Data Conversion Methods Sending data from one place to the next  Transform data into signals Formats of source vs. medium Format of the original data (analog/digital) Format used by the communication hardware (analog/digital) 4 possible combinations Digital data / digital signal  (computers over LAN) Analog data / digital signal  (long distance phone) Digital data / analog signal  (computers over phone lines) Analog data / analog signal  (radio broadcast) Winter 2011 ECE 766 Computer Interfacing and Protocols

Data Encoding / Modulation Baseband Digitally Encoded Resources shared by Time Division Multiplexing Broadband Analog Modulation Resources shared by Frequency Division Multiplexing Frequency Time Should I have called the vertical axis bandwidth? PSTN Winter 2011 ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Terminology Data rate (bps) Baud rate, “modulation rate” (signal elements/sec) Mark (1) and space (0) conditions (from telegraphy) Connection types Simplex: One way Half Duplex: Two way, but only one way at a time (Full) Duplex: Two way simultaneously Winter 2011 ECE 766 Computer Interfacing and Protocols

Criteria for a Good Encoding Scheme Signal Spectrum Minimize high frequency components No DC components Synch capability (find bit positions) Signal error detection capability Signal interference and noise immunity Cost and complexity Winter 2011 ECE 766 Computer Interfacing and Protocols

Absolute vs. Differential Encoding / Modulation Schemes Each signal corresponds to a predetermined information unit The meaning of a signal sequence is fixed, not relative. Differential: Information is encoded by difference between current and previous signal element The meaning of a signal sequence is relative, not absolute. Winter 2011 ECE 766 Computer Interfacing and Protocols

Digital Encoding Schemes Digital information is converted to a sequence of voltage pulses that propagate over the link Three subcategories by voltage use: Unipolar (Zero and Positive) Polar (Negative and Positive) Bipolar (Negative, Zero, and Positive) Winter 2011 ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Unipolar Encoding Uses zero and positive voltage pulses to encode binary data Not really “encoded” at all! Amplitude Time 1 Winter 2011 ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Polar Encoding Polar encoding uses a positive and a negative voltage level to represent bits  Solves the DC component problem(if balanced) Categories: Nonreturn to Zero (NRZ) NRZ-L (L=Level) NRZ-I (I=Inverted) Return to Zero (RZ)(as shown in book) Biphase Manchester Differential Manchester Winter 2011 ECE 766 Computer Interfacing and Protocols

Nonreturn to Zero (NRZ) The voltage level is constant during a bit interval, i.e., no returns to zero Absolute and differential versions Absolute NRZ: NRZ-L (L=Level)(like ntl) 0 = Positive voltage 1 = Negative voltage Amplitude Time 1 Winter 2011 ECE 766 Computer Interfacing and Protocols

Nonreturn to Zero (NRZ) Differential NRZ: NRZ-I (I=Inverted) A bit is represented by the transition of the voltage level, not the voltage level itself! 0 = No inversion at beginning of bit interval 1 = Inversion at beginning of bit interval Amplitude Time 1 Winter 2011 ECE 766 Computer Interfacing and Protocols

Nonreturn to Zero (NRZ) Evaluation No DC component Simple Few high frequency components Synchronization No synchronization at large (consider a string of the same bit) NRZ-I provides synchronization for every 1 encountered  can handle strings of 1s (superior to NRZ-L) Winter 2011 ECE 766 Computer Interfacing and Protocols

Return to Zero (RZ)(bipolar form) Targets to solve the synchronization problem A scheme that handles both strings of both 1s and 0s Voltage level change for every bit value  three levels: +,-, 0 0 = Transition from negative to zero 1 = Transition from positive to zero Amplitude Time 1 Winter 2011 ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Return to Zero (RZ) Variations used also for magnetic recording (no synchronization capability) Evaluation Solves synchronization problem Two signal changes / bit  More transitions  Occupies more bandwidth Winter 2011 ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Biphase Signal changes in the middle of the bit interval, but does not return to zero Signal change  bit representation  synchronization Manchester: 0 = Transition from positive to negative 1 = Transition from negative to positive Amplitude Time 1 Winter 2011 ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Biphase Differential Manchester: 0 = Transition at the beginning of bit period 1 = No transition at the beginning of bit period Evaluation: Not as simple Higher frequency components (as RZ) Synchronization capability No DC component Amplitude Time 1 Winter 2011 ECE 766 Computer Interfacing and Protocols

Computer Interfacing and Protocols Bipolar Like in RZ, three voltage levels are used Zero voltage level used for binary 0 Categories: Alternate Mark Inversion (AMI) Bipolar 8-Zero Substitution (B8ZS)  North America High Density Bipolar 3 (HDB3)  Europe and Japan Winter 2011 ECE 766 Computer Interfacing and Protocols

Alternate Mark Inversion (AMI) Uses three voltage levels 0 = Zero volts 1 = Non-zero voltage, opposite in polarity to the last logical 1 Evaluation No DC component Synchronized only for 1s, not 0s Error detection Amplitude Time 1 Winter 2011 ECE 766 Computer Interfacing and Protocols

Bipolar 8-Zero Substitution (B8ZS) Adds synchronization for long strings of 0s North American system Same working principle as AMI except for eight consecutive 0s Evaluation Adds synchronization without changing the DC balance Error detection possible 10000000001  +000+-0-+01 in general 00000000000V(-V)0(-V)V Amplitude Time 1 Violation Winter 2011 ECE 766 Computer Interfacing and Protocols

High Density Bipolar 3 (HDB3) Goal like B8ZS to improve Sync of AMI Just like AMI except 4 0’s are replaced by code For 0000 use 000V or B00V Where B and V are + or – And V is AMI violation, B is Balance Bit Use 000V if ODD number of + and – pulses so far Use B00V if EVEN, and B is opposite last pulse Winter 2011 ECE 766 Computer Interfacing and Protocols

High Density Bipolar 3 (HDB3) Same goal as B8ZS Based on AMI Replaces every four consecutive 0s based on Number of pulses since last substitution Polarity of last logical 1 Last 1 polarity # of 1s + - ODD(revised 2011) 0000000+ 0000000- Even(revised 2011) 0000-00- 0000+00+ Winter 2011 ECE 766 Computer Interfacing and Protocols

High Density Bipolar 3 (HDB3) Example: (revised 1-6-11) Number of 1s since last substitution is even, last 1 negative (before this string) Encode 100000000001 1 1 Amplitude Time Winter 2011 ECE 766 Computer Interfacing and Protocols