Data Communication Digital Transmition

Data Communication Digital Transmition
Behrouz A. Forouzan Data Communication - Digital Transmition

Data Communication - Digital Transmition
Index Digital to Digital Conversion Analog to Digital Conversion Transmission Modes Data Communication - Digital Transmition

Digital to Digital Conversion
Techniques line coding Always needed block coding May or may not needed Scrambling Data Communication - Digital Transmition

Digital to Digital Conversion Line Coding
At sender, digital data are encoded into digital signal At receiver, digital data are recreated by decoding the digital signal Data Communication - Digital Transmition

Digital to Digital Conversion Line Coding Characteristics
Signal Element Versus Data Element Data Rate Versus Signal Rate Required Bandwidth Baseline Wandering DC Components Self-synchronization Built-in Error Detection Immunity to Noise and Interference Complexity Data Communication - Digital Transmition

Digital to Digital Conversion Data element Versus Signal Element
smallest entity that can represent a piece of information (Bit) Carried Signal Element the shortest unit of a digital signal Carrier r : number of data elements carried by each signal element Data Communication - Digital Transmition

Digital to Digital Conversion Data element Versus Signal Element
Data Communication - Digital Transmition

Digital to Digital Conversion Data Rate Versus Signal Rate
number of data bits sent in 1 Sec. (bps) Speed of transmition Signal Rate number of signal elements sent in 1 Sec. (baud) Also called pulse rate or modulation rate More signal rate more bandwidth requirement Interest to increase the data rate while decreasing the signal rate Data Communication - Digital Transmition

Digital to Digital Conversion Data Rate Versus Signal Rate
Relationship depend on: data stream (all 0, all 1 or alternate 0 1) r (data element / signal element) Relationship Formula: define three cases: the worst (maximum signal rate), best (minimum signal rate ), and average N = data rate (bps); c is the case factor S is signal rate r previously defined Data Communication - Digital Transmition

Digital to Digital Conversion Required Bandwidth
actual bandwidth of digital signal is infinite but effective bandwidth is finite baud rate, not bit rate, determines required bandwidth for a digital signal. More changes means more baud rate means more frequency range means more required bandwidth Minimum bandwidth for a given data rate: Maximum data rate for a given Bandwidth: . Data Communication - Digital Transmition

Digital to Digital Conversion Required Bandwidth
Compare Nyquist formula with previous formula c = ½ (average case) and L = 2 , Then two formulas are the same Data Communication - Digital Transmition

Digital to Digital Conversion 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) Baseline wandering make it difficult for the receiver to decode correctly A good line coding scheme needs to prevent baseline wandering. Data Communication - Digital Transmition

Digital to Digital Conversion DC components
When voltage level in a digital signal is constant for a while, spectrum creates very low frequencies (around zero), called DC components Creates problems for a system that cannot pass low frequencies a telephone line cannot pass frequencies below 200 Hz. Data Communication - Digital Transmition

Digital to Digital Conversion Self-synchronization
receiver's bit intervals must correspond exactly to the sender's bit intervals self-synchronizing digital signal includes timing information in the data being transmitted. This achieved if there are transitions in the signal that alert the receiver to the beginning, middle, or end of the pulse. Data Communication - Digital Transmition

Digital to Digital Conversion Built-in Error Detection
built-in error-detecting capability in the generated code to detect some of or all the errors that occurred during transmission Differentiated coding Data Communication - Digital Transmition

Digital to Digital Conversion Immunity to Noise and Interference
Good coding that is immune to noise and other interferences Lower levels Data Communication - Digital Transmition

Digital to Digital Conversion Complexity
complex scheme is more costly to implement than a simple one Lower levels , lower signal change Data Communication - Digital Transmition

Digital to Digital Conversion Unipolar NRZ
Unipolar Scheme NRZ (Non-Return-to-Zero) Polar Schemes NRZ-L NRZ-I Return to Zero (RZ) Biphase Manchester Differential Manchester Bipolar Schemes (multilevel binary) AMI Pseudoternary Multilevel Schemes 2B/IQ, 8B/6T, and 4U-PAM5 Multitransition Multiline Transmission MLT-3 Data Communication - Digital Transmition

Digital to Digital Conversion unipolar - NRZ
In a unipolar scheme, all the signal levels are on one side of the time axis, either above or below non-return-to-zero (NRZ) Bit 0: zero voltage Bit 1: positive voltage NRZ means the signal does not return to zero at the middle of the bit Data Communication - Digital Transmition

