Digital Line Encoding
•
•
Converting standard logic level to a form
more suitable to telephone line
transmission.
Six factors must be considered when
selecting a line encoding format;
1.
2.
3.
4.
5.
6.
transmission voltage & DC component
Duty cycle
Bandwidth consideration
Clock and framing bit recovery
Error detection
Ease of detection and decoding
Why Digital Signaling?
• Low cost digital circuits
• The flexibility of the digital approach
(because digital data from digital
sources may be merged with digitized
data derived from analog sources to
provide general purpose
communication system)
Digital Modulation
• Using Digital Signals to Transmit Digital Data
– Bits must be changed to digital signal for transmission
– Unipolar encoding
• Positive or negative pulse used for zero or one
– Polar encoding
• Uses two voltage levels (+ and - ) for zero or one
– Bipolar encoding
• +, -, and zero voltage levels are used
Non-Return to Zero-Level (NRZ-L)
• Two different voltages for 0 and 1 bits.
• Voltage constant during bit interval.
– no transition, no return to zero voltage
• More often, negative voltage for one value and positive for the
other.
Non-Return to Zero Inverted (NRZ-I)
• Nonreturn to zero inverted on ones
• Constant voltage pulse for duration of bit
• Data encoded as presence or absence of signal transition
at beginning of bit time
• Transition (low to high or high to low) denotes a binary 1
• No transition denotes binary 0
• An example of differential encoding
Multilevel Binary(Bipolar-AMI)
•
•
•
•
zero represented by no line signal
one represented by positive or negative pulse
one pulses alternate in polarity
No loss of sync if a long string of ones (zeros still a
problem)
• No net dc component
• Lower bandwidth
• Easy error detection
0
1
0
0
1
1
0
0
0
1
1
Pseudoternary
• One represented by absence of line signal
• Zero represented by alternating positive and negative
• No advantage or disadvantage over bipolar-AMI
0
1
0
0
1
1
0
0
0
1
1
Manchester
• There is always a mid-bit transition {which is used as a
clocking mechanism}.
• The direction of the mid-bit transition represents the
digital data.
• 1  low-to-high transition
• 0  high-to-low transition
• Consequently, there may be a second transition at the
beginning of the bit interval.
• Used in 802.3 baseband coaxial cable and CSMA/CD
twisted pair.
Differential Manchester
• mid-bit transition is ONLY for clocking.
• 1  absence of transition at the beginning of the bit
interval
• 0  presence of transition at the beginning of the bit
interval
• Differential Manchester is both differential and bi-phase.
[Note – the coding is the opposite convention from NRZI.]
• Used in 802.5 (token ring) with twisted pair.
• * Modulation rate for Manchester and Differential
Manchester is twice the data rate  inefficient encoding
for long-distance applications.
Bipolar With 8 Zeros Substitution
(B8ZS)
• Based on bipolar-AMI
• If octet of all zeros and last voltage pulse preceding was
positive encode as 000+-0-+
• If octet of all zeros and last voltage pulse preceding was
negative encode as 000-+0+• Causes two violations of AMI code
• Unlikely to occur as a result of noise
• Receiver detects and interprets as octet of all zeros
B8ZS and HDB3
High Density Bipolar 3 Zeros
(HDB3)
• Also based on bipolar-AMI
• String of four zeros replaced with one or
two pulses