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CMPT 371

CMPT 371. Data Communications and Networking Digital Encoding. Encoding. Digital and analog data can be encoded as digital or analog signals. Digital data encoded to digital signals Analog data encoded to digital signals Digital data encoded to analog signals

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CMPT 371

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  1. CMPT 371 Data Communications and Networking Digital Encoding

  2. Encoding • Digital and analog data can be encoded as digital or analog signals. • Digital data encoded to digital signals • Analog data encoded to digital signals • Digital data encoded to analog signals • Analog data encoded to analog signals

  3. Definitions • Data rate, R • Rate of data transmission in bits per second • Duration or length of a bit,tb= 1/R • Time taken for transmitter to emit the bit • Modulation rate, D = R/b • Measured in baud (signal elements or symbols per second) • b is the number of bits per signal element • Mark and Space • Binary 1 and Binary 0 respectively

  4. Encoding and Modulation Stallings 2003: Figure 5.1

  5. Digital Signaling • The source data is a stream of data bits, g(t) is encoded into voltage pulses. • The particular encoding method will determine how the information is translated into voltage pulses • Encoding methods use multiple voltage levels • Information is carried using the voltage levels and sometimes the transitions between voltage levels. • Factors affecting efficiency of encoding include • Number of voltage levels • Signal bandwidth • Error detection efficiency • Maximum duration without a transition (Loss of Sync) • Amount of DC signal produced ( transformer coupling only with no dc signal)

  6. Analog Signaling • carrier signal: continuous signal with frequency, fc • Modulation: the process of encoding the source data stream (baseband signal or modulating signal) onto the carrier signal • Modulation involves superimposing variations in amplitude, phase or frequency on the carrier signal. These variations carry the information in the data. • The Modulated Signal (output from modulation) is transmitted as an analog signal • The Receiver will demodulate the Modulated Signal and extract the information • WHY? To change the signals bandwidth and frequency so it can be transmitted on the specified limited width communication channel.

  7. Digital to Digital Encoding Schemes • Nonreturn to Zero-Level (NRZ-L) • Nonreturn to Zero Inverted (NRZI) • Bipolar -AMI • Pseudoternary • Manchester • Differential Manchester • B8ZS • HDB3 • 4B/5B • MLT-3 • 8B/10 Schemes

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

  9. Nonreturn to Zero – Level (NRZI) • Data encoding of binary ones and zeros • Two signal levels used for encoding • 1 – Negative voltage • 0 – Positive voltage • Constant voltage pulse for duration of bit • no return to zero voltage during pulse • Synchronization may be lost during a long string of zeros or ones. A change is signal level is needed to determine where a bit starts or ends

  10. Nonreturn to Zero - Level 1 1 0 0 0 1 10 1 1 1 1 0 0 0 1 0 1 0 1 1 1 0 -1

  11. Properties of NRZ- level signals • Maximum possible frequency (bit rate R bps) • Period T=2/R, f = 1/T = R/2 (alternating 1s and 0s) • Minimum possible frequency • Period T=∞, f = 1/T = 0 (all 0s or all 1s): DC component • Bandwidth = Maximum Frequency – Minimum Frequency = R/2 – 0 = R/2 • May have a net dc component 1 0 -1 0 3/R 6/R 7/R 1/R 4/R 5/R 2/R 9/R 8/R time ±1 time 0 3/R 6/R 7/R 1/R 4/R 5/R 2/R 8/R 9/R

  12. Bipolar Alternate Mark Inversion (Bipolar AMI) • Multilevel binary data encoding of 1s and 0s • 0 represented by no signal (0 voltage) • 1 represented by a signal (+ve or –ve voltage) • Signals for ones alternate in sign (+ve and –ve) • Constant voltage pulse for duration of bit • No loss of sync if a long string of ones • Sync may be lost during a long string of zeros • Easy error detection: when 2 successive bits are the same (+1 or -1) an error has occurred.

  13. Bipolar Alternate Mark Inversion (Bipolar AMI) 1 1 0 0 0 1 1 0 1 1 1 1 0 0 0 1 0 1 0 1 1 0 -1

  14. Properties of Bipolar AMI signals • Maximum possible frequency (bit rate R bps) • Period T=2/R, f = 1/T = R/2 (all 1s) • Minimum possible frequency • Period T=∞, f = 1/T = 0 (all 0s) • Bandwidth = Maximum Frequency – Minimum Frequency = R/2 – 0 = R/2 • No net dc component (+ve and –ve bits alternate) 1 0 0 -1 0 3/R 6/R 7/R 1/R 4/R 5/R 2/R 8/R 9/R time 0 0 3/R 6/R 7/R 1/R 4/R 5/R 2/R 8/R 9/R time

