© Janice Regan, CMPT 128, CMPT 371 Data Communications and Networking Digital Encoding

Janice Regan © 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

Janice Regan © Definitions  Data rate, R  Rate of data transmission in bits per second  Duration or length of a bit, t b = 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

Janice Regan © Encoding and Modulation Stallings 2003: Figure 5.1

Janice Regan © 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)

Janice Regan © Analog Signaling  carrier signal: continuous signal with frequency, f c  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.

Janice Regan © 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

Janice Regan ©  Digital signal  Discrete, discontinuous voltage pulses  Each pulse is a signal element  Binary data encoded into signal elements

Janice Regan ©  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 Nonreturn to Zero – Level (NRZI)

Janice Regan © Nonreturn to Zero - Level

Janice Regan © 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 time 0 1/R 4/R 2/R 3/R 5/R 6/R7/R 8/R 9/R time 0 1/R 4/R 2/R 3/R 5/R 6/R7/R 8/R9/R 0 1 ±1±1

Janice Regan © 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.

Janice Regan © Bipolar Alternate Mark Inversion (Bipolar AMI)

Janice Regan © 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) time /R 4/R 2/R 3/R 5/R 6/R7/R 8/R9/R 0 1/R 4/R 2/R 3/R 5/R 6/R7/R 8/R9/R

Janice Regan © Pseudoternary

Janice Regan © 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

Janice Regan © Manchester

Janice Regan © 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 time 0 1/R 4/R 2/R 3/R 5/R 6/R7/R 8/R9/R time 0 2/R 1/R 3/R 4/R5/R 0 7/ R 6/R 8/R 9/R10/R

Janice Regan © Theoretical Bit Error Rate Stallings 2003: Figure 5.4

Janice Regan © 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

Janice Regan © Bipolar AMI

Janice Regan © 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  if the preceding voltage pulse was +ve  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

Janice Regan © B8ZS

23 Properties of B8ZS signals  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 time 0 1/R 4/R 2/R 3/R 5/R 6/R7/R 8/R9/R time 0 7/R 31/R 15/R 23/R 39/R 47/R55/R 63/R71/R

Janice Regan © B/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

Janice Regan © B/5B: code groups Stallings 2003: Table 16.6

Janice Regan © B/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

Janice Regan © B/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

Janice Regan © State Machine for MLT-3 encoding Stallings 2003: Figure 16.12

Janice Regan © MLT-3 (4B/5B 3-Level )