Transcript - Muhazam

EEC4113
Data Communication &
Multimedia System
Chapter 2: Baseband Encoding
by Muhazam Mustapha, July 2010
Learning Outcome
• By the end of this chapter, students are
expected to be able to explain link level
baseband encoding for transmission
Chapter Content
• Polarity in baseband encoding
• Encoding techniques
– NRZ-L, NRZI
– Bipolar
– Biphase
• Modulation rate
• Scrambling techniques
Polarity in Baseband Encoding
Baseband Encoding
• Definition: encoding of the signal in the
spectrum range from 0 Hz to the data rate
frequency
• Use: encoding of data for short distances,
LAN, Ethernet
Polarity in Encoding
• Unipolar
– All signals follow the values of the binary
Amplitude
1
0
1
1
0
0
0
1
Time
Polarity in Encoding
• Polar
– One signal sign follows one data binary
Amplitude
1
0
1
1
0
0
0
1
Time
Polarity in Encoding
• Bipolar
– 3 levels of signal: +ve, −ve, 0
– Binary 0 is level 0; binary 1 alternates sign
Amplitude
1
0
1
1
0
0
0
1
Time
Encoding Techniques
Nonreturn to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits
– Negative voltage for 1
– Positive voltage for 0
Amplitude
1
0
1
1
0
0
0
1
Time
Nonreturn to Zero-Inverted (NRZI)
• Bit 1: Transition at the beginning of bit time
• Bit 0: No transition
• A kind of differential encoding – data is represented by transition
rather than level
Amplitude
1
transitions
0
1
1
0
0
0
1
Time
Advantages of NRZ Coding
• Easiest to engineer
• Make efficient use of bandwidth
– Most of the energy in NRZ-L & NRZ-I signals
(80%) is between DC and half of the bit rate
– e.g. If NRZ code is used to generate a signal
with data rate of 9600 bps, then most of the
energy in the signal is concentrated between
DC & 4800 Hz
Spectral Density of Various
Schemes
B8ZS, HDB3
AM, pseudoternary
Manchester,
differential Manchester
NRZ
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Normalized frequency (f/R)
1.6
1.8
2.0
Disadvantages of NRZ Coding
• Presence of DC component (zero
frequency)
– Presents problems for a system that cannot
pass low frequencies
• e.g. Telephone line can’t pass frequencies below
300 Hz
Disadvantages of NRZ Coding
– Also presents problems for a system that uses
electrical coupling via transformer
• There must be direct physical attachment of
transmission component
• Electrical (AC) coupling via transformer, which
provides excellent electrical isolation that reduces
interference, is not possible
• e.g. A long distance link may use one or more
transformers to isolate different parts of the line
electrically
Disadvantages of NRZ Coding
• Lack of synchronization capability
– Consider long string of 1-s and 0-s for NRZ-L
or long string of 0-s for NRZI
– The output is a constant voltage over a long
period of time
– A drift between the timing of transmitter &
receiver will result in loss of synchronization
between both devices
Disadvantages of NRZ Coding
• Due to these lacking, it is unattractive for
signal transmission applications
• Due to these shortcomings, it is only used
in direct devices connection like in digital
magnetic recording
Bipolar-AMI
•
•
•
•
Alternate Mark Inversion
A kind of multilevel binary encoding
Binary 0: No line signal
Binary 1: +ve or –ve pulses alternately
1
0
1
1
0
0
0
1
Time
Advantages of Bipolar-AMI
• No loss of synchronization if a long string
of 1-s occur
– Each 1 introduces a transition
– Receiver can resynchronize on that transition
– Long string of 0-s would still be a problem
• No net DC component
– 1-s signals alternate in voltage
– 0-s is at zero volt
– Hence 0 DC component
Pseudoternary
•
•
•
•
Inversion of AMI
A kind of multilevel binary encoding
Binary 1: No line signal
Binary 0: +ve or –ve pulses alternately
1
0
1
1
0
0
0
1
Time
Disadvantages of Multilevel Binary
• Long string of 0-s (AMI) and 1-s
(pseudoternary) still present a problem
– Common technique: insert additional bits that
force transition – called scrambling
• Less efficient than NRZ
– The receiver has to distinguish 3 levels
– Requires ~3dB of power for the same BER as
NRZ
– BER is higher for the same SNR as NRZ
