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Lecturer: Tamanna Haque Nipa

Data Communication. Lecturer: Tamanna Haque Nipa. Data Communications and Networking, 4rd Edition, Behrouz A. Forouzan. Chapter 4: Digital Transmission. Introduction. The user data can be in one of two formats: Analog: Human voice as converted by typical home telephones

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Lecturer: Tamanna Haque Nipa

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  1. Data Communication Lecturer: Tamanna Haque Nipa • Data Communications and Networking, 4rd Edition, Behrouz A. Forouzan

  2. Chapter 4: Digital Transmission

  3. Introduction The user data can be in one of two formats: Analog: Human voice as converted by typical home telephones Digital: Computer files The transmitted signals, representing the data, can also be in one of two formats: Analog or Digital The conversion of the user data into a transmission signal is called Encoding. 3

  4. Encoding Techniques In data communications, the user data must be put in a format (signal) suitable for the transmission media (nature, quality, length, etc.) 4

  5. Encoding: Data-Signal Conversion There are four possible cases: Digital data, digital signals: We use Line Coding. Less complex and less expensive. Analog data, digital signals: We use A/D conversion for voice and video. Digital data, analog signals: We use Digital Modulation for optical fiber and unguided media. Analog data, analog signals:We use Analog Modulation to transmit base-band signal easily and cheaply. 5

  6. Analog Signals Carrying Analog and Digital Data

  7. Digital Signals Carrying Analog and Digital Data

  8. Digital Data, Digital Signal Digital signal Discrete, discontinuous voltage pulses Each pulse is a signal element Binary data encoded into signal elements 8

  9. Advantages of Digital Transmission • Digital technology • Low cost LSI/VLSI technology • Data integrity • Longer distances over lower quality lines • Capacity utilization • High bandwidth links economical • High degree of multiplexing easier with digital techniques • Security & Privacy • Encryption • Integration • Can treat analog and digital data similarly

  10. Disadvantages of Digital Signals • greater attenuation • digital now preferred choice

  11. DIGITAL-TO-DIGITAL CONVERSION • In this section, we see how we can represent digital data by using digital signals. The conversion involves three techniques: • Line coding • Block coding • Scrambling. • Line coding is always needed; block coding and scrambling may or may not be needed. Data Communication and Computer Networks 1303330

  12. Line coding is the process of converting digital data to digital signals. We assume that data, in the form of text, numbers, graphical images, audio, or video, are stored in computer memory as sequences of bits. Figure 4.1Line coding and decoding Signal Element Versus Data Element In data communications, our goal is to send data elements. A data element is the smallest entity that can represent a piece of information: this is the bit. In digital data communications, a signal element carries data elements. A signal element is the shortest unit (timewise) of a digital signal. In other words, data elements are what we need to send; signal elements are what we can send. Data elements are being carried; signal elements are the carriers.

  13. We define a ratio r which is the number of data elements carried by each signal element. Figure 4.2 shows several situations with different values of r. Figure 4.2 Signal element versus data element Data Rate Versus Signal Rate The data rate defines the number of data elements (bits) sent in is. The unit is bits per second (bps). The signal rate is the number of signal elements sent in Is. The unit is the baud. The data rate is sometimes called the bit rate; the signal rate is sometimes called the pulse rate, the modulation rate, or the baud rate.

  14. We now need to consider the relationship between data rate and signal rate (bit rate and baud rate). This relationship, of course, depends on the value of r. It also depends on the data pattern C. If we have a data pattern of all 1 s or all Os, the signal rate may be different from a data pattern of alternating Os and 1 s. Example 4.1 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 c is between 0 and 1? Solution We assume that the average value of c is 1/2 . The baud rate is then

  15. Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite. we can say that the bandwidth (range of frequencies) is proportional to the signal rate (baud rate). The minimum bandwidth can be given as We can solve for the maximum data rate if the bandwidth of the channel is given.

  16. Baseline Wandering In decoding a digital signal, the receiver calculates a running average of the received signal power. This average is called the baseline. The incoming signal power is evaluated against this baseline to determine the value of the data element. A long string of Os or 1 s can cause a drift in the baseline (baseline wandering) and make it difficult for the receiver to decode correctly. A good line coding scheme needs to prevent baseline wandering. DC ComponentsWhen the voltage level in a digital signal is constant for a while, the spectrum creates very low frequencies . These frequencies around zero, called DC (direct-current) components, present problems for a system that cannot pass low frequencies or a system that uses electrical coupling (via a transformer). For example, a telephone line cannot pass frequencies below 200 Hz. Also a long-distance link may use one or more transformers to isolate different parts of the line electrically. For these systems, we need a scheme with no DC component. Self-synchronizationTo correctly interpret the signals received from the sender, the receiver's bit intervals must correspond exactly to the sender's bit intervals. If the receiver clock is faster or slower, the bit intervals are not matched and the receiver might misinterpret the signals.

