Computer Networks Chapter 3 Data Transmission

Terminology ÑTransmitter ÑReceiver ÑMedium (Guided/Unguided) ÑDirect link ÑPoint-to-point ÑDirect link ÑMulti-point ÑSimplex/Half Duplex/Full Duplex

Frequency, Spectrum and Bandwidth ÑTime domain concepts ÑContinuous/Discrete signals ÑPeriodic/Aperiodic signals

Sine Wave ÑPeak Amplitude (A) Ñmaximum strength of signal Ñvolts ÑFrequency (f) ÑRate of change of signal ÑHertz (Hz) or cycles per second ÑPeriod = time for one repetition (T) ÑT = 1/f  Phase (  ) ÑRelative position in time

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

Frequency Domain Concepts ÑSignal usually made up of many frequencies ÑComponents are sine waves ÑCan be shown (Fourier analysis) that any signal is made up of component sine waves ÑCan plot frequency domain functions

Frequency Domain

Spectrum & Bandwidth ÑSpectrum Ñrange of frequencies contained in signal ÑAbsolute bandwidth Ñwidth of spectrum ÑEffective bandwidth Ñ Often just bandwidth ÑNarrow band of frequencies containing most of the energy ÑDC Component ÑComponent of zero frequency

Signal with DC Component

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

Analog and Digital Data Transmission ÑData ÑEntities that convey meaning ÑSignals ÑElectric or electromagnetic representations of data ÑTransmission ÑCommunication of data by propagation and processing of signals

Data ÑAnalog ÑContinuous values within some interval Ñe.g. sound, video ÑDigital ÑDiscrete values Ñe.g. text, integers

Acoustic Spectrum (Analog)

Signals ÑMeans by which data are propagated ÑAnalog ÑContinuously variable ÑVarious media Ñwire, fiber optic, space ÑSpeech bandwidth 100Hz to 7kHz ÑTelephone bandwidth 300Hz to 3400Hz ÑVideo bandwidth 4MHz ÑDigital ÑUse two DC components

Data and Signals ÑUsually use digital signals for digital data and analog signals for analog data ÑCan use analog signal to carry digital data ÑModem ÑCan use digital signal to carry analog data ÑCompact Disc audio

Analog Signals Carrying Analog and Digital Data

Digital Signals Carrying Analog and Digital Data

Analog Transmission ÑAnalog signal transmitted without regard to content ÑMay be analog or digital data ÑAttenuated over distance ÑUse amplifiers to boost signal ÑAlso amplifies noise

Digital Transmission ÑConcerned with content ÑIntegrity endangered by noise, attenuation etc. ÑRepeaters used ÑRepeater receives signal ÑExtracts bit pattern ÑRetransmits ÑAttenuation is overcome ÑNoise is not amplified

Advantages of Digital Transmission ÑDigital technology ÑLow cost LSI/VLSI technology ÑData integrity ÑLonger distances over lower quality lines ÑCapacity utilization ÑHigh bandwidth links economical ÑHigh degree of multiplexing easier with digital techniques ÑSecurity & Privacy ÑEncryption ÑIntegration ÑCan treat analog and digital data similarly

Transmission Impairments ÑSignal received may differ from signal transmitted ÑAnalog - degradation of signal quality ÑDigital - bit errors ÑCaused by ÑAttenuation and attenuation distortion ÑDelay distortion ÑNoise

Attenuation ÑSignal strength falls off with distance ÑDepends on medium ÑReceived signal strength: Ñmust be enough to be detected Ñmust be sufficiently higher than noise to be received without error ÑAttenuation is an increasing function of frequency

Delay Distortion ÑOnly in guided media ÑPropagation velocity varies with frequency

Noise (1) ÑAdditional signals inserted between transmitter and receiver ÑThermal ÑDue to thermal agitation of electrons ÑUniformly distributed ÑWhite noise ÑIntermodulation ÑSignals that are the sum and difference of original frequencies sharing a medium

Noise (2) ÑCrosstalk ÑA signal from one line is picked up by another ÑImpulse ÑIrregular pulses or spikes Ñe.g. External electromagnetic interference ÑShort duration ÑHigh amplitude

Channel Capacity ÑData rate ÑIn bits per second ÑRate at which data can be communicated ÑBandwidth ÑIn cycles per second of Hertz ÑConstrained by transmitter and medium

Required Reading ÑStallings chapter 3