Bab2 Transmisi Data

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Objektif Konsep dan Terminologi Transmisi data analog dan digital Gangguan transmisi (Impairment) Analisis Fourier Kuat sinyal (Signal Strength) dan Decibels

Sources of signal impairmentSignal carried on transmission medium affected by:

- Attenuation - Limited Bandwidth - Delay Distortion - Noise

Sources of signal impairment

AttenuationSignal Attenuation: Decrease in signal amplitude as it propagates along transmission medium- Consequence: Limit length of cable to be used - Solution : Amplifiers (Repeaters) used to restore signal to original level. - Property : Signal attenuation increases as a function of frequency.

Possible remedies : - Non Linear amplifiers or Equalizers

Attenuation (lanjutan)Signal Amplification ( Gain ): Increase in signal amplitude

Contoh (Attenuation)

Contoh (Lanjutan)

Signaling Rate vs. Data Bit Rate

Limited Bandwidth Bandwidth of a communication / transmission medium: band of sinusoidal frequency components (f1to f2) that will be transmitted by the channel unattenuated

Question: What is the effect of channel bandwidth on transmitted signal ?

Fourier analysis A periodic signal is made of an infinite series of sinusoidal frequency components - Fundamental frequency component : same frequency as initial periodic signal, cycles per sec (Hz) - Harmonics : other frequency components, multiples of fundamental frequency.

Fourier analysisg g

v(t ) ! a an cos n([ 0)t bn sin n([ 0)tn !1 n !1

V(t) = tegangan sinyal (periodis) sebagai fungsi waktu W0 = komponen frekuensi fondamental (rad/sec) T= 2 /W0 atau W0 = 2 f0

Aplikasi analisis Fourier(Untuk transmisi data)Possible binary sequences (periodic) : (1) 1 0 1 0 1 0 period = 2 bit cell intervals (2) 1 1 0 1 1 0 .. period = 3 bit cell intervals (3) 1 1 1 0 1 1 1 0 . period = 4 bit cell intervals Note: (1) has shortest period, highest fundamental frequency component: worst-case sequence

Aplikasi analisis Fourier(Untuk transmisi data) Basic Binary Signal Types: Unipolar Signal (Return to zero, RZ) Amplitudes : +V, 0; Mean Signal Level = V /2 Bipolar Signal (Non ReturnToZero , NRZ) Amplitudes : +V, -V; Mean Signal Level = 0

Aplikasi analisis Fourier(Untuk transmisi data)

Aplikasi analisis Fourier(Untuk transmisi data)

Aplikasi analisis Fourier(Untuk transmisi data) General Observations: A periodic binary sequence is made of an infinite series of sinusoidal signals made of-A fundamental frequency component, f0 -A third harmonic component , 3f0 -A fifth harmonic component , 5f0

Note: Odd harmonics only

Aplikasi analisis Fourier(Untuk transmisi data)/Lanjutan The amplitude of the harmonics diminishes with increasing frequency T (Signal Period) = 2*Tb (Bit Period)Tb = T/2; 1/ Tb = 2/T = 2 * 1/T R (bit rate) = 2 * f0 (fundamental frequency)

When binary data signal transmitted on a channel, only those frequency components that are within channel bandwidth will be received A channel with a bandwidth :From 0 to fundamental frequency (half the bit rate) can often give satisfactory performance

Aplikasi analisis Fourier (Untuk transmisi data)Analysis Effect of Limited Bandwidth:

Aplikasi analisis Fourier(Untuk transmisi data)Effect of Limited Bandwidth

Aplikasi analisis Fourier(Untuk transmisi data)Effect of Limited Bandwidth:

Contoh : A binary signal of R (bit rate) = 500 bps is transmitted on a communication channel What is the minimum bandwidth required, assuming(a) Fundamental frequency only (b) Fundamental and third harmonic (c) Fundamental,third and fifth harmonics

Solution :For R = 500bps, f0 = R/2 = 250 Hz (a) 0 250 Hz (b) 0 750 Hz (c ) 0 1250 Hz

Laju transfer informasi maksimumUntuk kanal transmisi (noiseless) (the Nyquist Formula) C = 2W log2M where W : bandwidth of the channel (Hz) M: number of levels per signaling element log2M : number of bits/signaling element

Laju transfer informasi maksimum (Contoh)Data is sent over a PSTN with : M = 8 levels /signaling element W = 3,000 Hz Q: What is the Nyquist maximum data transfer rate ? A:C = 2W log2M = 2*3000*log28 = 2*3000*3 = 18,000 bps

Bandwidth Efficiency of a Transmission ChannelB = R/W = 1/(W* Tb) bps Hz-1Observations: The higher the bit rate relative to the available bandwidth, the higher the bandwidth efficiency Typical values for B: 0.25 - 3.0bps Hz-1, (B=3.0 requires a high signal rate) Another expression for bandwidth efficiency:R = RS. m = RS log2 M B = R/W = (R S log2 M) /W

WhereM is number of levels per signaling element log2 M is number of bits per signaling element

Delay Distortion Rate of propagation of a signal over a channel is function of frequency of signal Consequence: Delay Distortion different frequency components arrive with different delays Delay distortion increases with bit rate Inter symbol interference : frequency components of a bit start to interfere with a later bit Use of an eye diagram : oscilloscope displays all possible signals superimposed

Delay Distortion (Lanjutan)

Noise Line Noise level : Random perturbations on the line, even when no signal present. Consequence: Interference between (attenuated) transmitted signal and line (background) noise Signal-to noise ratio: (SNR) SNR = 10log10(S/N) dBS : average power in received signal (watts) N : noise power (watts)

Noise (Lanjutan)Theoretical data rate of a transmission channel (Shannon Hartley law)

C = W log2(1 + S/N) bpsWhere W = Bandwidth (Hz). S = Average signal power (watts) N = Random noise power (watts)

Noise (Lanjutan)Theoretical data rate of a transmission channel (Shannon Hartley law)Given a PSTN with: W = 3000 Hz, and SNR = 20 dB determine maximum theoretical data rate A: SNR = 10log10(S/N) 20 = 10log10 (S/N) , untuk S/N = 102 = 100. C = W log2(1 + S/N) = 3000 log2(1+100) = 19,963 bps.

Noise (Lanjutan) Cross Talk: Noise caused by unwanted electrical coupling between adjacent lines Near End Cross Talk (NEXT) or Self Cross Talk: Strong signal output from transmitter circuit interferes with weak signal at receiver circuit Solution: Adaptive NEXT cancellers Impulse Noise: Caused by external electrical activity impulses (lightning, impulses from old switching systems) Observation : Both cross talk and impulse noise are caused by electrical activity external to transmission line