2.7 Digital Transmission

2.7 Digital Transmission • Discussing the schemes and techniques that are used to transmit data digitally • Digital-to-digital conversion techniques • Analog-to digital conversion techniques • Transmission modes 2.7.1 Digital-to-digital conversion • How to represents digital data by using digital signals. • 3 techniques used in the conversion • Line coding • Block coding • scrambling BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Line coding is the process of converting digital data to digital signals. • Data (text, numbers, graphical image, audio, video etc) are stored in computer memory as sequence of bits. • Line coding converts a sequence of bits to a digital signal. • At the sender, digital data are encoded into digital signal; at the receiver, the data are recreated by decoding the digital signal. BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Signal elements vs. data elements • In data communication, the goal is to send the data elements – smallest entity that can represent the information or simply the BIT • In digital data communications, a signal elements carries the data elements. • Data elements are need to be sent, while signal elements are what can be sent. • We defined a ratio r, which is the number of data elements carried by each signal element. BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Signal elements vs. data elements • Analogy : Suppose each data element is a person who needs to be carried from one place to another. A signal elements can be thought as a vehicle that can carry the people. r = 1 : each person is driving the vehicle r > 1 : more than one person is traveling in a vehicle (carpool) r = ½ : one person is driving a car and a trailer BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Data Rate vs. Signal Rate • Data rate defines the number of data elements (bits) sent in 1s (unit = bps) • Signal rate is the number of signal elements sent in 1s (unit = baud) • Data rate = bit rate • Signal rate = pulse rate = modulation rate = baud rate • In data communications, the goal is to increase the data rate while decreasing the signal rate (bring more people with fewer vehicles) • Increasing the data rate = increases the speed of transmission • Decreasing the signal rate = decrease the bandwidth requirement • Relationship between data rate and signal rate : • S : number of signal elements; N : data rate (bps); c : case factor; r : ratio BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Data Rate vs. Signal Rate • Ex : a signal is carrying data in which one data element is encoded as one signal element (r = 1). If the bit rate is 100 kbps, what is the average value of the baud rate if the c is between 0 and 1 ? BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Bandwidth • Most of nonperiodic digital signals encountered in the daily life have a bandwidth with finite values. • In other words, the bandwidth is theoretically infinite, but many of the components have such a small amplitude and can be ignored. • Means the effective bandwidth is finite. • Baud rate determines the required bandwidth (the vehicles affects the traffic, not the people !) • More changes in the signal = injecting more frequencies into the signal. • More frequencies = wider range of frequencies = wider bandwidth • Thus bandwidth (frequency range) is proportional to the signal rate (baud rate) • The minimum bandwidth can be defined as BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Bandwidth • Thus the maximum data rate (Nmax) can be solved if the bandwidth of the channel is given BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Design Consideration for Line Coding Scheme • Baseline wandering • DC components • Self-synchronization • Built-in error detection • Immunity to noise and interference • complexity BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Baseline Wandering • In decoding a digital signal, the receiver calculates a running average of the received signal power – called as a baseline. • The incoming signal power is evaluated against this baseline to determine the value of the data element. • A long string of 0s and 1s can cause a drift in the baseline (baseline wandering) and make it difficult for the receiver to decode correctly. • A good line coding scheme is needed to prevent baseline wandering. BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • DC Components • When the voltage level in the digital signal is constant for a while, the spectrum creates a very low frequencies. • These frequencies around zero (DC components), present a problem for a system that cannot pass low frequencies or a system that uses electrical coupling (via a transformer). • Ex : telephone line cannot pass frequencies below 200 Hz. • Ex : a long-distance link may use one or more transformers to isolate different parts of the line electrically. • For these systems, a scheme with no DC component is necessary. BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Self-synchronization • to correctly interpret the signals received from the sender, the receiver’s bit intervals must correspond exactly to the sender’s bit intervals. BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Self-synchronization • A self-synchronizing digital signal includes timing information in the data being transmitted. • It can be achieved if there are transitions in the signals that alert the receiver to the beginning, middle or end of the pulse. • If the receiver’s clock is out of synchronization, these points can reset the clock. • Built-in Error Detection • It is desirable to have a built-in error-detecting capability in the generated code to detect some or all the errors that occurred during transmission. BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Immunity to noise and interference • It is desirable that the code is immune to noise and other interference. • Complexity • A complex scheme is more costly to implement than a simple one. Ex : a scheme that uses four signal levels is more difficult to interpret than one that uses only two levels BENG 4522 Data Communications & Computer Networks

