Winter 2011 ECE 766Computer Interfacing and Protocols
104 -
Data Conversion MethodsData 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 766Computer Interfacing and Protocols
204 -
Data Encoding / ModulationData Encoding / Modulation• Baseband• Digitally Encoded• Resources shared by Time
Division Multiplexing
• Broadband• Analog Modulation• Resources shared by
Frequency Division Multiplexing
PSTN
Freq
uenc
y
Time
Should I have called the vertical axis bandwidth?
Winter 2011 ECE 766Computer Interfacing and Protocols
304 -
TerminologyTerminology• 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 766Computer Interfacing and Protocols
404 -
Criteria for a Criteria for a Good Encoding SchemeGood 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 766Computer Interfacing and Protocols
504 -
Absolute vs. Differential Absolute vs. Differential Encoding / Modulation SchemesEncoding / 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 766Computer Interfacing and Protocols
604 -
Digital Encoding SchemesDigital 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 766Computer Interfacing and Protocols
704 -
Unipolar EncodingUnipolar Encoding• Uses zero and positive voltage pulses to
encode binary data
• Not really “encoded” at all!
Amplitude
Time
0 1 0 1 1 1 0 0
Winter 2011 ECE 766Computer Interfacing and Protocols
804 -
Polar EncodingPolar 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 766Computer Interfacing and Protocols
904 -
Nonreturn 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
Amplitude
Time
0 1 0 1 1 1 0 0
Winter 2011 ECE 766Computer Interfacing and Protocols
1004 -
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
Amplitude
Time
0 1 0 1 1 1 0 0
Winter 2011 ECE 766Computer Interfacing and Protocols
1104 -
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)
Winter 2011 ECE 766Computer Interfacing and Protocols
1204 -
Return to Zero (RZ)(Return to Zero (RZ)(bipolar formbipolar 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
0 1 0 1 1 1 0 0
Winter 2011 ECE 766Computer Interfacing and Protocols
1304 -
Return 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 bandwidth
Winter 2011 ECE 766Computer Interfacing and Protocols
1404 -
BiphaseBiphase• 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
0 1 0 1 1 1 0 0
Winter 2011 ECE 766Computer Interfacing and Protocols
1504 -
BiphaseBiphase• 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
0 1 0 1 1 1 0 0
Winter 2011 ECE 766Computer Interfacing and Protocols
1604 -
BipolarBipolar• 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 766Computer Interfacing and Protocols
1704 -
Alternate 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 detection
Amplitude
Time
0 1 0 1 1 1 0 0
Winter 2011 ECE 766Computer Interfacing and Protocols
1804 -
Bipolar 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 possible
Amplitude
Time
0 0 0 0 0 0 0 01 0 1
Violation Violation
10000000001 +000+-0-+01 in general 00000000000V(-V)0(-V)V
Winter 2011 ECE 766Computer Interfacing and Protocols
1904 -
High 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 ODD number of + and – pulses so far
• Use B00V if EVEN, and B is opposite last pulse
Winter 2011 ECE 766Computer Interfacing and Protocols
2004 -
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 1
Last 1 polarity# of 1s + -
ODD(revised 2011) 0000000+ 0000000-
Even(revised 2011) 0000-00- 0000+00+
Winter 2011 ECE 766Computer Interfacing and Protocols
2104 -
High Density Bipolar 3 (HDB3)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 100000000001Amplitude
Time
0 0 0 0 0 0 0 0 0 0 11