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Analog Communication System Chapter- 2 Analog Communication System

Data Communication & Computer Network-1

What is communication?

Communication : Communication is about sending and receiving information or the transmission of information and meaning from one party to another through using shared symbols or messages.

Communication process model Sender Message Channel Medium Receiver Feedback © PhotoDisc

Elements of a Communication System Communication involves the transfer of information or intelligence from a source to a recipient via a channel or medium. Basic block diagram of a communication system: Channel Source Transmitter Receiver Recipient

Brief Description Source: analogue or digital Transmitter: transducer, amplifier, modulator, oscillator, power amp., antenna Channel: e.g. cable, optical fibre, free space Receiver: antenna, amplifier, demodulator, oscillator, power amplifier, transducer Recipient: e.g. person, speaker, computer

Elements of a Communication System Output message Input message Input Transducer Transmitter Channel Receiver Output Transducer

Input Transducer: The message produced by a source must be converted by a transducer to a form suitable for the particular type of communication system. Example: In electrical communications, speech waves are converted by a microphone to voltage variation.

Transmitter: The transmitter processes the input signal to produce a signal suits to the characteristics of the transmission channel. Signal processing for transmission almost always involves modulation and may also include coding. In addition to modulation, other functions performed by the transmitter are amplification, filtering and coupling the modulated signal to the channel.

Channel: The channel can have different forms: The atmosphere (or free space), coaxial cable, fiber optic, waveguide, etc. The signal undergoes some amount of degradation from noise, interference and distortion Receiver: The receiver’s function is to extract the desired signal from the received signal at the channel output and to convert it to a form suitable for the output transducer. Other functions performed by the receiver: amplification (the received signal may be extremely weak), demodulation and filtering. Output Transducer: Converts the electric signal at its input into the form desired by the system user. Example: Loudspeaker, personal computer (PC), tape recorders.

Digital data have discrete states and take discrete values. Data can be analog or digital. Analog data are continuous and take continuous values. Digital data have discrete states and take discrete values.

Signals can be analog or digital Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital signals can have only a limited number of values.

Figure Comparison of analog and digital signals

Figure Two signals with the same phase and frequency, but different amplitudes

Frequency and period are the inverse of each other.

Figure Two signals with the same amplitude and phase, but different frequencies

Frequency is the rate of change with respect to time Frequency is the rate of change with respect to time. Change in a short span of time means high frequency. Change over a long span of time means low frequency.

Phase describes the position of the waveform relative to time 0.

Figure Three sine waves with the same amplitude and frequency, but different phases

Bandwidth Width of the spectrum of frequencies that can be transmitted if spectrum=300 to 3400Hz, bandwidth=3100Hz Greater bandwidth leads to greater costs Limited bandwidth leads to distortion

Figure 1.1 Components of a data communication system 1.#

Data flow (simplex, half-duplex, and full-duplex) 1.#

Asynchronous Transmission Used in serial communication Data transmitted 1 character at a time Character format is usually 1 start & 1+ stop bits, plus data of 5-8 bits Character may include parity bit Timing needed only within each character Resynchronization is accomplished with each start bit Uses simple, cheap technology Wastes 20-30% of bandwidth 3

Asynchronous Character Transmission 26

Asynchronous Transmission

Synchronous Transmission Used in parallel communication Large blocks of bits transmitted without start/stop codes Synchronized by a clock signal or clocking data Data framed by preamble (opening)/ postamble (closing) bit patterns More efficient than asynchronous Overhead typically below 5% Used at higher speeds than asynchronous 4

Synchronous Transmission 29

Bytes, Blocks, and Frames 30

Analog Communication System Chapter- 2 Analog Communication System

Amplitude Modulation Amplitude Modulation is a process where the amplitude of a carrier signal is altered according to information in a message signal. The frequency of the carrier signal is usually much greater than the highest frequency of the input message signal.

Figure 5.16 Amplitude modulation

Frequency Modulation The modulating signal changes the freq. fc of the carrier signal The bandwidth for FM is high It is approx. 10x the signal frequency

Figure Frequency modulation

Phase Modulation (PM) The modulating signal only changes the phase of the carrier signal. The phase change manifests itself as a frequency change but the instantaneous frequency change is proportional to the derivative of the amplitude. The bandwidth is higher than for AM.

Figure Phase modulation

Digital Communication System Chapter- 3 Digital Communication System

DIGITAL-TO-DIGITAL CONVERSION How we can represent digital data by using digital signals. The conversion involves three techniques: line coding, block coding, and scrambling. Line coding is always needed; block coding and scrambling may or may not be needed.

