Digital data transmission,line coding and pulse shaping

DIGITAL DATA
TRANSMISSION,LINE
CODING AND PULSE
SHAPING
Made By: AAYUSH KUMAR
Roll No.: 1209731002

What is Digital Data
Transmission?
• A mode of transmission in which all information to
be transferred, such as voice, image or text data, is
converted in the digital (mainly binary numbers)
before transmission. At the end point, binary code is
converted back into original format.
• Digital transmission can deliver a signal, free of
ghosts, interference, and picture noise. It provides
sharper, clearer and faster transmission, using less
bandwidth to transmit more information than
analog.

Parallel Transmission
•In Parallel transmission, by grouping,
we can send n data bits at a time
instead of 1.
•Its advantage over serial transmission
is in terms of speed.

Block diagram of parallel
transmission

Serial transmission
• In Serial transmission, one bit follows
another
• It advantage over parallel transmission is
that it reduces cost of transmission. But
communication with device is parallel
hence conversion devices are required at
the interface between the sender and the
line (p to S) and between the line and
receiver (S to P)

Block diagram of serial
transmission

Asynchronous, synchronous
and isochronous
• In telecommunications, asynchronous communication
is transmission of data, generally without the use of an
external clock signal, where data can
be transmitted intermittently rather than in a steady stream
• The transmission of data in which both stations are
synchronized by a clock is known as synchronous transmission
of data. Codes are sent from the transmitting station to the
receiving station to establish the synchronization, and the data
are then transmitted in continuous streams.
• Isochronous transmission transmits asynchronous data over a
synchronous data link so that individual characters are only
separated by a whole number of bit-length intervals

Line Coding
• Converting a string of 1’s and 0’s (digital data) into a sequence
of signals that denote the 1’s and 0’s.
• For example a high voltage level (+V) could represent a “1” and
a low voltage level (0 or -V) could represent a “0”.

Relationship between data rate
and signal rate
• The data rate defines the number of bits sent per
sec - bps. It is often referred to the bit rate.
• The signal rate is the number of signal elements
sent in a second and is measured in bauds. It is
also referred to as the modulation rate.
• Goal is to increase the data rate whilst reducing
the baud rate.

Signal element versus data element

Unipolar NRZ
• All signal levels are on one side of the time axis -
either above or below
• NRZ - Non Return to Zero scheme is an example of
this code. The signal level does not return to zero
during a symbol transmission.
• Scheme is prone to baseline wandering and DC
components. It has no synchronization or any
error detection. It is simple but costly in power
consumption.

Unipolar RZ
• In unipolar RZ form, the waveform has zero value when symbol
‘0’ is transmitted and waveform has ‘v’ volts when ‘1’ is
transmitted.
• The ‘v’ volts is present for half time period and for the remaining
time period the waveform returns to zero.

Polar - NRZ
• The voltages are on both sides of the time axis.
• Polar NRZ scheme can be implemented with two
voltages. E.g. +V for 1 and -V for 0.
• There are two versions:
• NZR - Level (NRZ-L) - positive voltage for one symbol and
negative for the other
• NRZ - Inversion (NRZ-I) - the change or lack of change in
polarity determines the value of a symbol. E.g. a “1”
symbol inverts the polarity a “0” does not.

Polar - RZ
• The Return to Zero (RZ) scheme uses three
voltage values. +, 0, -.
• Each symbol has a transition in the middle. Either
from high to zero or from low to zero.
• This scheme has more signal transitions (two per
symbol) and therefore requires a wider
bandwidth.
• No DC components or baseline wandering.
• Self synchronization - transition indicates symbol
value.
• More complex as it uses three voltage level. It has
no error detection capability.

Split-Phase Manchester
• In manchestere format,if symbol 1 is to be
transmitted,Then a positive half interval pulse is followed
by a negative half interval pulse.
• If symbol zero is to be transmitted,then a negative half
interval pulse is followed by a positive half interval pulse.
• Hence for any symbol the pulse takes +ve as weel as –ve
values.

Bipolar - AMI and Pseudoternary
• Code uses 3 voltage levels: - +, 0, -, to represent
the symbols (note not transitions to zero as in RZ).
• Voltage level for one symbol is at “0” and the
other alternates between + & -.
• Bipolar Alternate Mark Inversion (AMI) - the “0”
symbol is represented by zero voltage and the “1”
symbol alternates between +V and -V.
• Pseudoternary is the reverse of AMI.

Bipolar schemes: AMI and pseudoternary

Bipolar Characteristics
• It is a better alternative to NRZ.
• Has no DC component or baseline wandering.
• Has no self synchronization because long runs of “0”s results in
no signal transitions.
• No error detection.

• In electronics and telecommunications, pulse shaping is the
process of changing the waveform of transmitted pulses. Its
purpose is to make the transmitted signal better suited to its
purpose or the communication channel, typically by limiting
the effective bandwidth of the transmission.
• By filtering the transmitted pulses this way, the intersymbol
interference caused by the channel can be kept in control.
• Typically pulse shaping occurs after line
coding and modulation.

Need for Pulse Shaping
In communications systems, two important requirements of a
wireless communications channel demand the use of a pulse
shaping filter.
1) generating bandlimited channels
2) reducing inter symbol interference (ISI) from multi-path signal
reflections.

Time vs. Frequency Domain for a Sinc Pulse
The sinc pulse,above , meets both of these requirements because it efficiently
utilizes the frequency domain to utilize a smaller portion of the frequency domain,
and because of the windowing affect that it has on each symbol period of a
modulated signal.

Digital data transmission,line coding and pulse shaping

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