Download   Lecture 17: Digital Transmission Fundamentals  Advantages of Digital Transmission over Analog Transmission 

EE 400: Communication Networks (101)
Dr. Wajih A. Abu-Al-Saud
Ref: A. Leon Garcia and I. Widjaja, Communication Networks, 2nd Ed. McGraw Hill, 2006
Latest update of this lecture was on 15­11­2010 Lecture 17: Digital Transmission Fundamentals Advantages of Digital Transmission over Analog Transmission Converting an analog signal to digital format often produces large number of digital bits that may in theory require a higher bandwidth to transmit that would be required for the transmitting the analog signal. However, this problem can be easily solved and many of the disadvantages of transmitting analog signal can be overcome by using some digital transmission techniques. Digital transmission has many advantages over analog transmission. The following illustrates some of these advantages 1. Use of repeaters and digital regenerators: Although basic repeaters can be used with analog signals (to amplify them after retransmission over long distances), analog signals always suffer from noise that is added to the signal. Getting rid of noise from analog signals is a process that has limited success because you can only remove noise that occupies a spectrum outside that of the desired analog signal. This is illustrated in the following figure. Amplifying the analog signal results in amplifying the noise too. (Note: noise occupying the same band of the desired signal can be partially removed by using sophisticated statistical methods. Try applications that perform noise reduction to audio signals to verify this). This is illustrated in the following figure. With digital signals, the situation is different. Since digital signals take only discrete values, noise can be removed “completely” as long as the amount of added noise is small compared to the difference in voltage between the discrete values of the digital signal. For example, if the noise signal with voltage limited to values from –1 V to +1 V is added to a digital signal with two amplitudes of 0 V and 5 V. Since the amplitude of the noise is relatively small compared to 1 EE 400: Communication Networks (101)
Dr. Wajih A. Abu-Al-Saud
Ref: A. Leon Garcia and I. Widjaja, Communication Networks, 2nd Ed. McGraw Hill, 2006
Latest update of this lecture was on 15­11­2010 Lecture 17: Digital Transmission Fundamentals amplitude of the digital signal, there will be no confusion in detecting whither a 0 V was transmitted or a 5 V was transmitted, and therefore you can clan the digital signal completely from noise. Therefore, with digital signals, repeaters and digital regenerators can be used repeatedly as the digital signal is being transmitted to amplify and clean up the signal before high levels of noise are added to the digital signal. 2. Use of equalizers: Equalizers are devices with built in filters that are able to correct problems with a channel that simple amplification or attenuation cannot fix. Equalizers belong to a class of filters called Adaptive Filters meaning that they adapt (modify themselves) to perform a specific task and keep modifying themselves whenever their surroundings change. An ideal channel will have flat transfer function over the band of the transmitted signal. A non ideal channel may amplify some frequencies and attenuate other frequencies. The final effect of using an equalizer is to make the transfer function of the channel in cascade (in series) with the equalizer filter become as close to an ideal channel as possible. An equalizer may depend on the transmission of a known sequence of digital bits or on knowing some statistics of the transmitted information to first adjust its filter to reverse the effect of the non‐ideal channel, and then apply the adjusted filter to the received (unknown) signal to remove the effect of channel and make the combination of the channel transfer function with the equalizer transfer function become flat over the band of the transmitted signal. Part of the sounds you hear produced by a dialup modem at beginning of a dialup session (the sh‐sh‐sh‐sh‐sh sound) is what we call a training sequence that is known to both the transmitter and receiver. The purpose of this is to allow the receiver modem to figure out what distortion the channel did to that known sequence so that it can adjust its filter to reverse the effect of the channel. 3. Ability of multiplexing different forms of information without the need for modulation: Unlike analog signals which require modulation if we desire to transmit different types of signals such as audio and video signals on the same communication channel, digital signals allow the transmission of audio, video, and data signals on the same communication channel without modulation by simply multiplexing these signals with each other. That is, transmit for example 100 bits of video followed by 10 bits of audio, followed by 5 bits of data, and repeat the process again and again. 4. Error detection and correction, data compression, and data encryption techniques: After the transmission of digital signal, often few errors may occur in the transmitted data. Yet, even errors that occur in the received digital data can be detected/corrected if proper error detection/correction information has been added to the data before transmission. Also, data compression as seen in the previous lecture (with its lossy and lossless types) can be used to reduce the amount of transmitted data significantly. Furthermore, data encryption can be used to hide any sensitive information from the outside world by using many data encryption techniques. All of these features do not exist for analog signals (or at least data compression and encryption techniques for analog signals are not as powerful and not as easy to apply for analog signals as those techniques for digital signals). 2 EE 400: Communication Networks (101)
Dr. Wajih A. Abu-Al-Saud
Ref: A. Leon Garcia and I. Widjaja, Communication Networks, 2nd Ed. McGraw Hill, 2006
Latest update of this lecture was on 15­11­2010 Lecture 17: Digital Transmission Fundamentals Properties of Digital Transmission Systems Sometimes, a comparison between two digital transmission systems is needed. There are many parameters that can be used to compare between digital transmission systems, but some of the most important parameters of a digital transmission system are: • Transmission Rate (measured in bits per second): This is a measure of the number of bits that can be transmitted over the communication channel per unit time. • Bandwidth Requirements (measured in Hz): This is a measure of the spectrum that the communication system requires to transmit the information at the desired transmission rate. • Error probability (measured in percentages): This represents the percentage of bits that are in error relative to the overall number of bits that are transmitted by the communication system. • Transmission Power (or Bit Energy) (measured in Watts (or Jules/bit)): This represents the amount of power of the transmitted signal that would be required to achieve a particular desired error probability. • System Complexity (measured in cost of building or operating the system): This represents that amount of money that a manufacturer will have to spend to build the system and the amount of money that a user will have to pay to use the system. The performance of a digital transmission system is a function of the following factors: 1. Amount of energy in each digital bit (or pulse): Generally, the more energy a digital bit (or pulse) has, the better the performance that the system will have. 2. The distance between the transmitter and receiver: Because energy is spread or attenuated as it travels over the channel and more noise is added due to the existence of more noise sources over long channels, generally the longer the path that the digital transmitted signal has to travel, the worse the performance that the system will have. However, you do not always have control over the distance between the transmitted and receiver. 3. Amount of noise that is added to the signal: Certainly, the less the noise that is added to the transmitted signal, the better the performance of the communication system. We usually have limited control over the added noise. 4. Bandwidth of the transmission channel: By using larger bandwidth, we can either transmit at a higher transmission bit rate while keeping the same probability of bit error, or we can transmit at the same transmission bit rate but reduce the probability of bit error. Generally, the larger the bandwidth allocated to a communication system, the better the performance it will have. 3