  15. Pseudoternary 1 1 0 0 0 1 10 1 1 1 1 0 0 0 1 0 1 0 1

  16. Manchester • Biphase data encoding of binary ones and zeros • Transition between the two possible signal levels occurs in the middle of each bit period • Used for clocking and to encode information • 1 - signal transition low to high • 0 - signal transition high to low • Signal transitions may also occur at the beginning of a bit period (to allow for the correct mid bit transition) • Used by IEEE 802.3

  17. Manchester 1 1 0 0 0 1 10 1 1 1 1 0 0 0 1 0 1 0 1

  18. Properties of Manchester signals • Maximum possible frequency (bit rate R bps) • Period T=1/R, f = 1/T = R (all 1s or all 0s) • Minimum possible frequency • Period T=2/R, f = 1/T = R/2 (alternating 1s and 0s) • Bandwidth = Maximum Frequency – Minimum Frequency = R – R/2 = R/2 0 3/R 8/R 2/R 7/R 0 1/R 4/R 5/R 6/R 9/R 10/R time 0 3/R 6/R 7/R 1/R 4/R 5/R 2/R 8/R 9/R time

  19. Theoretical Bit Error Rate Stallings 2003: Figure 5.4

  20. Scrambling • Use scrambling to replace sequences that cause transmission at a constant level (voltage) for many bit durations, and may cause synchronization problems • Replace long constant sequences with a filling sequence • The filling sequence must be chosen to • Produce enough transitions to sync • Be recognized by receiver and replaced with original data • Same length as original • The filling sequence should not have • A dc component • Any long sequences of zero level line signal • Any reduction in data rate • Error detection capability

  21. Bipolar AMI 1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1

  22. Bipolar With 8 Zeros Substitution (B8ZS) Data encoding (Based on bipolar-AMI) • 0 represented by no signal for duration of bit • 1 represented by a signal for duration of bit • Signals for ones alternate in sign • An octet of zeros in the data is encoded as • 000+-0-+ if the preceding voltage pulse was +ve • 000-+0+- if the preceding voltage pulse was -ve • Transmitting station inserts these octets to replace each octet of zeros in the data. • Receiving station detects the octets inserted to replace sequences of zeros and interprets each such octet as a sequence of eight zeros • Insertion of each octet causes two violations of the bipolar-AMI code • These violations are unlikely to occur as a result of noise

  23. 1 1 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 1 B8ZS

  24. Properties of B8ZS signals 1 0 0 • Maximum possible frequency (bit rate R bps) • Period T=2/R, f = 1/T = R/2 (all 1s) • Minimum possible frequency • Period T=16/R, f = 1/T = R/16 (repeating pattern of 7 0s followed by a 1 ) • Bandwidth = R/2 – R/16 = 7R/16 • No net dc component (+ve and –ve bits alternate, except when substitutions occur) • Sync is maintained (need 1 bit in 8 not zero for hardware reliability) • Used in telecom (Voip ..) on DS1 and T1 lines -1 0 3/R 6/R 7/R 1/R 4/R 5/R 2/R 8/R 9/R time 0 0 23/R 47/R 55/R 7/R 31/R 39/R 15/R 63/R 71/R time

  25. 4B/5B • Used for 100BASE-X and FDDI LANs • 4 Data Bits Encoded into 5 Code Bits • At least 2 transitions occur in each group of code bits • No more than 2 consecutive 0’s in a group of code bits • No more than 3 consecutive 0’s in any coded sequence • 80% efficiency, 125Mbaud, 100Mbps

  26. 4B/5B: code groups Stallings 2003: Table 16.6

  27. 4B/5B MLT-3 • Used for 100BASE-TX Over Twisted Pair • Different way of encoding groups of 4B/5B Code bits so high frequencies, which cannot be transmitted through twisted pair cable, are removed from the transmitted signal • The 4B/5B bit stream is scrambled

  28. 4B/5B MLT-3 • The resulting bit stream is encoded using MLT-3 • If the next input bit is zero the next output bit is the same as the present output bit • If the next input bit is one the next output bit is different from the present output bit • If the present output bit is not 0 then the next output bit is 0 • If the present output bit is 0 the next output bit is not zero and has a sign opposite the previous nonzero output bit

  29. State Machine for MLT-3 encoding Stallings 2003: Figure 16.12

  30. MLT-3 (4B/5B 3-Level ) 1 1 0 0 0 1 0 1 1 1 1 0 0 0 0 1 0 1 0 1 1 1 0 10 0 1 0 1 1 1 1 1 0 0 0 1 0 0 1 0 1 0 1 1

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