Data Conversion Methods Sending data from one place

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 04 - 1

Data Encoding / Modulation • Broadband • Analog Modulation • Resources shared by Frequency Division Multiplexing Frequency • Baseband • Digitally Encoded • Resources shared by Time Division Multiplexing Time Should I have called the vertical axis bandwidth? PSTN Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 2

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 04 - 3

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 04 - 4

Absolute vs. Differential Encoding / Modulation Schemes • Absolute: – 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 04 - 5

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 04 - 6

Unipolar Encoding • Uses zero and positive voltage pulses to encode binary data Amplitude 0 1 1 1 0 0 Time • Not really “encoded” at all! Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 7

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 04 - 8

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 0 1 1 1 0 0 Time Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 9

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 0 1 1 1 0 0 Time Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 10

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 1 s (superior to NRZ-L) Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 11

Return to Zero (RZ)(bipolar form) • Targets to solve the synchronization problem • A scheme that handles both strings of both 1 s and 0 s • Voltage level change for every bit value three levels: +, -, 0 – 0 = Transition from negative to zero – 1 = Transition from positive to zero Amplitude 0 1 1 1 0 0 Time Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 12

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 04 - 13

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 0 1 1 1 0 0 Time Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 14

Biphase • Differential Manchester: – 0 = Transition at the beginning of bit period – 1 = No transition at the beginning of bit period Amplitude 0 1 1 1 0 0 Time • Evaluation: – – Not as simple Higher frequency components (as RZ) Synchronization capability No DC component Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 15

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 (B 8 ZS) North America – High Density Bipolar 3 (HDB 3) Europe and Japan Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 16

Alternate Mark Inversion (AMI) • Uses three voltage levels – 0 = Zero volts – 1 = Non-zero voltage, opposite in polarity to the last logical 1 Amplitude 0 1 1 1 0 0 Time • Evaluation – No DC component – Synchronized only for 1 s, not 0 s – Error detection Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 17

Bipolar 8 -Zero Substitution (B 8 ZS) • Adds synchronization for long strings of 0 s • North American system • Same working principle as AMI except for eight consecutive 0 s 1000001 +000+-0 -+01 Amplitude 1 0 0 0 in general 0 0 0000 000 V(-V)0(-V)V 0 0 1 Time Violation • Evaluation Violation – Adds synchronization without changing the DC balance – Error detection possible Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 18

High Density Bipolar 3 (HDB 3) • Goal like B 8 ZS to improve Sync of AMI • Just like AMI except 4 0’s are replaced by code • For 0000 use 000 V or B 00 V – Where B and V are + or – – And V is AMI violation, B is Balance Bit • Use 000 V if ODD number of + and – pulses so far • Use B 00 V if EVEN, and B is opposite last pulse Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 19

High Density Bipolar 3 (HDB 3) • Same goal as B 8 ZS • Based on AMI • Replaces every four consecutive 0 s based on – Number of pulses since last substitution – Polarity of last logical 1 Last 1 polarity # of 1 s ODD (revised 2011) Even( Winter 2011 ) revised 2011 + - 0000 000+ 0000 000 - 0000 -00 - 0000 +00+ ECE 766 Computer Interfacing and Protocols 04 - 20

High Density Bipolar 3 (HDB 3) • Example: (revised 1 -6 -11) – Number of 1 s since last substitution is even, last 1 negative (before this string) – Encode 1000001 Amplitude 1 0 0 0 0 0 1 Time Winter 2011 ECE 766 Computer Interfacing and Protocols 04 - 21