Digital to Digital Conversion Line Coding Schemes
Very costly because normalized power (power needed to send 1 bit) is double that for polar NRZ. this scheme is normally not used in data communications. Data Communication - Digital Transmition

Digital to Digital Conversion Polar / NRZ-L, NRZ-I
In polar schemes, the voltages are on the both sides of the time axis NRZ-Level level of the voltage determines the value of the bit Bit 0: positive voltage Bit 1: negative voltage NRZ-Invert change or lack of voltage change determines value of the bit Bit 0: there is no change Bit 1: there is a change Data Communication - Digital Transmition

Bit Stream: Data Communication - Digital Transmition

Digital to Digital Conversion Polar / NRZ-L, NRZ-I Characteristic
sudden change of polarity resulting in all 0s interpreted as 1s and all 0s interpreted as 1s Required Bandwidth Average: N/2 Baseline Wandering Yes twice as severe in NRZ-L (long sequence of 0s or 1s) compare to NRZ-I (long sequence of 0s) DC Components value of power density is very high around frequencies close to zero Self-synchronization NO more serious in NRZ-L Data Communication - Digital Transmition

Digital to Digital Conversion Polar / Return to Zero (RZ)
Solution to synchronization problem in NRZ methods uses three values: positive, negative, and zero signal goes to 0 in the middle of each bit. Advantages: There is no DC component problem DisAdvantages: it requires two signal changes to encode a bit and occupies greater bandwidth a sudden change of sudden change of polarity resulting in all 0s interpreted as 1s and all 0s interpreted as 1s complexity three levels of voltage, which is more complex to create and discern Data Communication - Digital Transmition

Digital to Digital Conversion Polar / Return to Zero (RZ)

Digital to Digital Conversion Manchester, Differentiated Manchester
idea of RZ and idea of NRZ-L are combined always a transition at the middle of the bit, Voltage level is determined by bit value like NRZ-L Bit 0: positive voltage Bit 1: negative voltage combines the ideas of RZ and NRZ-I bit values are determined at the beginning of the bit Bit 0: transition Bit 1: no transition Data Communication - Digital Transmition

Advantage: Self- synchronization no baseline wandering no DC component Drawback: signal rate is double that for NRZ minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ Data Communication - Digital Transmition

Digital to Digital Conversion Bipolar / AMI and Pseudoternary
there are three voltage levels: positive, negative, and zero. Alternate mark inversion (AMI): Mark means 1 Bit 0: zero voltage Bit 1: alternating positive and negative voltages. pseudoternary Bit 0: alternating positive and negative voltages. Bit 1: zero voltage Data Communication - Digital Transmition

Alternative to NRZ but better Advantage: same signal rate as NRZ, but no DC component (why?) For a long sequence of 0s, voltage remains constant, but its amplitude is zero, which is the same as having no DC component We can prove it by using the Fourier transform energy in bipolar encoding is around frequency N/2 but in NRZ energy was around zero which unsuitable for transmission over channels with poor performance around this frequency commonly used for long-distance communication Drawback: Synchronization problem when a long sequence of 0s scrambling technique can solve this problem (learn later) Data Communication - Digital Transmition

Digital to Digital Conversion Multilevel Schemes
In mBnL schemes, a pattern of m data elements can be encoded as a pattern of n signal elements with L Levels in which ≤ If < data patterns occupy only a subset of signal patterns. The subset can be carefully designed to prevent baseline wandering, to provide synchronization, and to detect errors that occurred during data transmission B (binary data) for L (Level) (binary) for L =2, T (ternary) for L =3, Q (quaternary) for L =4. Data Communication - Digital Transmition

Digital to Digital Conversion Multilevel Schemes / 2B1Q
2BIQ: two binary, one quaternary used in DSL encodes the 2-bit data patterns as one signal element belonging to a four-level signal Bit 00: +1 volt Bit 01: +3 volt Bit 10: -1 volt Bit 11: - 3 volt Advantages: average signal rate of 2BlQ is S =N/4. Drawbacks: receiver has to discern four different thresholds (no noise immunity) Data Communication - Digital Transmition

Digital to Digital Conversion Multilevel Schemes / 8B6T
8B6T: eight binary, six ternary Used in 100BASE-4T cable 222 redundant signal elements that provide synchronization and error detection and DC balance DC balance: To make the whole stream Dc-balanced, the sender keeps track of the weight. Each signal pattern has a weight of 0 or +1 DC value If two groups of weight 1 are encountered one after another, the first one is sent as is, while the next one is totally inverted to give a weight of -1 Data Communication - Digital Transmition

