16-01-2013, 03:51 PM
Transmission of Digital Signal
1Transmission of Digital.pdf (Size: 815.18 KB / Downloads: 256)
Introduction
A computer network is used for communication of data from one station to another station in the network. We have seen that analog or digital data traverses through a communication media in the form of a signal from the source to the destination. The channel bridging the transmitter and the receiver may be a guided transmission medium such as a wire or a wave-guide or it can be an unguided atmospheric or space channel. But, irrespective of the medium, the signal traversing the channel becomes attenuated and distorted with increasing distance. Hence a process is adopted to match the properties of the transmitted signal to the channel characteristics so as to efficiently communicate over the transmission media. There are two alternatives; the data can be either converted to digital or analog signal. Both the approaches have pros and cons. What to be used depends on the situation and the available bandwidth.
Analog Data, Digital Signals
Analog data such as voice, video and music can be converted into digital signal communication through transmission media. This allows the use of modern digital transmission and switching equipment’s. The device used for conversion of analog data to digital signal and vice versa is called a coder (coder-decoder). There are two basic approaches:
-Pulse Code Modulation (PCM)
-Delta Modulation (DM)
Pulse Code modulation
Pulse Code Modulation involves the following three basic steps as shown in Fig. 2.4.16:
•Sampling – PAM
•Quantization
•Line coding
Delta Modulation (DM)
Delta Modulation is a very popular alternative of PCM with much reduced complexity. Here the analog input is approximated by a staircase function, which moves up or down by one quantization level (a constant amount) at each sampling interval. Each sample delta modulation process can be represented by a single binary digit, which makes it more efficient than the PCM technique. In this modulation technique, instead of sending the entire encoding of each and every sample, we just send the change from previous sample. If the difference between analog input and the feedback signal is positive, then encoded output is 1, otherwise it is 0. So, only one bit is to be sent per sample. Figure 2.4.20 shows the Delta modulation operation.