wish to record

16. A High Sampling Rate = much greater than 2X the highest frequency. This is 'Oversampling' that, while not "bad" will take time and create a large digital file.

17. Nyquist Sampling Rate = The minimum sample rate that captures the "essence" of the analog information. Note that while Nyquist is appropriate for sampling, it may not capture nuances in information.

18. Differences between Analog and Digital Signals Analog Signal An analog signal signifies a continuous signal that keeps changes with a time period. Analog signals are continuous sine waves. Analog signals describe the behavior of the wave with respect to amplitude, time period, & phase of the signal. Analog signal range will not be set. Analog signal is further horizontal toward distortion during the response to noise. An analog signal broadcasts the information in the signal form. The example of an analog signal is the human voice Digital Signal A digital signal signifies a discrete signal that carries binary data and has discrete values. Digital signal is square waves. Digital signals describe the behavior of the signal with respect to the rate of a bit as well as bit interval. Digital signal is limited as well as ranges from 0 to 1. A digital signal has resistance in response toward the noise, therefore, it does not often face distortion. A digital signal broadcasts the information in the form of binary that is bits.  The example of a digital signal is the data transmission in a computer.