Digital to Digital Conversion Multilevel Schemes / 8B6T
Example: The first 8-bit pattern is encoded as the signal pattern with weight 0 the second 8-bit pattern is encoded as with weight +1. The third bit pattern should be encoded as with weight +1 To create DC balance, the sender inverts the actual signal. The receiver can easily recognize that this is an inverted pattern because the weight is -1 Theory: Savg = ½ *6/8 *N Practice: Savg = 6/8 *N Data Communication - Digital Transmition

Digital to Digital Conversion Multilevel Schemes / 4D-PAMS
4D-PAMS: four dimensional five-level pulse amplitude modulation (4D-PAM5) 4D: data is sent over four wires at the same time It uses five voltage levels, such as -2, -1, 0, 1, and 2 level 0, is used only for forward error detection If we assume that the code is just one-dimensional, the four levels create something similar to 8B4Q. Signal rate is 4N/8 = N/2 With four channels (4 wires), signal rate can be reduced to N/8 Gigabit LANs use this technique to send 1-Gbps data over four copper cables with 125 Mbaud (4 * 250Mbps = 1Gbps) a lot of redundancy in the signal pattern Data Communication - Digital Transmition

Digital to Digital Conversion Multiline Transmission: MLT-3
NRZ-I and differential Manchester are differential encoding method with two transition rules (no inversion, inversion). signal with more than two levels, Then differential encoding with more than two transition rules. Data Communication - Digital Transmition

three level (MLT-3) scheme uses three levels (+V, 0, and - V) and three transition rules to move between the levels Bit 0: no transition Bit 1 and current level not 0 : level 0 Bit 1 and current level 0: opposite of last non-zero level Data Communication - Digital Transmition

1 bit for 1 Signal element So Signal rate = NRZ-I but greater complexity why choose this method? worst-case scenario: A sequence of Is. signal element pattern is +VO - VO is repeated every 4 bits. A nonperiodic signal has changed to a periodic signal with the period equal to 4 times the bit duration. This worst-case situation can be simulated as an analog signal with a frequency one-fourth of the bit rate. signal rate for MLT-3 is one-fourth the bit rate MLT-3 a suitable choice when we need to send 100 Mbps on a copper wire that cannot support more than 32 MHz Data Communication - Digital Transmition

Digital to Digital Conversion Summary of Line Coding Schemes

Digital to Digital Conversion Block Coding
Block coding gives redundancy to ensure synchronization and to provide error detecting. Block coding (mBlnB coding) replaces each m-bit group with an n-bit group, (n is larger than m) division substitution combination Methods: 4B/5B 8B/10B Data Communication - Digital Transmition

Digital to Digital Conversion Block Coding 4B/5B
designed to be used with NRZ-I which has a good signal rate, but synchronization problem Solution: 4B/5B Block Coding no more than one leading zero (left bit) and no more than two trailing zeros (right bits) Data Communication - Digital Transmition

4 bits  16 different combinations 5 bits  32 different combinations. there are 16 groups that are not used for 4B/5B encoding and used for control purposes (unused) error detection If a 5-bit group arrives that belongs to the unused portion of the table, the receiver knows that there is an error in the transmission Data Communication - Digital Transmition

add 20 percent more baud rate Still, signal rate is less than biphase (2-times of NRZ-I) don't solve DC component problem of NRZ-I If a DC component is unacceptable, use biphase or bipolar encoding Data Communication - Digital Transmition

Example: We need to send data at a 1-Mbps rate. What is the minimum required bandwidth, using a combination of 4B/5B and NRZ-I or Manchester coding? 4B/5B : increases bit rate to 1.25 Mbps. minimum bandwidth using NRZ-I is NI2 or 625 kHz. DC Problem Manchester: needs a minimum bandwidth of 1 MHz. No DC problem Data Communication - Digital Transmition

group of 8 bits data is substituted by a 10 bit 768 redundant groups Better built-in error-checking capability and better synchronization than 4B/5B a combination of 5B/6B and 3B/4B encoding, Data Communication - Digital Transmition

Digital to Digital Conversion Scrambling
Biphase (used in LAN ) are not suitable for long-distance communication because of their wide bandwidth requirement block coding with NRZ-I is not suitable for long-distance, because of the DC component Bipolar AMI has narrow bandwidth and does not create a DC component However, a long sequence of 0s upsets the synchronization Data Communication - Digital Transmition

Digital to Digital Conversion Scrambling
Scrambling in bipolar AMI does not increase the number of bits and does provide synchronization Used for long distances substitutes long zero-level pulses with a combination of other levels to provide synchronization scrambling, as opposed to block coding, is done at the same time as encoding Methods: B8ZS HDB3 Data Communication - Digital Transmition

