B.E SEMESTER: 4 INFORMATION TECHNOLOGY

Transcription

1 B.E SEMESTER: 4 INFORMATION TECHNOLOGY 1 Prepared by: Prof. Amish Tankariya SUBJECT NAME : DATA COMMUNICATION & NETWORKING 2 Subject Code

2 3 TOPIC: DIGITAL-TO-DIGITAL CONVERSION Chap: 5. ENCODING AND MODULATION DIGITAL-TO-DIGITAL CONVERSION o In this section, we see how we can represent digital data by using digital signals. o The conversion involves three techniques: o line coding, o block coding, and o scrambling. o Line coding is always needed; block coding and scrambling may or may not be needed. Topics discussed in this section: Line Coding Line Coding Schemes Block Coding Scrambling 4 2

3 LINE CODING: 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. Line coding converts a sequence of bits to a digital signal. Converting a string of 1 s and 0 s (digital data) into a sequence of signals that denote the 1 s and 0 s. For example a high voltage level (+V) could represent a 1 and a low voltage level (0 or -V) could represent a 0. 5 LINE CODING AND DECODING: At the sender, digital data are encoded into a digital signal; at the receiver, the digital data are recreated by decoding the digital signal. Converting a string of 1 s and 0 s (digital data) into a sequence of signals that denote the 1 s and 0 s. 6 3

4 SIGNAL ELEMENT VERSUS DATA ELEMENT Let us distinguish between a data element and a signal element. A data element is the smallest entity that can represent a piece of information: this is the bit. A data symbol (or element) can consist of a number of data bits: 1, 0 or 11, 10, 01, A signal element carries data elements. A signal element is the shortest unit (time-wise) of a digital signal. A data symbol can be coded into a single signal element or multiple signal elements 1 +V, 0 -V 1 +V and -V, 0 -V and +V 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. 7 CONTINUE... We define a ratio r which is the number of data elements carried by each signal element. Figure shows several situations with different values of r. 8 4

5 CONTINUE.. r = Data element/signal element In part a of the figure, one data element is carried by one signal element (r=1). In part b of the figure, we need two signal elements (two transitions) to carry each data element (r=1/2). In part c of the figure, a signal element carries two data elements (r=2). In part d, a group of 4 bits is being carried by a group of three signal elements (r=4/3). 9 AN ANALOGY Suppose each data element is a person who needs to be carried from one place to another. We can think of a signal element as a vehicle that can carry people. When r = 1, it means each person is driving a vehicle. When r > 1, it means more than one person is travelling in a vehicle (a carpool, for example). We can also have the case where one person is driving a car and a trailer (r=1/2). 10 5

6 RELATIONSHIP BETWEEN DATA RATE AND SIGNAL RATE The data rate defines the number of bits sent per sec - bps. It is often referred to the bit rate. The signal rate is the number of signal elements sent in a second and is measured in bauds. It is also referred to as pulse rate, the modulation rate or baud Rate. Goal is to increase the data rate whilst reducing the baud rate. Increasing the data rate increases the speed of transmission; Decreasing the signal rate decreases the bandwidth requirement. In our vehicle-people analogy, we need to carry more people in fewer vehicles to prevent traffic jams. We have a limited11 bandwidth in our transportation system. CONTINUE.. This relationship, depends on the value of r. It also depends on the data pattern. If we have a data pattern of all 1s or all 0s, the signal rate may be different from a data pattern of alternating 0s and 1s. To derive a formula for the relationship, we need to define three cases: Worst, Best, Average. The worst case is when we need the maximum signal rate; The best case is when we need the minimum signal rate. In data communications, we are usually interested in the average case. 12 6

7 CONTINUE.. We can formulate the relationship between data rate and signal rate as, Let S be signal rate, N is data rate, r is the number of data elements carried by each signal element S= N/r The baud or signal rate can be expressed as: S = c x N x 1/r bauds where N is data rate c is the case factor [worst(1), best(0) & avg.(1/2)] r is the ratio between data element & signal element We can say that the bandwidth (range of frequencies) is proportional to the signal rate (baud rate), The minimum bandwidth can be given as Bmin= c N 1/r We can solve for the maximum data rate if the bandwidth of the channel is given Nmax= 1/c B r 13 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 NOTE: Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite. 14 7

