Digital Transmissions

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Transmitting digital signals
How do we encode digital signals for
transmission?
How can we interpret those signals?
Baseband transmission
• Signal is sent without conversion to an analog
signal.
• Requires a transmission channel with bandwidth
that starts at 0Hz (a low-pass channel).
• For perfect preservation, requires a dedicated
channel with infinite bandwidth.
• Usually, we just approximate
• Wide bandwidth channel: Ignore frequencies at the borders
• Narrow bandwidth: Use an analog signal and adjust frequency
and phase to create an approximate match.
Figure 3.21 Rough approximation of a digital signal using the first harmonic
for worst case
Figure 3.22 Simulating a digital signal with first three harmonics
Broadband transmission
• First convert digital signal to analog signal.
• Uses a bandpass channel - one whose
bandwidth starts anywhere.
• The signal is modulated, which means we
manipulate the frequency or amplitude to
represent different components of digital
data.
Transmission problems
• There are three major roadblocks that we
need to be concerned about when exploring
the physical level:
– Attenuation: How rapidly does the signal fade
as it propagates.
– Distortion: How does the signal change as it is
transmitted across the medium?
– Noise: How do extraneous factors interfere
with the signal being sent?
Converting digital data to a
digital signal
• Data element: A bit is a data element - the shortest
piece of data that can be sent
– Data rate is the number of data elements sent in one
second (bits per second)
• Signal element: A signal element is the shortest
signal segment.
– Signal rate is the number of signal elements sent in one
second (baud).
• r is defined as the ratio of data elements to signal
elements - i.e., how many bits are sent in each
signal element.
Maximizing throughput
• High signal rates are obtained by using a broader
range of frequencies, thus allowing for a broader
range of possible signals
• Therefore, higher signal rates require higher
bandwidths.
• Ideally, we want to maximize data rate and
minimize signal rate.
• Increasing r, though, reduces reliability by
requiring more sophisticated hardware on both the
sending and receiving ends.
Bandwidth requirements
• The bandwidth required is related to the
signal rate, since that determines how many
frequencies are needed:
Bmin = c  N  1/r
Issues to be aware of
• Baseline wandering
• Self-synchronization
• Error Detection
• Complexity
Line Coding
• Line coding is the primary way we encode
digital data in digital signals. There are five
primary groups of line coding schemes:
–
–
–
–
–
Unipolar
Polar
Bipolar
Multilevel
Multitransition
Unipolar line coding
• Either all signals levels are positive or they
are all negative
• Non-Return-to-Zero (NRZ):
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Polar line coding
• Voltages can be both positive and negative.
Typically positive voltages are 0 and negative
voltages are 1.
• NRZ-L: Level of voltage determines bit value
• NRZ-I: Inversion of signal determines bit value
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• Return to Zero (RZ): Use
three values - positive,
negative, and 0. Signal
goes to 0 in the middle of
each bit.
• Biphase:
– Manchester: Transition in
middle of bit - voltage value
of start of bit determines bit
value (like RZ and NRZ-L)
– Differential Manchester:
Transition in the middle of
each bit whether there is a
transition at beginning of bit
determines value (like RZ
and NRZ-I)
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Bipolar Line Coding
• Always have three voltage levels - positive, 0, and
negative. 0 volts is typically the 0 bit, while the 1
bit alternates between positive and negative.
• Long-distance lines
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Multilevel line coding
• 2B1Q: (2 bit, 1
quaternary) 2-bit
combinations are
associated with
specific changes in
the input according
to a transition table.
• DSL
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• 8B6T (8 Bits, 6 Ternary): Groups of 8 bits
are represented as sets of six signal
elements with three possible levels.
• 100Base-4T cable.
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• 4D-PAM5 (4 Dimensional five-level pulse
amplitude modulation):For situations where
multiple wires are available for
simultaneous communication.
– Uses four wires and four voltage levels
– 8 bits can be sent simultaneously using one
signal element
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Multitransition line coding
MLT-3 (MultiLine
Transmission, three level)
–
–
Similar to NRZ-I and
differential Manchester in
that transitions define bits.
Uses three voltage levels
and three transition rules:
1. No transition means next bit
is 0.
2. If the current level is nonzero, a transition to 0 means
the next bit is 1.
3. If the current level is zero, a
transition to the opposite of
the last non-zero level is a 1.
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Which is best?
• Well, none of them are great.
• Most don’t provide synchronization. The ones that
do either require high bandwidth (Biphase, 8B6T)
or multiple wires (4D-PAM5).
• None of these can inherently provide for any sort
of error detection.
• To accomplish synchronization and, especially,
error detection, there need to be redundant
elements to the signal.
Block Coding
• Block coding tries to provide this
redundancy.
• In block coding, we take a set of m bits and
recode them as a longer group of n bits.
– i.e., 4B/5B coding converts groups of four bits
into groups of 5 bits.
• Isn’t this bad? Doesn’t it increase the size of
the data stream we need to send?
The benefits of redundancy
• Representing 4 bits requires 16 data patterns.
• Having 5 bits available gives us 32 possible
patterns. Among the benefits of this are:
– Eliminate baseline drift by eliminating long strings of
0’s.
– This also improves synchronization.
– Some sequences can be used as control sequences for
further synchronization.
– What happens if the receiver gets a sequence that does
not correspond to a valid one?
• Typically used with simpler line-coding schemes
like NRZ
Long-distance transmission
• NRZ, even with block coding, still has the
problem of DC components.
• Biphase schemes require too much
bandwith for high-capacity long-distance
transmission.
• Bipolar AMI coding avoids both these
problems, but can generate long strings of
0’s, which affects synchronization.
Scrambling
• Scrambling those long strings of 0’s is the solution.
• Recall that bipolar encoding requires the voltage to
alternate between high and low. With a long string of 0’s,
we can use a violation signal (one that doesn’t alternate) as
a substitute for a 0.
• B8ZS (Bipolar with 8 Zero Substitution) - North America
• HDB3 (High-Density Bipolar 3-zero) - outside NA
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