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COMP445 Data Communications and Computer Networks. Concepts in Signal Encoding Techniques. Monday, February 4 , 2013. Useful Terms; must know. Unipolar All signal elements have same sign 0, +1 Bipolar One logic state represented by positive voltage the other by negative voltage -1, +1

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COMP445 Data Communications and Computer Networks

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Comp445 data communications and computer networks
COMP445 Data Communications and Computer Networks

Concepts in

Signal Encoding Techniques

Monday, February 4, 2013


Useful terms must know
UsefulTerms;mustknow

  • Unipolar

    • All signal elements have same sign 0, +1

  • Bipolar

    • One logic state represented by positive voltage the other by negative voltage -1, +1

  • Data rate

    • Rate of data transmission in bits per second

  • Duration or length of a bit

    • Time taken for transmitter to emit the bit

  • Mark and Space

    • Mark 1

    • Space 0


B it rate
B it Rate

The Nyquist Theorem & Noiseless Channels

  • Baud Rate: the frequency with which components change

  • Each bit string is composed of n bits, and hence the signal component may have up to 2ndifferent amplitudes (one for each unique combination for b1, b2, …bn)

  • Are bit rate and baud rate the Same?

  • No, bit rate depends on the number of bits (n) as well as the baud rate; more precisely:

    Bit Rate = n * Baud Rate

  • Bit rate can then be increased by either increasing the baud rate or n; however only up to a point


B it rate continued
B it Rate (continued...)

  • This result is surprisingly old, back to 1920s, when Harry Nyquist developed his classic theory

  • Nyquist theory showed that if f is the maximum frequency a medium can transmit, then the receiver can reconstruct the signal by sampling it 2f times per second

  • For example, if the maximum frequency is 4000 Hz, then the receiver can completely construct it by sampling it at a rate of 8000 per second

  • Assuming that the transmitter baud rate is 2 f , in other words changes signal each 1 / 2f intervals, we can state

    Bit Rate = n * Baud Rate = n * 2 * f

  • This can also be stated based on component; if B is the number of different components, then

    B = 2norn = log2(B)

  • Hence,

    Bit Rate = 2 * f * log2(B)


B it rate continued1
B it Rate (continued...)

Noisy Channels

  • More components mean subtler change among them

  • Channels are subject to noise

    • The transmitted signal can be distorted due to the channel noise

    • If distortion is too large, the receiver may not be able to reconstruct the signal at all


B it rate continued2
B it Rate (continued...)

Shannon’s Result

  • How much noise is bad? This depends on its ratio to the signal

  • We define S/N (Signal-to-Noise-Ratio)

  • A higher S/N (less significant noise) indicates higher quality

  • Because S >> N, the ratio is often scaled down as

    R = log10(S/N) bels// bels is the measurement unit

    For example,

    If S is 10 times larger than N, then

    R = log10(10N/N) = 1 bel

    If S is 100 times larger than N, then

    R = log10(100N/N) = 2 bels

    Perhaps, a more familiar measurement is the decibel (dB)

    1 dB = 0.1 bel


B it rate continued3
B it Rate (continued...)

Shannon’s Result

  • In 1940, Claude Shannon went beyond Nyquist’s results and considered noisy channels

  • Shannon related the maximum bit rate not only to the frequency but also to the S/N ratio; specifically he showed that:

    Bit Rate = Bandwidth * log2(1 + S/N) bps

  • The formula states that a higher BW and S/N ratio allow higher bit rate

  • Hence, for the telephone system, which has a frequency of about 4000 Hz and S/N ≈ 35 dB, or 3.5 bels, Shannon’s result yields the following

    3.5 = log10(S/N)  S = 103.5N  S≈ 3162 N  S/N≈ 3162

    Bit Rate = Bandwidth * log2(1 + S/N)

    = 4000 * log2(1 + 3162)

    ≈ 4000 * 11.63 bps

    ≈ 46,506 bps ≈ 46.5 kbps


Encoding schemes
EncodingSchemes

  • Nonreturn to Zero-Level (NRZ-L)

  • Nonreturn to Zero Inverted (NRZI)

  • Bipolar –AMI (Alternate Mark Inversion)

  • Pseudoternary

  • Manchester

  • Differential Manchester

  • B8ZS (Bipolar With 8 Zero Substitution)

  • HDB3 (High Density Bipolar 3 Zeros)


Nonreturn to zero level nrz l
Nonreturn to Zero-Level (NRZ-L)

  • Two different voltages for 0 and 1 bits

  • Voltage constant during bit interval

    • no transition I.e. no return to zero voltage

  • e.g. Absence of voltage for zero, constant positive voltage for one

  • More often, negative voltage for one value and positive for the other

  • This is NRZ-L


Nonreturn to zero inverted
Nonreturn to Zero Inverted

  • Nonreturn to zero inverted on ones

  • Constant voltage pulse for duration of bit

  • Data encoded as presence or absence of signal transition at beginning of bit time

  • Transition (low to high or high to low) denotes a binary 1

  • No transition denotes binary 0

  • An example of differential encoding



Differential encoding
Differential Encoding

  • Data represented by changes rather than levels

  • More reliable detection of transition rather than level

  • In complex transmission layouts it is easy to lose sense of polarity


Nrz pros and cons
NRZ pros and cons

  • Pros +’s

    • Easy to engineer

    • Make good use of bandwidth

  • Cons –’s

    • DC component

    • Lack of synchronization capability

  • Used for magnetic recording (outdated)

