Data Transmission
Data transmission occurs between transmission and receiver over some
transmission medium. Successful transmission of data depends on two main
factors, quality of signal being transmitted and characteristics of transmission
medium. Data transmission always uses the form of electromagnetic waves and they
are classified into guided electromagnetic waves and unguided electromagnetic
waves. Examples of guided waves are twisted pair, coxial cable and optical
fiber. Unguided waves means transmitting electromagnetic waves but they are not
guided as example propagation through air, vacuum and seawater.
Transmission Configurations and Terminology
In general these are two types of transmission configuration used as given
below:
- Point to Point Transmission Configuration
In point to point configuration two devices propagate signals directly from
transmitter to receiver with no intermediate devices, other than amplifiers and
repeater used to increase signal strength.
- Multi-Point Transmission Configuration
In multipoint configuration, more than two devices share the same medium and
usage all devices.
Any of transmission configurations may be simple, half-duple or full-duplex.
Simplex
In simplex transmission, signals are transmitted in only one direction always.
They will never be changed.
Half-Duplex
In half-duplex, both stations can transmit but only one can work at a time. They
can't transmit simultaneously.
Full-Duplex
In full-duplex, both stations can transmit simultaneously.
Time Domain Concepts
An electromagnetic signal can be continuous, discrete or periodic.
Continuous Signal
A continuous signal is one in which the signal intensity or signal strength
varies in a smooth fashion over time. There is no break or discontinues in the
signal.
Discrete Signal
A discrete signal is one in which the signal intensity maintains a constant
level for some period of time and then changes to another constant level.
Periodic Signal
In periodic signal, same signal patterns repeats over time. There are two types
of periodic signals used as Analog Signal (Sine waves) and Digital Signal
(square waves).
Above figure is belongs to Analog Signal (Sine waves).
Above figure is belongs to Digital Signal (Square waves).
Frequency Domain Concepts
As we know any electromagnetic signal is made up of many frequencies. In the
figure given below, the second frequency is an integer multiple of the first
frequency. When all of the frequency components of a signal are integer
multiples of one frequency, the letter frequency is referred to as the
fundamental frequency.
Why Digital Communication
Any transmission system makes use of a physical transmission media or channel
that allows the propagation of electromagnetic energy in the form of pulses or
variations in voltage, current or light intensity.
In analog communication the objective is to transmit a signal in waveform which
is a function that varies continuously which time as given in figure below.
For example, the electrical signal coming out of a microphone corresponding to
the variation in air pressure corresponding to sound. This function of time must
be reproduced exactly at the receiver output of the analog communication system.
FM, TV transmissions are the example of analog transmission.
In digital transmission the main objective is to transmit a given system that is
selected from some finite set of possibilities, as in binary digital
transmission either 0 or 1. This can be done, by transmitting positive voltage
for a certain period of time to convey a 1 or a negative voltage to convey a 0
as in picture given below.
Repeater
Repeater is an electronic device that operates on only the physical layer of the
OSI model. There are two types of repeaters mostly used.- Analog Repeater
It is a device used in analog communication to regenerate a signal that
resembles as closely as possible the signal at the input of the repeater signal.
The input to the analog repeater is an attenuated (reduce electrical strength)
and distorted version of the original transmitted signal plus the random noise
added in the segment. The repeater next uses a device called an equalizer in an
attempt to eliminate the distortion (a change to worse or bad).
- Digital Repeater
In digital repeater, string of 0 and 1 conveyed by a sequence of positive and
negative voltages or signals. Digital transmission has superior performance over
analog transmission. Digital repeater eliminates the accumulation (natural
growth of noise) of noise.
Bandwidth, Data Rate, Channel Capacity, Error
Rate and Noise
Bandwidth
A communication channel utilizes frequency to transmit the electromagnetic
energy which represents the data. Transmitting information requires more than a
single bit of frequency and for this purpose a band of spectrum around the
nominal frequency is required which is known as bandwidth of the signal.