Digital to Digital Conversion Scrambling B8ZS
eight consecutive zero-level voltages are replaced by the sequence 000VB0VB V denotes violation: nonzero voltage that breaks AMI rule B denotes bipolar; nonzero voltage in accordance with AMI rule Data Communication - Digital Transmition

Digital to Digital Conversion Scrambling B8ZS
does not change the bit rate DC balance is maintained (two positives and two negatives) Data Communication - Digital Transmition

Digital to Digital Conversion Scrambling HDB3
four consecutive zero-level voltages are replaced with a sequence of 000V or B00V reason for two different substitutions is to maintain the even number of nonzero pulses after each substitution If number of nonzero pulses after the last substitution is odd, substitution pattern will be 000V If number of nonzero pulses after the last substitution is even, substitution pattern will be B00V Data Communication - Digital Transmition

Digital to Digital Conversion Scrambling HDB3

ANALOG-TO-DIGITAL CONVERSION
Pulse Code Modulation (PCM) Delta Modulation Data Communication - Digital Transmition

ANALOG-TO-DIGITAL CONVERSION PCM
1. analog signal is sampled. 2. sampled signal is quantized. 3. quantized values are encoded as bits Data Communication - Digital Transmition

ANALOG-TO-DIGITAL CONVERSION PCM Sampling
Sampling Rate is Sampling Interval Ts = 1/is According to the Nyquist theorem, to reproduce original analog signal, sampling rate must be at least 2 times highest frequency contained in signal Data Communication - Digital Transmition

Oversampling gives nothing more Under sampling lose information Data Communication - Digital Transmition

second hand of a clock has a period of 60 s. According to the Nyquist theorem, we need to sample the hand (take and send a picture) every 30 s 30 s Sampling (Sampling at Nyquist rate): 12, 6, 12, 6, 12, and 6. receiver of samples cannot tell if clock is moving forward or backward 15 s Sampling (Oversampling): 12,3,6, 9, and 12 clock is moving forward 45 s Sampling (Undersampling): 12, 9,6,3, and 12 clock is moving forward, receiver thinks that it is moving backward Data Communication - Digital Transmition

Example1: Telephone companies digitize voice by assuming a maximum frequency of 4000 Hz. The sampling rate therefore is 8000 samples per second Examle2: A complex low-pass signal has a bandwidth of 200 kHz. What is the minimum sampling rate for this signal? Sampling rate is therefore 400,000 samples per second. Example3: A complex bandpass signal has a bandwidth of 200 kHz. What is the minimum sampling rate for this signal? We cannot find the minimum sampling rate in this case because we do not know where the bandwidth starts or ends. We do not know the maximum frequency in the signal. Data Communication - Digital Transmition

ANALOG-TO-DIGITAL CONVERSION PCM Quantization
We assume that the original analog signal has instantaneous amplitudes between Vmin and Vmax We divide the range into L zones, each of height ∆ (delta). We assign quantized values of 0 to L - 1 to the midpoint of each zone We approximate the value of the sample amplitude to the quantized values Data Communication - Digital Transmition

-∆/2 ≤ Quantization Error ≤ ∆/2 quantization error changes signal-to-noise ratio, which in turn reduces the upper limit capacity according to Shannon SNRdB = 6.02 nb dB nb is the number of quantization bits SNRdB =6.02(3) = dB Data Communication - Digital Transmition

A telephone subscriber line must have an SNRdB above 40. What is the minimum number of bits per sample? SNRdB= 6.02nb = 40  n= 6.35 Telephone companies usually assign 7 or 8 bits per sample Data Communication - Digital Transmition

Uniform Versus Nonuniform Quantization For many applications, distribution of amplitudes in the analog signal is not uniform. Changes in amplitude often occur more frequently in the lower amplitudes than in the higher ones. nonuniform quantization effectively reduces the SNRdB of quantization. Nonuniform Quantization methods: Nonuniform zone height of ∆ is not fixed; it is greater near the lower amplitudes and less near the higher amplitudes. Companding and expanding signal is companded at the sender before conversion; it is expanded at the receiver after conversion Companding means reducing the instantaneous voltage amplitude for large values; Companding gives greater weight to strong signals and less weight to weak ones Data Communication - Digital Transmition

ANALOG-TO-DIGITAL CONVERSION PCM Encoding
nb = Bit rate = sampling rate * number of bits per sample = is * nb Data Communication - Digital Transmition

