Chapter 2 : Data Communications

Chapter 2 : Data Communications • Generally, data usable to a person or application are not in a form that can be transmitted over a network. • To be transmitted, data must be transformed to electromagnetic signals • All form of information – can be represented by electromagnetic signals • Voice, data, image, video etc BENG 4522 Data Communications & Computer Networks

2.1 Analog & Digital • Data can be analog or digital • Analog data • Refers to information that is continuous – sound made by human voice • Take on continuous values • Digital data • Refers to information that has discrete states – data stored in computer memory (0 and 1) • take discrete values. • Analog signal • have an infinite number of values in a range • Digital signal • Digital signal – have only a limited number of defined values (i.e. 0 or 1) BENG 4522 Data Communications & Computer Networks

2.1 Analog & Digital BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Nonperiodic signal • Periodic signal • The signal completes a pattern within a measurable time frame/period, and repeats that pattern over subsequent identical periods. • Nonperiodic signal • the signal changes without exhibiting a pattern/cycle that repeats over time • Both analog and digital signals can be periodic or nonperiodic • Refers to information that is continuous – sound made by human voice • In data communications, we commonly used periodic analog signals and nonperiodic digital signals BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Nonperiodic signal • Periodic analog signals • Periodic analog signals can be classified as • Simple – sine wave (cannot be decomposed into simpler signals) • Composite – composed of multiples sine waves • Sine wave • Sine wave can be represented by 3 parameters – peak amplitude, frequency and the phase BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Nonperiodic signal • Period and frequency • Period – refers to the amount of time, in seconds, a signals need to complete 1 cycle. • Frequency – refer to the number of periods in 1 s. BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Nonperiodic signal • Period and frequency BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Nonperiodic signal • Wavelength • Wavelength describes how far the wave can travel in 1 period time. • Depends on the frequency and the medium BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Nonperiodic signal • Wavelength • Propagation speed of electromagnetic signals depends on the medium and on the frequency of the signal • The relation between the wavelength and frequency of electromagnetic signals travel in vacuum • λ: wavelength, f : frequency, c : speed of light 3 x 108 ms-1 BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Non-periodic signal • Time and frequency domains • Time-domain – the waveform is plot in changes of signal amplitude with respect to time. • Frequency-domain – plot is concerned only with the peak value and frequency. BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Non-periodic signal • Time and frequency domain • The time domain and frequency domain of three sine waves. BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Non-periodic signal • Composite signal • According to fourier analysis, any composite signal is a combination of simple sine wave with different frequencies, amplitudes and phases. • Can be periodic or nonperiodic • If the composite signal is periodic, the decomposition gives a series of signal with discrete frequencies (integer value i.e. 1, 2, 3, ...). • If the signal is nonperiodic, the decomposition gives a combination of sine waves with continuous frequencies (real values). BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Non-periodic signal • Periodic Composite signal • The component sinusoids are at frequencies that are multiples of the basic frequency of periodicity. harmonics Fundamental frequency BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Non-periodic signal • Periodic composite signal BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Non-periodic signal • Nonperiodic composite signal BENG 4522 Data Communications & Computer Networks

2.2 Periodic & Non-periodic signal • Bandwidth • The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. BENG 4522 Data Communications & Computer Networks

2.3 Digital signal • In general if a signal has L levels, each level needs log2L bits ! BENG 4522 Data Communications & Computer Networks

2.3 Digital signal • Bit Rate • Most digital signals are nonperiodic, and thus period and frequency are not appropriate characteristics. • For digital signal, in stead of using frequency, bit rate is used. • Bit rate is the number of bits sent in 1s, expressed in bits per second (bps). • Examples • Bit Length • The bit length is the distance one bit occupies on the transmission medium. Bit length = propagation speed x bit duration BENG 4522 Data Communications & Computer Networks

2.3 Digital signal • Digital signal as a Composite Analog Signal • Fourier analysis can be used to decompose a digital signal. • If the digital signal is periodic (which is rare in data communications), the decomposed signal has a frequency domain representation with an infinite bandwidth and discrete frequencies. BENG 4522 Data Communications & Computer Networks

2.3 Digital signal • Digital signal as a Composite Analog Signal • If the digital signal is nonperiodic, the decompose signal still has an infinite bandwidth, but the frequencies are continuous. BENG 4522 Data Communications & Computer Networks

2.3 Digital signal • Relationship between bit rate and bandwidth • Consider the case binary data is encoded into digital signal, and to be transmitted by a transmission medium. • Digital signal contains an infinite bandwidth, but a real transmission medium has a finite bandwidth which can limit the data rate that can be carried on the transmission medium. • Limited bandwidth creates distortions of the input signal, which makes the task of interpreting the received signal more difficult. • The more limited bandwidth, the greater the distortion, and the greater the potential for error by the receiver. • The higher the bit rate of a signal is, the greater is its bandwidth. • The greater the bandwidth of a transmission system, the higher is the bit rate that can be transmitted. BENG 4522 Data Communications & Computer Networks

2.3 Digital signal • Relationship between bit rate and bandwidth BENG 4522 Data Communications & Computer Networks

2.3 Digital signal • Transmission of Digital Signals • Digital signals can be transmitted using two different approaches : • Baseband transmission • Broadband transmission (using modulation) • Baseband transmission • Signal is sent over a channel without changing the digital signal to analog signal. • Baseband transmission requires a low-pass channel, a channel with a bandwidth starts from zero. • We know that digital signal is a composite analog signal with an infinite bandwidth. • For that, baseband transmission of digital signal that preserves the shape of the digital signal is possible only if we have a low-pass channel with an infinite or very wide bandwidth. • In baseband transmission, the required bandwidth is proportional to the bit rate; if we need to send the bits faster, we need more bandwidth. BENG 4522 Data Communications & Computer Networks

2.3 Digital signal • Broadband transmission (using modulation) • The digital signal is sent over a channel by changing the digital signal to analog signal (using the modulation process). • Modulation allows the use of bandpass channel – a channel with a bandwidth that does not start from zero. BENG 4522 Data Communications & Computer Networks

2.3 Digital signal • Modulation of a digital signal for a transmission over a passband channel BENG 4522 Data Communications & Computer Networks

Chapter 2 : Data Communications