Computer Networks Chapter 3 Data Transmission Terminology Transmitter

Computer Networks Chapter 3 Data Transmission

Terminology Ñ Transmitter Ñ Receiver Ñ Medium (Guided/Unguided) Ñ Direct link Ñ Point-to-point Ñ Direct link Ñ Multi-point Ñ Simplex/Half Duplex/Full Duplex

Frequency, Spectrum and Bandwidth Ñ Time domain concepts Ñ Continuous/Discrete signals Ñ Periodic/Aperiodic signals

Sine Wave Ñ Peak Amplitude (A) Ñ maximum strength of signal Ñ volts Ñ Frequency (f) Ñ Rate of change of signal Ñ Hertz (Hz) or cycles per second Ñ Period = time for one repetition (T) Ñ T = 1/f Ñ Phase (f) Ñ Relative position in time

Varying Sine Waves Amplitude, Frequency and Phase fully determine a sine wave

Frequency Domain Concepts Ñ Signal usually made up of many frequencies Ñ Components are sine waves Ñ Can be shown (Fourier analysis) that any signal is made up of component sine waves Ñ Can plot frequency domain functions

Frequency Domain

Spectrum & Bandwidth Ñ Spectrum Ñ range of frequencies contained in signal Ñ Absolute bandwidth Ñ width of spectrum Ñ Effective bandwidth Ñ Often just bandwidth Ñ Narrow band of frequencies containing most of the energy Ñ DC Component Ñ Component of zero frequency

Signal with DC Component

Data Rate and Bandwidth Ñ Any transmission system has a limited band of frequencies Ñ This limits the data rate that can be carried

Analog and Digital Data Transmission Ñ Data Ñ Entities that convey meaning Ñ Signals Ñ Electric or electromagnetic representations of data Ñ Transmission Ñ Communication of data by propagation and processing of signals

Data Ñ Analog Ñ Continuous values within some interval Ñ e. g. sound, video Ñ Digital Ñ Discrete values Ñ e. g. text, integers

Acoustic Spectrum (Analog)

Signals Ñ Means by which data are propagated Ñ Analog Ñ Continuously variable Ñ Various media Ñ wire, fiber optic, space Ñ Speech bandwidth 100 Hz to 7 k. Hz Ñ Telephone bandwidth 300 Hz to 3400 Hz Ñ Video bandwidth 4 MHz Ñ Digital Ñ Use two DC components

Data and Signals Ñ Usually use digital signals for digital data and analog signals for analog data Ñ Can use analog signal to carry digital data Ñ Modem Ñ Can use digital signal to carry analog data Ñ Compact Disc audio

Analog Signals Carrying Analog and Digital Data

Digital Signals Carrying Analog and Digital Data

Analog Transmission Ñ Analog signal transmitted without regard to content Ñ May be analog or digital data Ñ Attenuated over distance Ñ Use amplifiers to boost signal Ñ Also amplifies noise

Digital Transmission Ñ Concerned with content Ñ Integrity endangered by noise, attenuation etc. Ñ Repeaters used Ñ Repeater receives signal Ñ Extracts bit pattern Ñ Retransmits Ñ Attenuation is overcome Ñ Noise is not amplified

Advantages of Digital Transmission Ñ Digital technology Ñ Low cost LSI/VLSI technology Ñ Data integrity Ñ Longer distances over lower quality lines Ñ Capacity utilization Ñ High bandwidth links economical Ñ High degree of multiplexing easier with digital techniques Ñ Security & Privacy Ñ Encryption Ñ Integration Ñ Can treat analog and digital data similarly

Transmission Impairments Ñ Signal received may differ from signal transmitted Ñ Analog - degradation of signal quality Ñ Digital - bit errors Ñ Caused by Ñ Attenuation and attenuation distortion Ñ Delay distortion Ñ Noise

Attenuation Ñ Signal strength falls off with distance Ñ Depends on medium Ñ Received signal strength: Ñ must be enough to be detected Ñ must be sufficiently higher than noise to be received without error Ñ Attenuation is an increasing function of frequency

Delay Distortion Ñ Only in guided media Ñ Propagation velocity varies with frequency

Noise (1) Ñ Additional signals inserted between transmitter and receiver Ñ Thermal Ñ Due to thermal agitation of electrons Ñ Uniformly distributed Ñ White noise Ñ Intermodulation Ñ Signals that are the sum and difference of original frequencies sharing a medium

Noise (2) Ñ Crosstalk Ñ A signal from one line is picked up by another Ñ Impulse Ñ Irregular pulses or spikes Ñ e. g. External electromagnetic interference Ñ Short duration Ñ High amplitude

Channel Capacity Ñ Data rate Ñ In bits per second Ñ Rate at which data can be communicated Ñ Bandwidth Ñ In cycles per second of Hertz Ñ Constrained by transmitter and medium

Required Reading Ñ Stallings chapter 3