Data Communications PDF

Data Communications Assistant Professor Department of Computer Science & Engineering IIIT Naya Raipur Digital Transmission Analog to Digital Conversion Analog-to-digital conversion is a process in which a continuously variable, or analog, signal is changed into...

Data Communications Assistant Professor Department of Computer Science & Engineering IIIT Naya Raipur Digital Transmission Analog to Digital Conversion Analog-to-digital conversion is a process in which a continuously variable, or analog, signal is changed into a multilevel digital signal without altering its essential content.  Analog to Digital Conversion Pulse code modulation is a method that is used to convert an analog signal into a digital signal so that a modified analog signal can be transmitted through the digital communication network. Block Diagram of Pulse Code Modulation Basic Elements of Pulse Code Modulation System Sampling is a process of measuring the amplitude of a continuous-time signal at discrete instants, converts the continuous signal into a discrete signal. According to the Nyquist theorem, the sampling rate must be at least 2 times the highest frequency contained in the signal. Quantization In quantization, an analog sample with an amplitude that converted into a digital sample with an amplitude that takes one of a specifically defined set of quantization values. Quantization is done by dividing the range of possible values of the analog samples into some different levels and assigning the center value of each level to any sample in the quantization interval. Quantization approximates the analog sample values with the nearest quantization values. Low Pass Filter (LPF) This LPF is used to remove the high frequency (HF) components that are present within the input analog signal. Here this signal is higher as compared to the highest frequency message signal so that it avoids aliasing of the message signal. Regenerative Repeater The signal strength can be enhanced through this regenerative repeater. So, the channel’s output also includes a regenerative repeater circuit to balance the signal loss, renovate the signal & also increases the signal strength. Decoder The main function of a decoder circuit is to decode the pulse-coded signal to repeat the actual signal. This circuit works like a demodulator. Figure Components of a PCM decoder Analog to Digital Conversion Delta modulation PCM is a very complex technique. Other techniques have been developed to reduce the complexity of PCM. The simplest is delta modulation. PCM finds the value of the signal amplitude for each sample; DM finds the change from the previous sample. Analog Transmission 5.11 Digital to Analog Conversion It is the process of changing one of the characteristics of an analog signal based on the information in the digital data. Figure Types of digital-to-analog conversion Note Bit rate is the number of bits per second. Baud rate is the number of signal elements per second. In the analog transmission of digital data, the baud rate is less than or equal to the bit rate. Example An analog signal carries 4 bits per signal element. If 1000 signal elements are sent per second, find the bit rate. Solution In this case, r = 4, S = 1000, and N is unknown. We can find the value of N from  Carrier Signal The receiving device is tuned to the frequency of the carrier signal that expects from the sender. Digital information then changes the carrier signal by modify. ing one or more of its characteristics (amplitude, frequency, or phase). This kind of modification is called modulation (shift keying).  Amplitude shift keying In ASK, the amplitude of the carrier signal is changed to create signal element. In this technique, frequency and phase remain constant. In ASK, one voltage level represent binary 1 and another voltage level represents binary 0. Thus, ASK that is implemented using only two levels is called binary amplitude shift keying (BASK). There is another type of BASK scheme called on-off-keying (OOK). In OOK one of the bit values is represented by no voltage. In ASK, the minimum bandwidth required for transmission is equal to the baud rate.Also, in ASK, bit rate is same as baud rate. Figure Binary amplitude shift keying Frequency shift keying Frequency Shift Keying FSK is the digital modulation technique in which the frequency of the carrier signal varies according to the digital signal changes. The output of a FSK modulated wave is high in frequency for a binary High input and is low in frequency for a binary Low input. Binary FSK is the most common form of FSK. With binary FSK, the center or carrier frequency is shifted by the binary input signal.  Figure Binary frequency shift keying \ Phase shift keying Phase Shift Keying PSK is the digital modulation technique in which the phase of the carrier signal is changed by varying the sine and cosine inputs at a particular time. PSK is of two types, depending upon the phases the signal gets shifted. They are − Binary Phase Shift Keying BPSK is also called as 2-phase PSK or Phase Reversal Keying. In this technique, the sine wave carrier takes two phase reversals such as 0° and 180 Quadrature Phase Shift Keying QPSK is the phase shift keying technique, in which the sine wave carrier takes four phase reversals such as 0°, 90°, 180°, and 270°.  Figure Implementation of BASK 5.23 Figure Binary phase shift keying  Example We have an available bandwidth of 100 kHz which spans from 200 to 300 kHz. What are the carrier frequency and the bit rate if we modulated our data by using ASK with d = 1? Solution The middle of the bandwidth is located at 250 kHz. This means that our carrier frequency can be at fc = 250 kHz. We can use the formula for bandwidth to find the bit rate (with d = 1 and r = 1). Modulation Changing some characteristics (amplitude, f re q u e n c y o r pha s e ) of a carrier s ignal in accordance with the amplitude of the message signal. This process is called modulation. Modulation is the process of mixing a low energy message signal with the high energy carrier signal to produce a new high energy signal which carries information to a long distance. The Message signal contains information whereas the carrier signal contains no information. Carrier signal is used just to transmit the information to a long distance. In modulation process, the characteristics of the carrier signal is changed but the message signal characteristics will not be changed. Analog to analog Conversion nalog to analog conversion can be accomplished in three way Figure Amplitude modulation Note The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BAM = 2B. Figure Frequency modulation Note The total bandwidth required for FM can be determined from the bandwidth of the audio signal: BFM = 2(1 + β)B. Figure Phase modulation Note The total bandwidth required for PM can be determined from the bandwidth and maximum amplitude of the modulating signal: BPM = 2(1 + β)B.