8 Example 4.2 The maximum data rate of a channel is Nmax = 2 B log 2 L (defined by the Nyquist formula). Does this agree with the previous formula for Nmax? Solution A signal with L levels actually can carry log 2 L bits per level. If each level corresponds to one signal element and we assume the average case (c = 1/2), then we have NOTE: We can say that the baud rate, not the bit rate, determines the required bandwidth for a digital signal. If we use the transportation analogy, the number of vehicles affects the traffic, not the number of people being carried. 15 CONSIDERATIONS FOR CHOOSING A GOOD SIGNAL ELEMENT REFERRED TO LINE ENCODING 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 0s or 1s can cause a drift in the baseline (baseline wandering) and make it difficult for the receiver to decode correctly. If the incoming signal varies over a long period of time, the baseline will drift and thus cause errors in detection of incoming data elements. A good line encoding scheme will prevent long runs of fixed amplitude. 16 8

9 CONTINUE.. DC components When the voltage level in a digital signal is constant for a while, the spectrum creates very low frequencies. These frequencies around zero, called DC (directcurrent) components, present problems for a system that cannot pass low frequencies. For example, a telephone line cannot pass frequencies below 200 Hz. For these systems, we need a scheme with no DC component. 17 CONTINUE.. 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. If the receiver clock is faster or slower, the bit intervals are not matched and the receiver might misinterpret the signals. 18 9

10 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. 19 EXAMPLE: 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: Sender clock: x Receiver clock: x+0.001x = 1.001x For data rate 1kbps, taking x=1, In 1 second Sender transmits 1000 bit Then in second Receiver receives how many bits? At 1 kbps, the receiver receives (1.001*1000=1001) bps instead of 1000 bps. At 1 Mbps, the receiver receives 1,001,000 bps instead of 20 1,000,000 bps. 10

11 CONTINUE.. Built-in Error detection : Errors occur during transmission, it is desirable to have a built-in error-detecting capability to detect some of or all the errors that occurred during transmission. Some codes are constructed such that when an error occurs it can be detected. For example: a particular signal transition is not part of the code. When it occurs, the receiver will know that a symbol error has occurred. 21 CONTINUE.. Noise and interference Another desirable code characteristic is a code, that is immune to noise and other interferences. Some encoding schemes will have this capability. There are line encoding techniques that make the transmitted signal immune to noise and interference. This means that the signal cannot be corrupted, it is stronger than error detection. Complexity: A complex scheme is more costly to implement than a simple one. For example, a scheme that uses four signal levels is more difficult to interpret than one that uses only two levels

12 LINE CODING SCHEMES 23 LINE CODING SCHEMES: We can roughly divide line coding schemes into five broad categories, as shown in 24 12

13 1)UNIPOLAR In a unipolar scheme, All signal levels are on one side of the time axis - either above or below NRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission. Scheme is prone to baseline wandering and DC components. It has no synchronization or any error detection. It is simple but costly in power consumption. 25 Unipolar NRZ scheme 2)POLAR Polar encoding has the following Categories: Nonreturn to Zero (NRZ) NRZ-L (L=Level) NRZ-I (I=Inverted) Return to Zero (RZ) Manchester Differential Manchester 26 13

14 POLAR - NRZ In polar schemes, the voltages are on both sides of the time axis. Polar NRZ scheme can be implemented with two voltages. E.g. +V for 0 and -V for 1. There are two versions: NZR - Level (NRZ-L) - positive voltage for one symbol and negative for the other NRZ - Inversion (NRZ-I) - the change or lack of change in polarity determines the value of a symbol. E.g. a 1 symbol inverts the polarity a 0 does not. 27 NOTE: In NRZ-L the level of the voltage determines the value of the bit. In NRZ-I the inversion or the lack of inversion determines the value of the bit. NRZ-L and NRZ-I both have an average signal rate of N/2 Bd. NRZ-L and NRZ-I both have a DC component problem and baseline wandering, it is worse for NRZ-L. Both have no self synchronization & no error detection. Both are relatively simple to implement

