chapter 2 basic communication theory basic communications theory w understand the basic transmission...
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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
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