  • Not often used for signal transmission


Multilevel binary
Multilevel Binary

  • Use more than two levels

  • Bipolar-AMI (Alternate Mark Inversion)

    • zero represented by no line signal

    • one represented by positive or negative pulse

    • one pulses alternate in polarity

    • No loss of sync if a long string of ones (zeros still a problem)

    • No net DC component

    • Lower bandwidth

    • Easy error detection


Pseudoternary
Pseudoternary

  • One represented by absence of line signal

  • Zero represented by alternating positive and negative

  • No advantage or disadvantage over bipolar-AMI



Trade off for multilevel binary
Trade Off for Multilevel Binary

  • Not as efficient as NRZ

    • Each signal element only represents one bit (con)

    • In a 3 level system could represent log23 = 1.58 bits

    • Receiver must distinguish between three levels (+A, -A, 0) (con)

    • Requires approximately 3dB more signal power for same probability of bit error (con)


Biphase
Biphase

  • Manchester

    • Transition in middle of each bit period

    • Transition serves as clock and data

    • Low to high represents one

    • High to low represents zero

    • Used by IEEE 802.3

  • Differential Manchester

    • Midbit transition is clocking only

    • Transition at start of a bit period represents zero

    • No transition at start of a bit period represents one

    • Note: this is a differential encoding scheme

    • Used by IEEE 802.5




Biphase pros and cons
Biphase Pros and Cons

  • Cons

    • 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

      • Absence of expected transition



Scrambling gate to filling
Scrambling;gate to filling

  • Use scrambling to replace sequences that would produce constant voltage

  • Filling sequence

    • Must produce enough transitions to sync

    • Must be recognized by receiver and replace with original

    • Same length as original

  • No DC component

  • No long sequences of zero level line signal

  • No reduction in data rate

  • Error detection capability


Comp445 data communications and computer networks
B8ZS

  • Bipolar With 8 Zeros Substitution

  • Based on bipolar-AMI

  • If octet of all zeros and last voltage pulse preceding was positive encode as 000+-0-+

  • If octet of all zeros and last voltage pulse preceding was negative encode as 000-+0+-

  • Causes two violations of AMI code

  • Unlikely to occur as a result of noise

  • Receiver detects and interprets as octet of all zeros


Comp445 data communications and computer networks
HDB3

  • High Density Bipolar 3 Zeros

  • Based on bipolar-AMI

  • String of four zeros replaced with one or two pulses


B8zs and hdb3
B8ZSandHDB3


Digital data analog signal
Digital Data, Analog Signal

  • Public telephone system

    • 300Hz to 3400Hz

    • Use modem (modulator-demodulator)

  • Amplitude shift keying (ASK)

  • Frequency shift keying (FSK)

  • Phase shift keying (PSK)



Amplitude shift keying
Amplitude Shift Keying

  • Values represented by different amplitudes of carrier

  • Usually, one amplitude is zero

    • i.e. presence and absence of carrier is used

  • Susceptible to sudden gain changes

  • Inefficient

  • Up to 1200bps on voice grade lines

  • Used over optical fiber


Binary frequency shift keying
Binary Frequency Shift Keying

  • Most common form is binary FSK (BFSK)

  • Two binary values represented by two different frequencies (near carrier)

  • Less susceptible to error than ASK

  • Up to 1200bps on voice grade lines

  • High frequency radio

  • Even higher frequency on LANs using co-ax



Phase shift keying
Phase Shift Keying

  • Phase of carrier signal is shifted to represent data

  • Binary PSK

    • Two phases represent two binary digits

  • Differential PSK

    • Phase shifted relative to previous transmission rather than some reference signal


Differential psk
Differential PSK

  • Is the next bit different than the current bit?


Quadrature psk
Quadrature PSK

  • More efficient use by each signal element representing more than one bit

    • e.g. shifts of /2 (90o)

    • Each element represents two bits

    • Can use 8 phase angles and have more than one amplitude

    • 9600bps modem use 12 angles , four of which have two amplitudes

  • Offset QPSK (orthogonal QPSK)

    • Delay in Q stream




Digitizing analog data recording your voice and storing in pc
Digitizing Analog Data(recording your voice and storing in PC)


Pulse code modulation pcm 1
Pulse Code Modulation(PCM) (1)

  • If a signal is sampled at regular intervals at a rate higher than twice the highest signal frequency, the samples contain all the information of the original signal

  • If fs > 2fc OK 

  • Voice data limited to below 4000Hz

  • Requires 8000 samples per second

  • Analog samples (Pulse Amplitude Modulation, PAM)

  • Each sample assigned digital value


Pulse code modulation pcm 2
Pulse Code Modulation(PCM) (2)

  • 4 bit system gives 16 levels (24=16)

  • Quantized

    • Quantizing error or noise

    • Approximations mean it is impossible to recover original exactly

  • 8 bit sample gives 256 levels (28=256)

  • Quality comparable with analog transmission

  • 8000 samples per second of 8 bits each gives 64kbps



Delta modulation
Delta Modulation

  • Analog input is approximated by a staircase function

  • Move up or down one level () at each sample interval

  • Binary behavior

    • Function moves up or down at each sample interval



Delta modulation performance
DeltaModulation - Performance

  • Good voice reproduction

    • PCM - 128 levels (7 bit) (again 27=128)

    • Voice bandwidth 4khz

    • Should be 8000 x 7 = 56kbps for PCM


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