ANALOG-TO-DIGITAL CONVERSION PCM Original Recovery

ANALOG-TO-DIGITAL CONVERSION PCM Bandwidth
Suppose we are given the bandwidth of a low-pass analog signal. If we then digitize the signal, what is the new minimum bandwidth of the channel that can pass this digitized signal? Bmin = c * N * 1/r = c * nb * fs * 1/r = 1/2 * nb * 2 * Banalog = nb * Banalog c = ½ average situation N = nb * fs = data rate; bit per second r = 1 in NRZ or bipolar This is the cost of digitization Data Communication - Digital Transmition

ANALOG-TO-DIGITAL CONVERSION PCM Bandwidth
Example: We have a low-pass analog signal of 4 kHz. If we send the analog signal, we need a channel with a minimum bandwidth of 4 kHz. If we digitize the signal and send 8 bits per sample, we need a channel with a minimum bandwidth of 8 X 4 kHz =32 kHz. Data Communication - Digital Transmition

ANALOG-TO-DIGITAL CONVERSION Digitization
In spite of cost of digitization, digital techniques continue to grow in popularity for transmitting analog data: Repeaters instead of amplifiers; no additive noise TDM instead of FDM; no intermodulation noise, use of more efficient digital switching techniques Data Communication - Digital Transmition

Maximum Data Rate/ Minimum Required Bandwidth of Digital Signal
Maximum Data Rate of a Channel Minimum Required Bandwidth Data Communication - Digital Transmition

ANALOG-TO-DIGITAL CONVERSION Delta Modulation (DM)
PCM is a very complex technique PCM finds the value of the signal amplitude for each sample; DM finds the change from the previous sample Data Communication - Digital Transmition

DM important parameters : size of the step assigned to each binary digit, δ sampling rate. When the analog waveform is changing very slowly, there will be quantizing noise, which increases as δ is increased when the analog waveform is changing rapidly, there is slope-overload noise which increases as δ is decreased. Data Communication - Digital Transmition

accuracy of the scheme can be improved by increasing the sampling rate However this increases the data rate of the output signal Data Communication - Digital Transmition

PCM versus DM: DM is simpler in implementation than PCM Less quantization error in DM PCM exhibits better S/N characteristics Data Communication - Digital Transmition

ANALOG-TO-DIGITAL CONVERSION DM Modulator / demodulator

ANALOG-TO-DIGITAL CONVERSION Adaptive DM
In adaptive DM, value of δ changes according to amplitude of analog signal A better performance can be achieved Data Communication - Digital Transmition

Data Communication - Digital Transmition
TRANSMISSION MODES serial transmission (1 bit in each clock tick) asynchronous, synchronous isochronous parallel transmission (multiple bits in each clock tick) Data Communication - Digital Transmition

TRANSMISSION MODES Parallel Transmission
Use n wires to send n bits at one time. Advantage: Speed disadvantage: cost. is usually limited to short distances Data Communication - Digital Transmition

TRANSMISSION MODES Serial Transmission
Advantage: cost disadvantage: parallel to serial convertor in sender serial to parallel convertor in receiver Data Communication - Digital Transmition

TRANSMISSION MODES Asynchronous Serial Transmission
Asynchronous means "asynchronous at the byte level;‘ but the bits are still synchronized; information is received and translated by agreed upon patterns based on grouping the bit stream into bytes. usually 8 bits data, 1 start bit (0) at the beginning and 1 or more stop bits (Is) at the end of each byte. gap between each byte can be represented either by an idle channel or by a stream of additional stop bits. advantages : cheap and effective attractive choice for low-speed communication connection of a keyboard to a computer is a natural application for asynchronous transmission. Data Communication - Digital Transmition

TRANSMISSION MODES Asynchronous Serial Transmission

TRANSMISSION MODES Synchronous Serial Transmission
bit stream is combined into longer "frames,“ send bit stream without start or stop bits or gaps. responsibility of receiver to group bits. Advantage: speed useful for high-speed applications such as transmission of data from one computer to another Byte synchronization is accomplished in the data link layer. Although no gap between characters, may be uneven gaps between frames Data Communication - Digital Transmition

TRANSMISSION MODES Synchronous Serial Transmission

TRANSMISSION MODES Isochronous Serial Transmission
In real-time audio and video, uneven delays between frames are not acceptable TV images are broadcast at the rate of 30 images per second; they must be viewed at the same rate. the entire stream of bits must be synchronized. The isochronous transmission guarantees that the data arrive at a fixed rate. Data Communication - Digital Transmition

Data Communication Digital Transmition

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