15 Example: A system is using NRZ-I to transfer 1-Mbps data. What are the average signal rate and minimum bandwidth? Solution The average signal rate is S= c x N x 1/r = 1/2 x 10,00,000 x 1 = 500 kbaud. The minimum bandwidth for this average baud rate is B min = S = 500 khz. Note c = 1/2 for the avg. case as worst case is 1 and best case is 0 29 POLAR RZ: The Return to Zero (RZ) scheme uses three voltage values. +, 0, -. Each symbol has a transition in the middle. Either from high to zero or from low to zero. This scheme has more signal transitions (two per symbol) and therefore requires a wider bandwidth. No DC components or baseline wandering. Self synchronization - transition indicates symbol value. More complex as it uses three voltage level. It has no error detection capability. 30 Polar RZ scheme 15

16 POLAR BIPHASE: Manchester coding consists of combining the NRZ-L and RZ schemes. In Manchester encoding, the duration of the bit is divided into two halves. The voltage remains at one level during the first half and moves to the other level in the second half. The transition at the middle of the bit provides synchronization. 31 CONTINUE.. Differential Manchester coding consists of combining the NRZ-I and RZ schemes. There is always a transition at the middle of the bit, but the bit values are determined at the beginning of the bit. If the next bit is 0, there is a transition; if the next bit is 1, there is none

17 NOTE: The Manchester scheme overcomes several problems associated with NRZ-L, And differential Manchester overcomes several problems associated with NRZ-I. In Manchester and differential Manchester encoding, the transition at the middle of the bit is used for synchronization. The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ. The is no DC component and no baseline wandering. None of these codes has error detection. 33 3)BIPOLAR SCHEMES: In bipolar encoding (sometimes called multilevel binary), there are three voltage levels: positive, negative, and zero. Code uses 3 voltage levels: +, 0, -, to represent the symbols The voltage level for one data element is at zero, while the voltage level for the other element alternates between positive and negative. Two variations of bipolar encoding are: AMI 34 and pseudoternary. 17

18 AMI AND PSEUDOTERNARY A common bipolar encoding scheme is called bipolar Alternate Mark Inversion (AMI). In the termalternatemarkinversion,theword markcomes from telegraphy and means 1. So AMI means alternate 1 inversion. A neutral zero voltage represents binary 0 And Binary 1s are represented by alternating positive and negative voltages. 35 CONTINUE.. A variation / opposite of AMI encoding is called PSEUDOTERNARY, in which the 1 bit is encoded as a zero voltage and the 0 bit is encoded as alternating positive and negative voltages. Pseudoternary is the reverse of AMI 36 18

19 CONTINUE.. It is a better alternative to NRZ. It has no DC component or baseline wandering. It has no self synchronization because long runs of 0 s results in no signal transitions. No error detection. 37 4)MULTILEVEL SCHEMES The desire to increase the data speed or decrease the required bandwidth has resulted in the creation of many schemes. The goal is to increase the number of bits per baud by encoding a pattern of m data elements into a pattern of n signal elements. In these schemes we increase the number of data bits per symbol thereby increasing the bit rate. Since we are dealing with binary data we only have 2 types of data element a 1 or a 0. We can combine the 2 data elements into a pattern of m elements to create 2 m symbols. Example: 00,01,10,11 If we have L signal levels, we can use n signal elements to create L n signal elements. Now we have 2 m symbols and L n signals