Disadvantages of Multilevel Binary
AMI, pseudoternary,
ASK, FSK
BER
NRZ, biphase, PSK,
QPSK
3 dB
SNR (dB)
Manchester
•
•
•
•
A kind of biphase encoding
Transition in the middle of each bit period
Binary 1: Low to High transition
Binary 0: High to Low transition
1
0
1
1
0
0
0
1
Time
Differential Manchester
• A kind of biphase & differential encoding
• Binary 0: Transition at start of bit period
• Binary 1: No transition at start of bit period
1
0
1
1
0
0
0
1
Time
Advantages of Biphase Encoding
• Synchronization
– Biphase codes are self-clocking codes
– Predictable transition during each bit period
– Receiver can synchronize on that transition
• No DC component
• Error detection
– Absence of expected transition can be used
to detect errors
• Due to these advantages it is popular for
LAN connection
Disadvantages of Biphase
Encoding
• Requires double the bandwidth of nonbiphase encoding
• Requires more signaling power
• Due to these disadvantages it is not
popular in long distance connection
Modulation Rate
Data Rate
• Also known as BIT Rate
• Definition: The rate at which data (or bits)
are communicated per second
• Unit: bit per second (bps)
• Example: 1000 bps means 1000 bit is
transmitted and received in 1 second
Modulation Rate
• Also known as BAUD rate or SYMBOL rate
• Definition: The MAXIMUM no. symbol at which
the signal in communication channel can have
per second
• Unit: baud per second
• Example: Consider an NRZ optical signaling
between red & green. If the system has to
produce the colors at max 2400 times per
second then it is 2400 baud per second
Baud vs Bit Rate
• 1 signal symbol may represent more that 1 bits
• Hence this gives room for more than 1 bps in
each baud rate
– bps = baud × no. bit per baud
• Example: Consider an NRZ optical signaling
between green (00), red (01), yellow (10) and
blue (11). If the system has to produce the
colors at max 2400 times per second then it is
2400 baud per second. Since there are 2 bits
per symbol, then it is 4800 bps.
Baud vs Bit Rate
In general:
D =R/L
= R / log2M
D = Modulation rate, baud
R = Data rate, bps
L = Number of bits per
symbol or signal element
M = Number of different
symbols used = 2L
Baud vs Bit Rate
Data rate = 1 Mbps
NRZL
Time
1 bit/μs
1 sym/μs
1 bit/μs
2 sym/μs
Modulation rate = 1 Mbaud
Data rate = 1 Mbps
Manchester
Time
Modulation rate = 2 Mbaud
Scrambling Techniques
Scrambling Techniques
• Multilevel binary with scrambling
techniques
– Commonly used in long-distance transmission
• Sequences that would result in a constant
voltage level would be replaced with a new
filling sequence
• The filling sequence would provide enough
transitions for the receiver’s clock to
maintain synchronization
Scrambling Techniques
• The filling sequence must be recognized
by the receiver & to be replaced with the
original data sequence
• The filling sequence is the same length as
the original sequence, hence there is no
data rate penalty
Scrambling Techniques
• Design goals:
– No DC component
– No long sequence of zero-level line signal
– No reduction in data rate
– Error detection capability
• Two scrambling techniques commonly
used in long-distance transmission
– B8ZS
– HDB3
B8ZS
• Bipolar with 8-Zeros Substitution
• Based on Bipolar-AMI encoding
– Long string of 0-s may result in loss
synchronization
• Replaces strings of eight 0-s with:
If the last voltage pulse preceding this 8 0-s was +ve, then they
are encoded with 0 0 0 + − 0 − +
If the last voltage pulse preceding this 8 0-s was -ve, then they
are encoded with 0 0 0 − + 0 + −
B8ZS
1
1
0
0
0
0
0
0
0
0
1
1
0
0
0
0
0
1
0
Bipolar-AMI
B8ZS
This technique forces 2 code violations, which is unlikely to occur due to
noise, and the parity is also maintained
HDB3
• High-Density Bipolar 3-Zeros
• Based on Bipolar-AMI encoding
• Replaces strings of four 0-s with:
No. bipolar pulses since last substitution
Polarity of Preceding Pulse
Odd
Even
−
000−
+00+
+
000+
−00−
HDB3
1
1
0
Bipolar-AMI
HDB3
Consider pulses count at
this point is odd
0
0
0
0
0
0
0
1
1
0
0
0
0
0
1
0