Line Coding The process of converting digital data to digital signals Digital data – sequences of bits

Line coding and decoding

Characteristics of Line Coding Signal element vs. data element A signal element is the shortest time unit of a digital signal. Data elements are what to need to send as information. Data rate vs. signal rate Data rate (bit rate) – the number of data elements in a unit time (bps). Signal rate (pulse, modulation rate, baud rate) – the number of signal element in a unit time (baud). The goal is two-fold of “high data rate” and “low signal rate”. Bandwidth Although the actual bandwidth of a digital signal is infinite, the effective bandwidth is finite. Baseline wandering DC components – unable to pass a low-pass filter Self-synchronization Built-in error detection Immunity to noise and interferences Complexity

Figure Signal element versus data element

Figure Effect of lack of synchronization

Figure Line coding schemes

Unipolar All signal levels are on one side of the time axis - either above or below NRZ - Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission. Scheme is prone to baseline wandering and DC components. It has no synchronization or any error detection. It is simple but costly in power consumption.

Figure Unipolar NRZ scheme

Polar - NRZ The voltages are on both sides of the time axis. Polar NRZ scheme can be implemented with two voltages. E.g. +V for 1 and -V for 0. There are two versions: NZR - Level (NRZ-L) - positive voltage for one symbol and negative for the other NRZ - Inversion (NRZ-I) - the change or lack of change in polarity determines the value of a symbol. E.g. a “1” symbol inverts the polarity a “0” does not.

Figure Polar NRZ-L and NRZ-I schemes

Polar - RZ The Return to Zero (RZ) scheme uses three voltage values. +, 0, -. Each symbol has a transition in the middle. Either from high to zero or from low to zero. This scheme has more signal transitions (two per symbol) and therefore requires a wider bandwidth. No DC components or baseline wandering. Self synchronization - transition indicates symbol value. More complex as it uses three voltage level. It has no error detection capability.

Figure Polar RZ scheme

Polar Bi-phase: Manchester and Differential Manchester Manchester coding consists of combining the NRZ-L and RZ schemes. Every symbol has a level transition in the middle: from high to low or low to high. Uses only two voltage levels. Differential Manchester coding consists of combining the NRZ-I and RZ schemes. Every symbol has a level transition in the middle. But the level at the beginning of the symbol is determined by the symbol value. One symbol causes a level change the other does not.

Figure Polar biphase: Manchester and differential Manchester schemes

Line Coding Schemes, con’t Bipolar. Ex: AMI and Pseudoternary use three levels: positive, zero, and negative. Multilevel mBnL a pattern of m data elements is encoded as a pattern of n signal elements in which 2m ≤ Ln. Ex) 2B1Q: 2 binary 1 quaternary (=2B4L) 8B6T: 8 binary 6 ternary. 28 data patterns, and 36 signal pattern 4D-PAM5 (4-D 5-level pulse amplitude mod.) Multitransition MLT-3 : multiline transmission, three level.

Figure Bipolar schemes: AMI and pseudoternary

Figure Multilevel: 2B1Q scheme

Table line coding schemes

Chapter 4 Transmission Media

Figure 4.1 Transmission medium and physical layer

Figure 4.2 Classes of transmission media

4.1 Guided Media Twisted-Pair Cable Coaxial Cable Fiber-Optic Cable

Figure Twisted-pair cable

Figure UTP and STP

Table Categories of unshielded twisted-pair cables Category Bandwidth Data Rate Digital/Analog Use 1 very low < 100 kbps Analog Telephone 2 < 2 MHz 2 Mbps Analog/digital T-1 lines 3 16 MHz 10 Mbps Digital LANs 4 20 MHz 20 Mbps 5 100 MHz 100 Mbps 6 (draft) 200 MHz 200 Mbps 7 (draft) 600 MHz 600 Mbps

Figure UTP connector

Figure Coaxial cable

Fiber-Optic Cable Fiber-optic cable uses light to transmit data signals. The core of the fiber-optic cable is composed of one or more thin tubes of glass or plastic. Each tube is called the optical fiber and is as thin as the human hair. A light-emitting diode (LED) or a laser is used to send light through the fibers.

Figure Optical fiber

Figure Propagation modes

Figure Modes

Multimode, graded-index Table Fiber types Type Core Cladding Mode 50/125 50 125 Multimode, graded-index 62.5/125 62.5 100/125 100 7/125 7 Single-mode

Figure Fiber construction

Figure Fiber-optic cable connectors

Note: Microwaves are used for unicast communication such as cellular telephones, satellite networks, and wireless LANs.

Note: Infrared signals can be used for short- range communication in a closed area using line-of-sight propagation.