Data Communication Digital Transmition Behrouz A. Forouzan 1Data Communication - Digital...

Data Communication - Digital Transmition 1

Data CommunicationDigital Transmition

Behrouz A. Forouzan


Index

• Digital to Digital Conversion• Analog to Digital Conversion• Transmission Modes


Digital to Digital Conversion

• Techniques– line coding• Always needed

– block coding• May or may not needed

– Scrambling• May or may not needed


Digital to Digital ConversionLine Coding

• At sender, digital data are encoded into digital signal

• At receiver, digital data are recreated by decoding the digital signal


Digital to Digital ConversionLine 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


Digital to Digital ConversionData element Versus Signal Element

• Data 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


Digital to Digital ConversionData element Versus Signal Element


Digital to Digital ConversionData Rate Versus Signal Rate

• Data 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


Digital to Digital ConversionData 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


Digital to Digital ConversionRequired 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:

• .

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Digital to Digital ConversionRequired 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 ConversionBaseline 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.


Digital to Digital ConversionDC 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.


Digital to Digital ConversionSelf-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.


Digital to Digital ConversionBuilt-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


Digital to Digital ConversionImmunity to Noise and Interference

• Good coding that is immune to noise and other interferences

• Lower levels


Digital to Digital ConversionComplexity

• complex scheme is more costly to implement than a simple one

• Lower levels , lower signal change

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Digital to Digital ConversionUnipolar 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 Conversionunipolar - 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


Digital to Digital ConversionLine 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.


Digital to Digital ConversionPolar / 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



Bit Stream: 01001110


Digital to Digital ConversionPolar / 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

– Yes– value of power density is very high around frequencies close to zero

• Self-synchronization– NO– more serious in NRZ-L


Digital to Digital ConversionPolar / 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


Digital to Digital ConversionPolar / Return to Zero (RZ)


Digital to Digital ConversionManchester, Differentiated Manchester

• 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

• Manchester :– combines the ideas of RZ and NRZ-I– always a transition at the middle of the bit, – bit values are determined at the beginning of the bit

• Bit 0: transition• Bit 1: no transition


Digital to Digital Conversion Manchester, Differentiated Manchester


Digital to Digital Conversion Manchester, Differentiated Manchester

• 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


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



• 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)


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.


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)


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


Digital to Digital Conversion Multilevel Schemes / 8B6T

• Example: – The first 8-bit pattern 00010001 is encoded as the signal pattern

-0-0++ with weight 0– the second 8-bit pattern 010 10011 is encoded as - + - + + 0 with

weight +1.– The third bit pattern should be encoded as + - - + 0 + 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


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


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.



• 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



• 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


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


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)



• 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



• 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



• 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



• 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,


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


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


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


Digital to Digital Conversion Scrambling B8ZS

• does not change the bit rate• DC balance is maintained (two positives and two negatives)


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


Digital to Digital Conversion Scrambling HDB3


ANALOG-TO-DIGITAL CONVERSION

• Pulse Code Modulation (PCM)• Delta Modulation


ANALOG-TO-DIGITAL CONVERSIONPCM

• 1. analog signal is sampled.• 2. sampled signal is quantized.• 3. quantized values are encoded as bits


ANALOG-TO-DIGITAL CONVERSIONPCM 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



Oversampling gives nothing more

Under sampling lose information



• 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



• 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.


ANALOG-TO-DIGITAL CONVERSIONPCM Quantization

1. We assume that the original analog signal has instantaneous amplitudes between Vmin and Vmax

2. We divide the range into L zones, each of height ∆ (delta).

3. We assign quantized values of 0 to L - 1 to the midpoint of each zone

4. We approximate the value of the sample amplitude to the quantized values



• -∆/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 + 1.76 dB

• nb is the number of quantization bits• SNRdB =6.02(3) + 1.76 = 19.82 dB



• A telephone subscriber line must have an SNRdB above 40. What is the minimum number of bits per sample?

• SNRdB= 6.02nb + 1.76= 40 n= 6.35• Telephone companies usually assign 7 or 8 bits

per sample



• 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


ANALOG-TO-DIGITAL CONVERSIONPCM Encoding

• nb = • Bit rate = sampling rate * number of bits per

sample = is * nb


ANALOG-TO-DIGITAL CONVERSIONPCM Original Recovery


ANALOG-TO-DIGITAL CONVERSIONPCM 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


ANALOG-TO-DIGITAL CONVERSIONPCM 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.


ANALOG-TO-DIGITAL CONVERSIONDigitization

• 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


Maximum Data Rate/ Minimum Required Bandwidth of Digital Signal

• Maximum Data Rate of a Channel

• Minimum Required Bandwidth


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



• 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.



• accuracy of the scheme can be improved by increasing the sampling rate

• However this increases the data rate of the output signal



• PCM versus DM:– DM is simpler in implementation than PCM– Less quantization error in DM

– PCM exhibits better S/N characteristics


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


TRANSMISSION MODES

• serial transmission (1 bit in each clock tick)– asynchronous,– synchronous– isochronous

• parallel transmission (multiple bits in each clock tick)


TRANSMISSION MODESParallel Transmission

• Use n wires to send n bits at one time.

• Advantage:– Speed

• disadvantage: – cost.

• is usually limited to short distances


TRANSMISSION MODESSerial Transmission

• Advantage:– cost

• disadvantage: – parallel to serial convertor in sender – serial to parallel convertor in receiver


TRANSMISSION MODESAsynchronous 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.


TRANSMISSION MODESAsynchronous Serial Transmission


TRANSMISSION MODESSynchronous 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


TRANSMISSION MODESSynchronous Serial Transmission


TRANSMISSION MODESIsochronous 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 Behrouz A. Forouzan 1Data Communication - Digital...

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