Line coding
Line coding is the process of
converting digital data into digital signals; this conversion of digital data
into digital signal takes place at sender side and this digital signal is
decoded into digital data at receiver side. These line coding schemes are
important part of digital-to-digital transmission besides block coding and
scrambling.
1. Unipolar line coding schemes
1(a). NRZ
NRZ refers to non-return-to-zero i.e.
the signal does not return to 0 voltage at the middle of the bit. Change of
voltage takes place only when first bit finishes and next bit starts. This
scheme is normally not used in data communications these days.
2. Polar line coding schemes
As the name refers a polar line coding
scheme is one which has both positive and negative values of voltages.
2(a). NRZ
Polar line coding NRZ are of two types:
NRZ-Level and NRZ-Invert.
NRZ-Level
In non-return-to-zero Level the value
of the bit is determined by the level of the voltage. i.e., among 0 and 1, one
bit have positive value of voltage and other have negative value of voltage.
NRZ-Invert
In non-return-to-zero Invert the value
of bit is determined by either inversion or lack of inversion. i.e., there is
inversion on encountering any one bit among 0 and 1, and for other bit there is
no inversion.
2(b). RZ
RZ refers to return to zero i.e., the
bit travels half time for either positive or negative value of the voltage and
for the rest of the half time it travels along the x-axis for 0 value of
voltage.
2(c). Biphase
As the name refers in biphase encoding
half of the bit travels for positive value of voltage and half of it travels
for negative value of voltage. It is of two types: Manchester encoding and
differential
Manchester encoding.
Manchester
It is a combination of RZ and NRZ-I.
Like RZ there is always a transition between the bit and 0 is represented
starting from positive and ending in negative and 1 is represented starting
from negative and ending in positive as shown in diagram.
Differential Manchester
There is always a transition in between
the bit and, on encountering any one bit among 0 and 1 there occurs transition
and for the other bit there is no transition as shown in diagram.
3. Bipolar
In bipolar encoding, there are three
levels viz. positive, negative, and zero. The term bipolar refers that any one
data element alternates between positive and negative voltages; while the other
element is at voltage level zero.
AMI
AMI is Alternate Mark Inversion where
'Mark' means 1. Thus bit 1 alternates between positive and negative voltages
while 0 has zero voltage level.
Pseudoternary
In pseudoternary bipolar encoding 0 bit
is encoded as alternating positive and negative voltages while 1 has zero
voltage level.
