nullData Conversion MethodsData Conversion MethodsSending 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)Data Encoding / ModulationData Encoding / ModulationBaseband
Digitally Encoded
Resources shared by Time Division MultiplexingBroadband
Analog Modulation
Resources shared by Frequency Division MultiplexingShould I have called the vertical axis bandwidth?TerminologyTerminologyData 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 simultaneouslyCriteria for a
Good Encoding SchemeCriteria for a
Good Encoding SchemeSignal Spectrum
Minimize high frequency components
No DC components
Synch capability (find bit positions)
Signal error detection capability
Signal interference and noise immunity
Cost and complexityAbsolute vs. Differential
Encoding / Modulation SchemesAbsolute vs. Differential
Encoding / Modulation SchemesAbsolute:
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.Digital Encoding SchemesDigital Encoding SchemesDigital 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)Unipolar EncodingUnipolar EncodingUses zero and positive voltage pulses to encode binary data
Not really “encoded” at all!
Polar EncodingPolar EncodingPolar 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 ManchesterNonreturn to Zero (NRZ)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
Nonreturn to Zero (NRZ)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
Nonreturn to Zero (NRZ)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)Return to Zero (RZ)(bipolar form)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 zeroReturn to Zero (RZ)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 bandwidthBiphaseBiphaseSignal 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 positiveBiphaseBiphaseDifferential 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 componentBipolarBipolarLike 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 JapanAlternate Mark Inversion (AMI)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 detectionBipolar 8-Zero Substitution (B8ZS)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 possible10000000001 +000+-0-+01 in general 00000000000V(-V)0(-V)VHigh Density Bipolar 3 (HDB3)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 EVEN number of + and – pulses so far
Use B00V if ODD, and B is opposite last pulse
High Density Bipolar 3 (HDB3)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 1High Density Bipolar 3 (HDB3)High Density Bipolar 3 (HDB3)Example: (revised 1-13-06)
Number of 1s since last substitution is even, last 1 negative (before this string)
Encode 100000000001AmplitudeTime000000001001