  17. Figure 4.3 Effect of lack of synchronization A self-synchronizing digital signal includes timing information in the data being transmitted. This can be achieved if there are transitions in the signal 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.

  18. Example 4.3 In a digital transmission, the receiver clock is 0.1 percent faster than the sender clock. How many extra bits per second does the receiver receive if the data rate is 1 kbps? How many if the data rate is 1 Mbps? Solution At 1 kbps, the receiver receives 1001 bps instead of 1000 bps. At 1 Mbps, the receiver receives 1,001,000 bps instead of 1,000,000 bps.

  19. Figure 4.4 Line coding schemes

  20. Unipolar Encoding Unipolar uses only one signal level (one polarity) High voltage is binary “1” No voltage is binary “0” Unipolar encoding is easy to implement. However: Not self-synchronized Has a DC component Compared with its polar counterpart, this scheme is very costly. The normalized power (power needed to send 1 bit per unit line resistance) is double that for polar NRZ. For this reason, this scheme is normally not used in data communications today. 20

  21. Types of Polar Encoding Polar encoding uses two signal levels Positive & Negative Polarities 21

  22. Non-Return to Zero (NRZ) Encoding NRZ encoding can be of two types: NRZ-Level (NRZ-L) “0” is encoded with one polarity, say “+5V” “1” is encoded with another polarity, say “-5V” NRZ-Invert (NRZ-I) “0” is encoded with no change in polarity from previous bit “1” is encoded with a change in polarity from previous bit NRZ-I provides better synchronization than NRZL if “1” bits exist in data stream A stream of many “0” can still cause synch. problems 22

  23. NRZ-L and NRZ-I Encoding 23

  24. NRZ pros and cons Pros Easy to engineer Make good use of bandwidth Cons dc component Lack of synchronization capability Used for magnetic recording Not often used for signal transmission 24

  25. Return to Zero (RZ) Encoding We have seen that: NRZ-L has poor synch. Performance NRZ-I has better synch. for streams of “1” but faces the same problem for streams of “0” RZ encoding overcomes this synch. issue by using three voltage levels: Positive, Negative and Zero “1” is encoded as: (“+V”, Transition “+V ↓ 0V”) “0” is encoded as: (“ −V”, Transition “−V ↑ 0V”) RZ is less spectrally efficient than NRZ because it has more transitions i.e. higher freq. components. 25

  26. RZ Encoding 26

  27. Manchester Encoding Manchester uses a polarity inversion in the middle of each bit period Low to high represents one High to low represents zero This transition is used for bit representation as well as synch. purposes. Manchester achieves the same level of synch. as RZ but with two voltage levels only 27

  28. Diff. Manchester Encoding Polarity inversion in the middle of each bit period (Tb) is used for synch. only Transition at start of a bit period represents zero No transition at start of a bit period represents one Diff. Manchester requires two signal changes to represent “0” and one signal change to represent “1” 28

  29. Manchester and Diff. Manchester Encoding 29

  30. Manchester vs. Diff. Manchester Both Manchester and Diff. Manchester encoding rely on signal transition to encode data Both have better performance in the presence of noise than any encoding scheme that relies on the absolute voltage level to encode data However, it is easy to lose sense of the polarity of a signal in a complex transmission layout 30

  31. Biphase Pros and Cons Con At least one transition per bit time and possibly two Maximum modulation rate is twice NRZ Requires more bandwidth Pros Synchronization on mid bit transition (self clocking) No dc component Error detection 31

  32. Bipolar Encoding Bipolar encoding uses three voltage levels: Positive, Negative and Zero “0” is encoded as: (“0V”) “1” is encoded by alternating between (“+V”) and (“−V”) If the first “1” is encoded as (“+V”) then the next “1” is encoded as (“−V”), and so on. This alternation occurs in the case whether these “1”s are consecutive or not Types of Bipolar Encoding Alternate Mark Inversion (AMI) Bipolar n-Zero Substitution (BnZS) High Density Bipolar 3-Zero (HDB3) 32

  33. Alternate Mark Inversion (AMI) “Mark” means “1” in telegraphy AMI means Alternate “1” Inversion AMI alternates the voltage polarity for successive “1” bits “0” bits will be represented by “0V” AMI lacks self-synchronization for long streams of “0” AMI encoding has no DC component 33

  34. Bipolar AMI Encoding Example 34

  35. In bipolar encoding(sometimes called multilevel binary), we use three levels: positive, zero, and negative. Figure 4.9 Bipolar schemes: AMI and pseudoternary The bipolar scheme was developed as an alternative to NRZ. The bipolar scheme has the same signal rate as NRZ, but there is no DC component. The NRZ scheme has most of its energy concentrated near zero frequency, which makes it unsuitable for transmission over channels with poor performance around this frequency. The concentration of the energy in bipolar encoding is around frequency N/2.

