Chapter 2

Basic Communication Theory

Basic Communications Theory

Understand the basic transmission theory, and figure out the maximum data rate.

Identify the three transmission modes: Simplex, Half-duplex and Full-duplex

Classify the differences between serial and parallel transmission in terms of cost, data rate and suitability

Describe various analogue and digital modulation techniques

Information representation

Information can be transmitted in a transmission medium as a representation of passing information to the receiver.

Telephone wire Air Optical fiber Coaxial cable

Advantages using Fourier series

any complex real-time signal bandwidth can be identified and analyzed in frequency domain

signal distortion against frequency spectrum could be shown in frequency domain

signal amplification against frequency spectrum could also be analyzed.

Signal Analysis

Amplitude: Absolute measure of the height of the wave

Phase: Relative measure of the difference in time between waves

Frequency: Absolute measure of the number of times a wave repeats per unit time

Bandwidth

Any analogue signal is not formed by a single frequency if it is expanded in terms of Fourier series.

The waveform such as voice produced by human being consists of waveforms of many different frequencies.

Bandwidth = fh - fl

Characteristics of Bandwidth

The more bandwidth, the higher the quality of signal to be delivered across the medium.

Signal outside the bandwidth will be distorted by the transmission medium.

Examples of harmonics

Signals are usually grouped into broadband or baseband depending on the signal characteristics.

Baseband transmission refers to sending the digital data along the transmission channel by means of voltage fluctuation.

Channel capacity

The maximum data rate for a finite bandwidth transmission medium in the presence of random noise.

Maximum data rate = W x log 2 (1 + S/N)• W is the bandwidth of transmission medium• S/N is the signal to noise power ratio• Maximum data rate is measured in bits/second

Decibel

As the signal to power ratio is usually quite significant, a better representation in communications is used to express the ratio of two values in logarithmic format.

dB = 10 log 10 P1/P2 Where:

• dB number of decibels

• P1 the first value of the power

• P2 the second value of the power

Coding Data in Signals

As discussed above, the transmission rate is related to the bandwidth of transmission medium and signal to noise ratio.

To increase the transmission rate, one can extend the signal to multiple level.

Restriction on Coding level

Physical properties of transmission medium Intelligence of machine to identify the

coding level Noise level in the medium

Channel type Irrespective of direction of data transfer, there are THREE

types of transmission channels.

• Simplex: One party in the communication can send data to the other, but cannot receive data from the other end : radio pager.

• Half duplex: Both parties can send and receive information from the other end but not at the same time: walkie talkie.

• Full duplex: Both parties can send and receive information at the same time: computer to computer communication

Examples of Channel type

Serial / Parallel transmission Serial transmission: transmit the data bit by

bit

Parallel transmission: transmit data byte by byte, word by word

Asynchronous / Synchronous Transmission In serial transmission, the transmission

format can be further classified into Asynchronous and Synchronous.

Asynchronous transmission format Guarded by start and stop bits and the

character to character space is random The efficiency is limited to 70% taking the

start bit, stop bit and parity bit into account.

Synchronous Transmission format The characters are packed together and

there is no gap between two characters.

Asynchronous Handshakes

There are two major handshaking methods being used by the computers for asynchronous data format.

Software and Hardware

Software

By sending appropriate characters to resume or suspend the data flow between two parties• ENQ/ACK

• DC1/DC3 <-- common method

• DC1/DC2/DC1

Hardware

By setting or resetting the control signals to resume/suspend data• RTS/CTS• DTR/DSR

Modulation

Modulation is used to translate digital signals to analog signals which can be transmitted over a transmissions medium without distorting the signals.

Type of modulation

Modulation can be grouped into two categories:

Analogue modulation• Data over telephone line

Digital modulation -- to convert the analogue signal into digital format (CODEC)• Voice over digital exchange

Analogue Modulation

THREE basic modulation techniques• Amplitude Modulation: use the amplitude of

carrier wave to represent binary data• Frequency Modulation: use the frequency of

carrier wave to represent binary data• Phase Modulation: use the phase of carrier

wave to represent binary data

Bits/Sec and Bauds

Bits/Sec: Refers to the actual information transfer rate that can be achieved on a given channel

Baud Rate: Refers to the fundamental signalling rate used on the circuit

Digital Transmission

The advantages offered by digital network are:• Lower error rate• Higher transmission speed: 625 Mb/s or even

up to 20G bps• Efficient use of channel by using digital

multiplexing techniques,

Pulse Code Modulation

appropriate for digital transmission over long distance

commonly used nowadays