2.7.1.1 Line Coding • Line coding scheme can be divided into five broad categories : BENG 4522 Data Communications & Computer Networks

2.7.1.1.1 Unipolar Sheme • In a unipolar scheme, all the signal levels are on one side of the time axis; either above or below. • NRZ (Non-Return-to-Zero) scheme • Designed so that positive voltage defines bit 1 and the zero voltage defines bit 0. • It is called NRZ because the signal does not return to zero at the middle of the bit. BENG 4522 Data Communications & Computer Networks

2.7.1.1.1 Unipolar Scheme • Unipolar NRZ (None-Return-to-Zero) is simple, but • DC component : Cannot travel through system that does not allow a low frequency component to passage (ex : Transformer) • Synchronization: Consecutive 0’s and 1’s are hard to be synchronized  Separate line for a clock pulse • Normalized power is double that for polar NRZ BENG 4522 Data Communications & Computer Networks

2.7.1.1.2 Polar Scheme • In a polar scheme, the voltages are on both sides of the time axis. • Ex : positive voltage level for bit 1, negative voltage level for bit 0. BENG 4522 Data Communications & Computer Networks

2.7.1.1.2 Polar Scheme : NRZ (NRZ-L & NRZ-I) • In a polar scheme, the voltages are on both sides of the time axis. • Ex : positive voltage level for bit 1, negative voltage level for bit 0. • Polar NRZ • NRZ-L (Non Return to Zero-Level) – Level of the voltage determines the value of the bit • NRZ-I (Non Return to Zero-Invert) - Inversion or the lack of inversion determines the value of the bit BENG 4522 Data Communications & Computer Networks

2.7.1.1.3 Polar Scheme : RZ • Provides synchronization for consecutive 0s/1s • Signal changes during each bit • Three values (+, -, 0) are used • Bit 1: positive-to-zero transition, bit 0: negative-to-zero transition BENG 4522 Data Communications & Computer Networks

2.7.1.1.4 Polar Scheme : Biphase • Combination of RZ and NRZ-L/NRZ-I ideas • Signal transition at the middle of the bit is used for synchronization • Manchester (combine RZ & NRZ-L) • Used for Ethernet LAN • Bit 1: negative-to-positive transition • Bit 0: positive-to-negative transition • Differential Manchester (combine RZ & NRZ-I) • Used for Token-ring LAN • Bit 1: no transition at the beginning of a bit • Bit 0: transition at the beginning of a bit • Minimum bandwidth is 2 times of that NRZ BENG 4522 Data Communications & Computer Networks

2.7.1.1.4 Polar Scheme : Biphase BENG 4522 Data Communications & Computer Networks

2.7.1.1.5 Bipolar Scheme • Three levels of voltage, called “multilevel binary” • Bit 0: zero voltage, bit 1: alternating +1/-1 • AMI (Alternate Mark Inversion) and pseudoternary • No DC component BENG 4522 Data Communications & Computer Networks

2.7.1.1.6 Multilevel Scheme • To increase the number of bits per baud by encoding a pattern of m data elements into a pattern of n signal elements • In mBnL schemes, a pattern of m data elements is encoded as a pattern of n signal elements in which 2m≤ Ln • mBnL : m (length of binary pattern), B (binary data), n (length of the signal pattern), L (number of levels in the signaling) • 2B1Q (two binary, one quaternary) • 8B6T (eight binary, six ternary) • 4D-PAM 5 (four-dimensional five-level pulse amplitude modulation) BENG 4522 Data Communications & Computer Networks

2.7.1.1.6 Multilevel Scheme (2B1Q) for DSL BENG 4522 Data Communications & Computer Networks

2.7.1.1.6 Multilevel Scheme (8B6T) • Used with 100Base-4T cable • Encode a pattern of 8 bits as a pattern of 6 (three-levels) signal elements • The average signal rate is theoretically, Save = 1/2 x N x 6/8; in practice the minimum bandwidth is very close to 6N/8 BENG 4522 Data Communications & Computer Networks

2.7.1.1.6 Multilevel Scheme 4D-PAM5: for Gigabit LAN BENG 4522 Data Communications & Computer Networks

2.7.1.1.7 Multiline Transition : MLT-3 • The signal rate for MLT-3 is one-fourth the bit rate • MLT-3 when we need to send 100Mbps on a copper wire that cannot support more than 32MHz BENG 4522 Data Communications & Computer Networks

2.7.1.1 Summary of Line Coding Scheme BENG 4522 Data Communications & Computer Networks

2.7 Digital Transmission