Basics of Data Communication: Part 3

Abhimanyu K Vatsa
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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