Chapter 2 Basic Communication Theory

Basic Communications Theory w Understand the basic transmission theory, and figure out the maximum data rate. w Identify the three transmission modes: Simplex, Half-duplex and Full-duplex w Classify the differences between serial and parallel transmission in terms of cost, data rate and suitability w Describe various analogue and digital modulation techniques

Information representation w Information can be transmitted in a transmission medium as a representation of passing information to the receiver. w Telephone wire w Air w Optical fiber w Coaxial cable

Advantages using Fourier series w any complex real-time signal bandwidth can be identified and analyzed in frequency domain w signal distortion against frequency spectrum could be shown in frequency domain w signal amplification against frequency spectrum could also be analyzed.

Signal Analysis w Amplitude: Absolute measure of the height of the wave w Phase: Relative measure of the difference in time between waves w Frequency: Absolute measure of the number of times a wave repeats per unit time

Bandwidth w Any analogue signal is not formed by a single frequency if it is expanded in terms of Fourier series. w The waveform such as voice produced by human being consists of waveforms of many different frequencies. w Bandwidth = f h - f l

Characteristics of Bandwidth w The more bandwidth, the higher the quality of signal to be delivered across the medium. w Signal outside the bandwidth will be distorted by the transmission medium.

Examples of harmonics w Signals are usually grouped into broadband or baseband depending on the signal characteristics. w Baseband transmission refers to sending the digital data along the transmission channel by means of voltage fluctuation.

Channel capacity w The maximum data rate for a finite bandwidth transmission medium in the presence of random noise. w 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 w 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. w dB = 10 log 10 P1/P2 w Where: dB number of decibels P1 the first value of the power P2 the second value of the power

Coding Data in Signals w As discussed above, the transmission rate is related to the bandwidth of transmission medium and signal to noise ratio. w To increase the transmission rate, one can extend the signal to multiple level.

Restriction on Coding level w Physical properties of transmission medium w Intelligence of machine to identify the coding level w Noise level in the medium

Channel type w 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 w Serial transmission: transmit the data bit by bit w Parallel transmission: transmit data byte by byte, word by word

Asynchronous / Synchronous Transmission w In serial transmission, the transmission format can be further classified into Asynchronous and Synchronous.

Asynchronous transmission format w Guarded by start and stop bits and the character to character space is random w The efficiency is limited to 70% taking the start bit, stop bit and parity bit into account.

Synchronous Transmission format w The characters are packed together and there is no gap between two characters.

Asynchronous Handshakes w There are two major handshaking methods being used by the computers for asynchronous data format. w Software and Hardware

Software w By sending appropriate characters to resume or suspend the data flow between two parties ENQ/ACK DC1/DC3 <-- common method DC1/DC2/DC1

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

Modulation w 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 w Modulation can be grouped into two categories: w Analogue modulation Data over telephone line w Digital modulation -- to convert the analogue signal into digital format (CODEC) Voice over digital exchange

Analogue Modulation w 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 w Bits/Sec: Refers to the actual information transfer rate that can be achieved on a given channel w Baud Rate: Refers to the fundamental signalling rate used on the circuit

Digital Transmission w 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 w appropriate for digital transmission over long distance w commonly used nowadays