Sinyal dan Data

Analog dan Digital

• DataAnalog data are continuous and take continuous

valuesDigital data have discrete states and take discrete

values• Signals

Analog signals can have an infinite number of values in a range

Digital signals can have only a limited number of values

Comparison Analog and Digital

• In data communication, we commonly use periodic analog signals and aperiodic digital signals.

A Periodic Sine Wave

Unit of Periods and Frequencies

Summary of Periods and Frequencies

• 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• If a signal does not change at all, its frequency is

zero• If a signal changes instantaneously, its frequency

is infinite

Phase

• Phase describes the position of the waveform relative to time zero

Example of Sine Wave

The Time-Domain and Frequency-Domain Plots

Summary of Sine Wave

• A single-frequency sine wave is not useful in data communications; we need to change one or more of itscharacteristics to make it useful

• When we change one or more characteristics of a single frequency signal, it becomes a composite signal made of many frequencies.

• According to Fourier analysis, any composite signal can be represented as a combination of simple sine waves with different frequencies, phases, and amplitudes.

A Composite Periodic Signal

• If the composite signal is periodic, the decomposition gives a series of signals with discrete frequencies; if the composite signal is nonperiodic, the decomposition gives a combination of sine waves with continuous frequencies

The Decomposition of A Composite PeriodicSignal

The Time and Frequency Domainsof A Nonperiodic Signal

Bandwidth/ Lebar pita

• Perbedaan antara frekuensi terendah dan frekuensi tertinggi dalam rentang tertentu.

• Lebar cakupan frekuensi yang digunakan oleh sinya dalam medium transmisi.

Example

• A signal has a spectrum with frequencies between 1000 and 2000 Hz (bandwidth of 1000Hz). A medium can pass frequencies from 3000 to 4000 Hz (a bandwidth of 1000 Hz). Can this signal faithfully pass through this medium?

Digital Signals

• Selain diwakili oleh sinyal analog , informasi juga dapat diwakili oleh sinyal digital . Sebagai contoh, 1 dapat dikodekan sebagai tegangan positif dan 0 sebagai tegangan nol . Sebuah sinyal digital dapat memiliki lebih dari dua tingkat . Dalam hal ini , kita dapat mengirim lebih dari 1 bit untuk setiap tingkat .

Two Signal Levels and Four Signal Levels

Example

• What is the bit rate for high-definition TV (HDTV)?• Solution• HDTV uses digital signals to broadcast high quality video

signals. The HDTV screen is normally a ratio of 16 : 9. There are 1920 by 1080 pixels per screen, and the screen is renewed 30 times per second. 24 bits represents one color pixel.

• The TV stations reduce this rate to 20 to 40 Mbps through compression.

Digital Signal as a Composite Analog Signal

Transmission of Digital Signals

• Baseband Transmission• Broadband Transmission (using modulation)

Baseband Transmission

Baseband Transmission using aDedicated Medium

Broadband Transmission (Using Modulation)

Modulation of a Digital Signal forTransmission on a Bandpass

Summary of Transmission of Digital Signals

• The analog bandwidth of a medium is expressed in hertz; the digital bandwidth, in bits per second

• Digital transmission needs a low-pass channel• Analog transmission can use a band-pass

channel

Transmission Impairment

Attenuation

Decibel

• Measures the relative strengths of two signals or one signal at two different point

dB = –3 + 7 – 3 = +1

Contoh

• Sebuah sinyal dikirim melalui media transmisi dan dayanya berkurang setengah dari daya sebelumnya. Ini berarti bahwa P2 = 1/2 P1. Dalam hal ini, atenuasi (kehilangan daya) dapat dihitung:

10 log10 (P2/P1) = 10 log10 (0.5P1/P1) = 10 log10 (0.5) = 10(–0.3) = –3 dB

Distortion

• The signal changes its form or shape

Noise

• Example: thermal noise, induced noise, crosstalk, and impulse noise

Signal-to-Noise Ratio (SNR)

Data Rate Limits

• A very important consideration in data communications is how fast we can send data, in bits per second, over a channel. Data rate depends on three factors:1. The bandwidth available2. The level of the signals we use3. The quality of the channel (the level of noise)

• Theorem:– Nyquist => Noiseless channel– Shannon => Noisy channel

Noiseless Channel: Nyquist Bit Rate

• Theoretical maximum bit rate

• Note:• Increasing the levels of a signal may reduce

the reliability of the system

Example 1

• Consider a noiseless channel with a bandwidth of 3000 Hz transmitting a signal with two signal levels. The maximum bit rate can be calculated.

Noisy Channel: Shannon Capacity

• In reality, channel is always noisy

Example

• Assume that SNRdB = 36 dB and the channel bandwidth is 2 MHz. The theoretical channel capacity can be calculated as

Performance

• Bandwidth• Throughput• Latency (Delay)• Bandwidth-Delay Product

Bandwidth

Two different context:• Bandwidth in hertz, refers to the range of

frequencies in a composite signal or the range of frequencies that a channel can pass

• Bandwidth in bits per second, refers to the speed of bit transmission in a channel or link

Throughput

• A measure of how fast we can actually send data through a network

Example 2: • A network with bandwidth of 10 Mbps can pass only

anaverage of 12,000 frames per minute with each framecarrying an average of 10,000 bits. What is the throughput of this network?