Data Rate
It is simply rate at which data can be communicated. The smallest rate of data
communication would be bit per second (bps).
Channel Capacity.
To carry data at a higher rate, a wider bandwidth is required. The rate at which
data can be transmitted over a communication channel is known as the channel
capacity.
Error Rate
It is a simply a rate at which errors occur. When an error is the reception of a
1, a 0 is transmitted or the reception of a 0, a 1 is transmitted.
Noise
When communication starts, its noises over the communication path. Noise is
movement of electrons over communication path. There are many types of noises
exist as follows:- White Noise or Thermal Noise
The vibration from the movement of electrons cause the emission (release or
flow) of electromagnetic waves of all frequencies. This phenomenon is called
white noise or thermal noise because it contains elements of all spectral
frequencies equally, just like white light. Basically white noise generated by
electrical noise.
- Electrical Noise
This is background noise and stems from high voltage and high frequency
interface on the alternating current line (AC), typically a power line. The
basic cause of this noise is Radio Frequency that comes from the Radio Station.
- Inter-modulation Noise
Many voice grade circuits are modulated onto a high capacity link. These
channels can interface with each other if the equipment is slightly non-linear.
Nyquist Sampling Rate
Assume that we have a channel that is noise-free and then data rate will be
bandwidth of the signal. We formulate this rate as Nyquist sampling rate and it
states that if the signal with frequencies components not greater than WHz
(Cycle per second) then nyquist sampling rate will be 2W (smp) sample per
second.
Shannon Channel Capacity
Data transmission rate and noise tied together in a formula developed by the
Mathematician Claude Shannon. For a given level of noise, we would expect that
greater signal strength would improve the ability to correctly receive data in
the presence of noise. The key parameter involved in this reasoning is the
Signal to Noise (SNR) ratio, which is the ratio of power. - High Signal-to-Noise (SNR)
2. Low Signal-to-Noise (SNR)
Line Coding
To convert binary information sequence into a digital signal in a digital
communication system line coding is used. The selection of line coding technique
involves several considerations. Maximizing bit rate is the main concern in
digital transmission when bandwidth is at a premium. Another important design
consideration is the ease (make easier) with which the bit timing information
can be recovered from the digital signal. Some line coding has built-in error
detecting capabilities. Finally, the complexity and the cost of the line code
implementations are always factors in the selection of a given application.
There are couple of line coding mechanism exists as- Unipolar NRZ (Non Return to Zero)
- Polar NRZ
- NRZ Inverted
- Bipolar Encoding
- Manchester Encoding
- Differential Manchester Encoding
But Unipolar NRZ is simplest and most used. It has following line coding method.
Let's take a binary sequence 101011100.
Modem and Digital Modulation
The telephone system is originally based on analog signaling. With the advantage
of technology, the long distance converted into digital but the local loops
between a telephone exchange and the telephone at the user are still analog.
Consequently, when computer wishes to send digital data, it produces over the
telephone line then the data must be converted into analog form by a device for
transmission over telephone line and at the receiver end this received analog
signal must be again converted into digital.
A device that accepts a serial stream of digital bits as input and produces
modulated analog carrier signal as output is called modem (modular de-modular).
The modem is inserted between the computer and telephone system. A continuous
tone in the 1000 to 2000 Hz range called sine wave carrier is modulated
according to the input digital signal at the transmitting end at the receiver
end the received modulated signal is converted back to digital stream of bits by
the processor of demodulation.
Various parameters of sine wave carrier like amplitude, frequency or phase can
be modulated to transmit information. There are following type of modulation
used as discussed below:- Binary Modulation
In Binary modulation, signal directly varies with bit which is being
transmitted.
- Amplitude Modulation
In Amplitude modulation, two different voltage levels are used to represent bits
0 and 1.
- Frequency Modulation
It is also known as frequency shift keying. In this two or more different tones
or frequencies are used.
- Phase Modulation
In this wave is systematically shifted 45, 135, 225, 315 degrees at un-infirmly
intervals.
HAVE A HAPPY CODING!