  36. Multilevel: 2B1Q scheme(two binary, one quaternary). uses data patterns of size 2 and encodes the 2-bit patterns as one signal element belonging to a four-level signal. Using 2B1Q, we can send data 2 times faster than by using NRZ-L. However, 2B 1Q uses four different signal levels, which means the receiver has to discern four different thresholds.

  37. Multilevel: 8B6T scheme eight binary, six ternary The idea is to encode a pattern of 8 bits as a pattern of 6 signal elements, where the signal has three levels (ternary). In this type of scheme, we can have 28 = 256 different data patterns and 36 = 478 different signal patterns. The mapping table is shown in Appendix D. There are 478 - 256 = 222 redundant signal elements that provide synchronization and error detection. Part of the redundancy is also used to provide DC balance. Each signal pattern has a weight of 0 or +1 DC values. This means that there is no pattern with the weight -1. To make the whole stream DC-balanced, the sender keeps track of the weight. The minimum bandwidth is very close to 6N/8.

  38. 4D-PAM5 is called four dimensional five-level pulse amplitude modulation. The 4D means that data is sent over four wires at the same time. It uses five voltage levels, such as -2, -1, 0, 1, and 2.However, one level, level 0, is used only for forward error detection . The technique is designed to send data over four channels (four wires). Figure 4.12 Multilevel: 4D-PAM5 scheme

  39. Multitransition: MLT-3 scheme 1. If the next bit is 0, there is no transition. 2. If the next bit is 1 and the current level is not 0, the next level is 0. 3. If the next bit is 1 and the current level is 0, the next level is the opposite of the lastnonzero level. The signal rate is the same as that for NRZ-I, but with greater complexity (three levels and complex transition rules). It turns out that the shape of the signal in this scheme helps to reduce the required bandwidth.

  40. Summary of line coding schemes

  41. Block Coding Block coding can ensure synchronization, provide error detection and improve the performance of line coding. In general, block coding changes a block of m bits into a block of n bits, where n is larger than m. Block coding is normally referred to as mB/nB coding; it replaces each m-bit group with an n-bit group.

  42. Block coding normally involves three steps: • Division : In the division step, a sequence of bits is divided into groups of m bits. For example, in 4B/5B encoding, the original bit sequence is divided into 4-bit groups. • Substitution: In substitution step, we substitute an m-bit group for an n-bit group. For example, in 4B/5B encoding we substitute a 4-bit code for a 5-bit group. • Combination: The n-bit groups are combined together to form a stream. The new stream has more bits than the original bits.

  43. 4B/5B mapping codes

  44. AMI used with scrambling • Bipolar AMI encoding, has a narrow bandwidth and does not create a DC component. However, a long sequence of Os upsets the synchronization. If we can find a way to avoid a long sequence of Os in the original stream, we can use bipolar AMI for long distances. • To provide synchronization but not increasing the number of bits, a solution is to substitutes long zero-level pulses with a combination of other levels to provide synchronization is called scrambling. • Note that scrambling, as opposed to block coding, is done at the same time as encoding. • The system needs to insert the required pulses based on the defined scrambling rules. • Two common scrambling techniques are B8ZS and HDB3.

  45. Two cases of B8ZS scrambling technique • B8ZS substitutes eight consecutive zeros with 000VB0VB. • HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution.

  46. ANALOG-TO-DIGITAL CONVERSION In this section we describe two techniques of analog signal to digital data, Pulse Code Modulation (PCM)Delta Modulation (DM)

  47. PCM The most common technique to change an analog signal to digital data (digitization) is called pulse code modulation (PCM). A PCM encoder has three processes. 1. The analog signal is sampled. 2. The sampled signal is quantized. 3. The quantized values are encoded as streams of bits. The analog signal is sampled every Ts, where Ts is the sample interval or period. The inverse of the sampling interval is called the sampling rate or sampling frequency and denoted by fs, where fs = 1/Ts.

  48. There are three sampling methods: ideal, natural, and flat-top. • In ideal sampling, pulses from the analog signal are sampled. This is an ideal sampling method and cannot be easily implemented. • In natural sampling, a high-speed switch is turned on for only the small period of time when the sampling occurs. The result is a sequence of samples that retains the shape of the analog signal. • The most common sampling method, called sample and hold, however, creates flat-top samples by using a circuit.

  49. We can sample a signal only if the signal is band-limited. In other words, a signal with an infinite bandwidth cannot be sampled. • The sampling rate must be at least 2 times the highest frequency, not the bandwidth. • If the analog signal is low-pass, the bandwidth and the highest frequency are the same value. If the analog signal is bandpass, the bandwidth value is lower than the value of the maximum frequency

  50. Example 4.10 A complex low-pass signal has a bandwidth of 200 kHz. What is the minimum sampling rate for this signal? Solution The bandwidth of a low-pass signal is between 0 and f, where f is the maximum frequency in the signal. Therefore, we can sample this signal at 2 times the highest frequency (200 kHz). The sampling rate is therefore 400,000 samples per second.

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