ELEMENTS OF A DIGITAL COMMUNICATION SYSTEM
Figure 1.1-1 illustrates the functional diagram and the basic elements of a digital communication system.
The source output may be either an analog signal, such as an audio or video signal, or a digital signal, such as the output of a teletype machine, that is discrete in time and has a finite number of output characters. In a digital communication system, the messages produced by the source
are converted into a sequence of binary digits.
are converted into a sequence of binary digits.
The process of efficiently converting the output of either an analog or digital source into a sequence of binary digits is called source encoding or data compression.The sequence of binary digits from the source encoder, which we call the information sequence, is passed to the channel encoder.
The purpose of the channel encoder is to introduce, in a controlled manner, some redundancy in the binary information sequence that can be used at the receiver to overcome the effects of noise and interference encountered in the transmission of the signal through the channel.
This increase the reliability of the received data and improves the fidelity of the received signal. The binary sequence at the output of the channel encoder is passed to the digital modulator, which serves as the interface to the communication channel. Since nearly all the communication channels encountered in practice are capable of transmitting electrical signals (waveforms), the primary purpose of the digital modulator is to map the binary information
sequence into signal waveforms.
To elaborate on this point, let us suppose that the coded information sequence is to be transmitted one bit at a time at some uniform rate R bits per second (bits/s). The digital modulator may simply map the binary digit 0 into a waveform so(t) and the binary digit 1 into a waveform s, (t). In this manner, each bit from the channel encoder is transmitted separately. We call this binary modulation. Alternatively, the modulator may transmit 6 coded information bits at a time by using M = 2h
distinct waveforms so(t), i = 0, 1, ..., M - 1, one waveform for each of the 26 possible b-bit sequences. We call this M-ary modulation (M > 2).
Note that a new b-bit sequence enters the modulator every b/R seconds.
Hence, when the channel bit rate R is fixed, the amount of time available to transmit one of the M waveforms corresponding to a b-bit sequence is b times the time period in a system that uses binary modulation.
The purpose of the channel encoder is to introduce, in a controlled manner, some redundancy in the binary information sequence that can be used at the receiver to overcome the effects of noise and interference encountered in the transmission of the signal through the channel.
This increase the reliability of the received data and improves the fidelity of the received signal. The binary sequence at the output of the channel encoder is passed to the digital modulator, which serves as the interface to the communication channel. Since nearly all the communication channels encountered in practice are capable of transmitting electrical signals (waveforms), the primary purpose of the digital modulator is to map the binary information
sequence into signal waveforms.
To elaborate on this point, let us suppose that the coded information sequence is to be transmitted one bit at a time at some uniform rate R bits per second (bits/s). The digital modulator may simply map the binary digit 0 into a waveform so(t) and the binary digit 1 into a waveform s, (t). In this manner, each bit from the channel encoder is transmitted separately. We call this binary modulation. Alternatively, the modulator may transmit 6 coded information bits at a time by using M = 2h
distinct waveforms so(t), i = 0, 1, ..., M - 1, one waveform for each of the 26 possible b-bit sequences. We call this M-ary modulation (M > 2).
Note that a new b-bit sequence enters the modulator every b/R seconds.
Hence, when the channel bit rate R is fixed, the amount of time available to transmit one of the M waveforms corresponding to a b-bit sequence is b times the time period in a system that uses binary modulation.
The communication channel is the physical medium that is used to send the signal from the transmitter to the receiver. In wireless transmission, the channel may be the atmosphere (free space).
On the other hand, telephone channels usually employ a variety of physical media, including wire lines, optical fiber cables, and wireless (microwave radio). Whatever the physical medium used for transmission of' the information, the essential feature is that the transmitted signal is corrupted in a random manner by a variety of possible mechanisms, such as additive thermal noise generated by electronic devices; man-made noise, e.g., automobile ignition noise; and atmospheric noise, e.g., electrical lightning discharges during thunderstorms.
At the receiving end of a digital communication system, the digital demodulator processes the channel-corrupted transmitted waveform and reduces the waveforms to a sequence of numbers that represent estimates of the transmitted data symbols (binary or M -ary).
This sequence of numbers is passed to the channel decoder, which attempts to reconstruct the original information sequence from knowledge of the code used by the channel encoder and the redundancy contained in the received data.
A measure of' how well the demodulator and decoder perform is the fre-quency with which errors occur in the decoded sequence. More precisely, the average probability of a bit-error at the output of the decoder is a measure of the performance of the demodulator decoder
combination.
In general, the probability of error is a function of the code characteristics, the types of waveforms used to transmit the information over the channel, the transmitter power, the characteristics of the channel (i.e., the amount Of noise, the Mature of the interference), and
the method of' demodulation and decoding.
The source decoder accepts the output sequence from the channel decoder and, from knowledge of the source encoding method used, attempts to reconstruct the original signal from tile source. Because of channel decoding errors and possible distortion introduced by the source encoder, and perhaps, the source decoder, the signal at the output of the source decoder is an approximation to the original source output The difference or some function of the difference between the original signal and the reconstructed signal is a measure of the distortion introduced by the digital communication system.
On the other hand, telephone channels usually employ a variety of physical media, including wire lines, optical fiber cables, and wireless (microwave radio). Whatever the physical medium used for transmission of' the information, the essential feature is that the transmitted signal is corrupted in a random manner by a variety of possible mechanisms, such as additive thermal noise generated by electronic devices; man-made noise, e.g., automobile ignition noise; and atmospheric noise, e.g., electrical lightning discharges during thunderstorms.
At the receiving end of a digital communication system, the digital demodulator processes the channel-corrupted transmitted waveform and reduces the waveforms to a sequence of numbers that represent estimates of the transmitted data symbols (binary or M -ary).
This sequence of numbers is passed to the channel decoder, which attempts to reconstruct the original information sequence from knowledge of the code used by the channel encoder and the redundancy contained in the received data.
A measure of' how well the demodulator and decoder perform is the fre-quency with which errors occur in the decoded sequence. More precisely, the average probability of a bit-error at the output of the decoder is a measure of the performance of the demodulator decoder
combination.
In general, the probability of error is a function of the code characteristics, the types of waveforms used to transmit the information over the channel, the transmitter power, the characteristics of the channel (i.e., the amount Of noise, the Mature of the interference), and
the method of' demodulation and decoding.
The source decoder accepts the output sequence from the channel decoder and, from knowledge of the source encoding method used, attempts to reconstruct the original signal from tile source. Because of channel decoding errors and possible distortion introduced by the source encoder, and perhaps, the source decoder, the signal at the output of the source decoder is an approximation to the original source output The difference or some function of the difference between the original signal and the reconstructed signal is a measure of the distortion introduced by the digital communication system.
Digital Communication advantages
1.Reliable communication; less sensitivity to changes in environmental conditions (temperature, etc.)
2.Easy multiplexing
3.Easy signaling
„Hook status, address digits, call progress information
4.Voice and data integration
5.Easy processing like encryption and compression
6.Easy system performance monitoring
„QOS monitoring
7.Integration of transmission and switching
8.Signal regeneration, operation at low SNR, superior performance
9.Integration of services leading to ISD
Digital Communication System Disadvantages
1.Increased bandwidth
„64 KB for a 4 KHz channel, without compression (However, less with compression)
„64 KB for a 4 KHz channel, without compression (However, less with compression)
2.Need for precision timing „Bit, character, frame synchronization needed
3.Analogue to Digital and Digital to Analogue conversions
„Very often non-linear ADC and DAC used, some performance degradation
4.Higher complexity