20 CONTINUE.. If 2 m > L n then we cannot represent the data elements, we don t have enough signals. If 2 m = L n then we have an exact mapping of one symbol on one signal. If 2 m < L n then we have more signals than symbols and we can choose the signals to represent the symbols and therefore have better noise immunity and error detection as some signals are not valid. 39 CONTINUE.. Multilevel The code designers have classified these types of coding as mbnl where m is the length of the binary pattern, B means binary data, n is the length of the signal pattern, and L is the number of levels in the signaling. L can be represented as: L = 2 for B (Binary) L = 3 for T (Ternary) L = 4 for Q (Quaternary) Note that the first two letters define the data pattern, and the second two define the signal pattern. A pattern of m data elements is encoded as a pattern of n signal elements in which 2 m L n. Example: 2B1Q: 2 binary 1 quaternary (2 2 data patterns and 4 1 signal pattern) 8B6T: 8 binary 6 ternary. 2 8 data patterns, and 3 6 signal pattern 4D-PAM5 (4-D 5-level Pulse Amplitude Mod.) 40 20

21 2B1Q The first mbnl scheme, two binary, one quaternary (2B1Q), uses data patterns of size 2 and encodes by one signal element of four-level signal. However, 2B1Q uses four different signal levels, The average signal rate of 2B1Q iss=n/4. This means that using 2B1Q, we can send data 2 times faster than by using NRZ-L. The reduced bandwidth comes with a price. There are no redundant signal patterns in this scheme because = B6T A very interesting scheme is eight binary, six ternary (8B6T). 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 2 8 = 256 different data patterns and 3 6 = 478 different signal patterns. There are = 222 redundant signal elements that provide synchronization and error detection. Part of the redundancy is also used to provide DC balance. Figure shows an example where 8 bit data pattern is encoded as 6 signal patterns. The average signal rate of the scheme is theoretically S avg = ½ Ν 6/8 ; 42 21

22 4D-PAM5 The last signaling scheme we discuss in this category is called Fourdimensional-five-level Pulse Amplitude Modulation (4D-PAM5). 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. The technique is designed to send data over four channels (four wires). All 8 bits can be fed into a wire simultaneously and sent by using one signal element. The point here is that the four signal elements comprising one signal group are sent simultaneously in a four-dimensional setting. Figure shows the imaginary one-dimensional and the actual fourdimensional implementation. 43 5)MULTITRANSITION CODING If we have a signal with more than two levels, we can design a differential encoding scheme with more than two transition rules. MLT-3 is one of them. The multiline transmission, three level (MLT-3) scheme uses three levels (+V, 0, and V) and three transition rules to move between the levels. 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 last nonzero level

23 FIGURE : MULTITRANSITION: MLT-3 SCHEME 45 TABLE: SUMMARY OF LINE CODING SCHEMES 46 23

24 BLOCK CODING For a code to be capable of error detection and synchronization, we need to add redundancy, i.e., extra bits to the data bits. Block coding changes a block ofmbits into a block of n bits, where n is larger than m. Block coding is referred to as an mb/nb encoding technique. Block coding is done in three steps: division, substitution and combination. The resulting bit stream prevents certain bit combinations that would result in DC components or poor sync. Quality when used with line encoding. 47 FIGURE: BLOCK CODING CONCEPT Using block coding 4B/5B with NRZ-I line coding scheme 48 24

25 SCRAMBLING The best code is one that does not increase the bandwidth for synchronization and has no DC components. Scrambling is a technique used to create a sequence of bits that is required for transmission - self clocking, no low frequencies, no wide bandwidth. We are looking for a solution that substitutes long zero-level pulses with a combination of other levels to provide synchronization. Scrambling replaces unfriendly runs of bits with a violation code that is easy to recognize and removes the unfriendly sequence. 49 B8ZS For example: B8ZS substitutes eight consecutive zeros with 000VB0VB. The V stands for violation, it violates the line encoding rule B stands for bipolar, it implements the bipolar line encoding rule

26 HDB3 HDB3 substitutes four consecutive zeros with 000V or B00V depending on the number of nonzero pulses after the last substitution. 1) If the number of nonzero pulses after the last substitution is odd, the substitution pattern will be 000V, which makes the total number of nonzero pulses even. 2) If the number of nonzero pulses after the last substitution is even, the substitution pattern will be B00V, which makes the total number of nonzero pulses even. 51 REFERENCE BOOK: Data communication & Networking by Bahrouz Forouzan

27 QUESTIONS 53 27