Take a digital signal and convert it into a suitable form for sending on a wire
Modulation
Take a signal and modifies a carrier signal with it
Modulation Rate
Rate at which signal level change
Measured in baud
Mark | Space
Binary 1 | Binary 0
Baseband vs Broadband
Baseband
Digital Signals: Entire medium carries only one signal at a time
Analog Signals: Original Frequency range of an analog signal before it’s modulated
Broadband
Carries two or more data in separate channels
Encoding Schemes
Nonreturn to Zero-Level (NRZ-L)
Nonreturn to Zero Inverted (NRZI)
Bipolar -AMI
Pseudoternary
Manchester
Differential Manchester
B8ZS
HDB3
Nonreturn to Zero-Level (NRZ-L)
Two different voltages for 0 and 1
Voltage constant during bit interval
Zero voltage for 0, Positive voltage for 1
Sometimes negative voltage for zero
Nonreturn to Zero Inverted
Nonreturn to zero inverted on ones
Constant voltage pulse for duration bit
Data encoded as presence or absence of signal transition at beginning of bit time
Transition denotes a 1
No transition denote a 0
An example of Differential Encoding
Differential Encoding
Data represented by changes
Advantages
More reliable detection of transition
Disadvantages
In complex layouts it’s easy to lose sense of polarity
NRZ pros and cons
Pros
Easy to engineer
Make good use of bandwidth
Cons
DC component
Lack of Synchronization
Used for magnetic recording
Not often used for signal transmission
Multilevel Binary
Use more than two levels
Bipolar AMI (Alternate Mark Inversion)
0 No signal
1 Positive or negative pulse
Pulse alternate in polarity
No loss of sync if a long string of ones, (not good for long zeroes)
No net DC component
Low bandwidth
Easy error detection
Pseudoternary
1 No signal
0 Alternating positive and negative
No advantage or disadvantage over bipolar-AMI
Trade Off for multilevel binary
Not as efficient as NRZ
Each signal only represent one bit
In 3 level system could represent bits
Receiver must distinguish between three levels
Requires 3dB more signal power for same probability of bit error
Biphase
Manchester
Transition in middle of each bit period
Transition serves as clock and data
LOW HIGH 1
HIGH LOW 0
Differential Manchester
Midbit transition is clocking only
Transition at start of a bit 0
No transition at start of bit 1
This is a differential encoding scheme
Biphase Pros and Cons
Con
At least one transition per bit time or two
Max modulation rate is twice NRZ
Require more bandwidth
Pros
Synchronization on mid bit transition
No dc component
Error detection
Scrambling
Use scrambling to replace sequences that produce constant voltage
Filling sequence
Must produce enough transitions to sync
Must be recognized by receiver and replace with original
Same length as original
No dc component
No long sequences of zero level line signal
No reduction in data rate
Error detection capability
B8ZS
Bipolar with 8 zeros substitution
Based on bipolar-AMI (Alternate Mark Inversion)
Solves the problem where
If 8 continuous zeroes and previous pulse is positive, encode as 000+-0-+
If a pulse is + and there are 8 zeroes after that,
then encode it as 000+-0-+
this causes an AMI violation because the 4th item
is + and the previous pulse is also +
the second AMI violation occurs at position 7
If 8 continuous zeros and previous pulse is negative, encode as 000-+0+-
If a pulse is - and there are 8 zeroes after that,
then encode is as 000-+0+-
this causes an AMI violation because the 4th item
is - and the previous pulse is also -
the second AMI violation occurs at position 7
Causes two violations of AMI code
Unlikely to occur as a result of noise
Receiver detects and interprets as octet of all zeros
HDB3
High Density Bipolar 3 Zeros
Based on Bipolar-AMI
String of four zeroes replaced with one or two pulses
Digital Data, Analog Signal
Amplitude shift keying (ASK)
Frequency shift keying (FSK)
Phase shift keying (PK)
Amplitude Shift Keying
Values represented by different amplitudes of the carrier wave
One amplitude is zero
Susceptible to sudden gain changes
Inefficient
Up to 1200bps on voice grade lines
Used over optical fiber
Binary Frequency Shift Keying
Two binary values represented by two different frequencies
Less susceptible to error
Upto 1200bps on voice grade lines
High frequency radio
Even higher frequency on LANs using co-ax
Phase Shift Keying
Phase of carrier signal is shifted to represent data
Binary PSK
Two phases represent two binary digits
Differential PSK
Phase shifted relative to previous transmission rather than some reference signal
BPSK
DPSK
Quadrature PSK
More efficient
Each element represent more than one bit
Eg: Shifts of can represent two bits
Can use 8 phase angles and have more than one amplitude
9600bps modems use 12 angles, four of which have two amplitudes
Performance of Digital to Analog Modulation Schemes
Bandwidth
for ASK and PSK bandwidth is directly related to bit rate
FSK bandwidth related to
Data rate for lower freq
Offset of modulated freq from carrier at high freq
Quadrature Amplitude Modulation
Used on ADSL and some wireless
Combination of ASK and PSK
Logical extension of QPSK
Send two different signals simultaneously on same frequency
Use two copies of career, one shifted
Each carrier is ASK modulated
Two independent signals over the same medium
Demodulate and combine for original binary input
Analog Data, Digital Signal
Digitization
Conversion of analog data into digital data
Pulse Code Modulation (PCM)
Signal is sampled at regular intervals, each sample assigned a digital value
Ex: 4 Bit signal gives 16 levels
Quantized
Quantizing error or noise
Approximation : Can’t recover original exactly
Ex: 8000 samples per second of 8 bits gives 64kbps
Nonlinear Encoding
Quantization levels not evenly spaced
Reduce overall signal distortion
Can also be done by companding
Delta Modulation
Analog output is approximated by a staircase function
Move up or down one level at each sample interval
Binary behavior
Function moves up or down at each sample interval
Good Voice reproduction
PCM - 128 levels (7bit)
Voice bandwidth 4KHz
= 56kbps
Data compression can improve on this
Eg: Interframe coding techniques for video
Analog Data, Analog Signals
Why modulate analog signals?
Higher frequency can give efficient transmission
Permit frequency division multiplexing
Types of modulation
Amplitude
Frequency
Phase
Amplitude Modulation (AM)
Amplitude of a carrier signal is altered
Frequency of the carrier is usually greater than the highest frequency of the input signal*
