Data and Signals & Analouge Signaling

Data and Signals One of major functions of physical layer is to move data in the form of electromagnetic signals across a transmission medium. Data useable to a person or application are not in form that can be transmitted over a network. i.e. a photograph must first be changed to a form that transmission media can accept.

Data and Signals Transmission media work by conducting energy along a physical path. For transmission, data need to be changed to signals.

Data and Signals Communication at application, transport, network and data link layer is logical. But communication at the physical layer is physical. When two entities need to exchange data over Internet, communication at physical layer means exchanging of signals. Both data and signals can be either analog o digital.

Analog Data The term analog data refers to information that is continuous. So, the analog signals are show in continuous form. There are no sudden change in analog signals

Analog Data: Example An analog clock that has hour, minute and second hands, gives information in continuous form. The movement of hands is continuous.

Analog Data: Example Analog data, such as the sounds made by a human voice, take on continuous values. When someone speaks, an analog wave is created in the air. This can be captured by a microphone and converted to an analog signal or sampled and converted to a digital signal.

Digital Data The term digital data refers to information that has discrete states. There are sudden change happened in digital signals

Digital Data: Example A digital clock that reports the hours and the minutes will change suddenly from 8:05 to 8:06 or 4:59 to 5:00 Digital data take on discrete values. For example, data are stored in computer memory in the form of 0s and 1s. Digital data can be converted to a digital signal or modulated into an analog signal for transmission across a medium.

Analog and Digital Signals An analog signal has infinitely many levels of intensity over a period of time. As the wave moves from value A to value B, it passes through and includes an infinite number of values along its path. A digital signal, on the other hand, can have only a limited number of defined values. Although each value can be any number, it is often as simple as 1 and O.

Analog and Digital Signals The simplest way to show signals is by plotting them on a pair of perpendicular axes. – The vertical axis represents the value or strength of a signal. – The horizontal axis represents time.

Analog and Digital Signals Both analog and digital signals can take one of two forms: periodic or non-periodic/aperiodic. A periodic signal completes a pattern within a measurable time frame, called a period, and repeats that pattern over subsequent identical periods. The completion of one full pattern is called a cycle. A non-periodic signal changes without exhibiting a pattern or cycle that repeats over time.

Analog and Digital Signals In data communications we commonly use:- – Periodic analog signals because they need less bandwidth. – Non-periodic digital signals because they can represent variation in data.

Periodic analog signals Periodic analog signals can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. A composite periodic analog signal is composed of multiple sine waves. Topics discussed in this section: – Sine Wave – Phase – Wavelength – Composite Signals – Bandwidth

Periodic analog signals: Sine Wave The sine wave is the most fundamental form of a periodic analog signal. When we visualize it as a simple oscillating curve, its change over the course of a cycle is smooth and consistent, a continuous, rolling flow.

Periodic analog signals: Sine Wave

Peak Amplitude: The peak amplitude of a signal is the absolute value of its highest intensity, proportional to the energy it carries. For electric signals, peak amplitude is normally measured in volts.

Periodic analog signals: Sine Wave

Peak Amplitude: The voltage of battery is a constant; this constant value can be considered a sine wave. The peak value of an AA battery is normally 1.5 V.

Periodic analog signals: Sine Wave Period and Frequency: Period refers to the amount of time, in seconds, a signal needs to complete 1 cycle. Frequency refers to the number of periods in 1s. Note that period and frequency are just one characteristic defined in two ways.

Periodic analog signals: Sine Wave Period and Frequency: Frequency and period are the inverse of each other. Period is formally expressed in seconds. Frequency is formally expressed in Hertz (Hz), which is cycle per second.

Periodic analog signals: Sine Wave

Periodic analog signals: Phase The term phase describes the position of the waveform relative to time 0. If we think of the wave as something that can be shifted backward or forward along the time axis, phase describes the amount of that shift. It indicates the status of the first cycle.

Periodic analog signals: Phase A phase shift of 360° corresponds to a shift of a complete period A phase shift of 180° corresponds to a shift of one-half of a period A phase shift of 90° corresponds to a shift of one-quarter of a period

Periodic analog signals: Phase Three sine waves with the same amplitude and frequency, but different phases

Periodic analog signals: Wavelength Wavelength binds the period or the frequency of a simple sine wave to the propagation speed of the medium The wavelength is the distance a simple signal can travel in one period. Wavelength =propagation speed x period = propagation speed/frequency

Periodic analog signals: Wavelength While the frequency of a signal is independent of the medium, the wavelength depends on both the frequency and the medium.

Periodic analog signals: Composite signals A single-frequency sine wave is not useful in data communications We need to send a composite signal, a signal made of many simple sine waves. According to Fourier analysis, any composite signal is a combination of simple sine waves with different frequencies, amplitudes, and phases. Jean-Baptiste Fourier was a French mathematician

Periodic analog signals: Composite signals A composite signal can be periodic or nonperiodic. A periodic composite signal can be decomposed into a series of simple sine waves with discrete frequencies, frequencies that have integer values (1, 2, 3, and so on). A non-periodic composite signal can be decomposed into a combination of an infinite number of simple sine waves with continuous frequencies, frequencies that have real values.

Periodic analog signals: Bandwidth The range of frequencies contained in a composite signal is its bandwidth. The bandwidth is normally a difference between two numbers. For example, if a composite signal contains frequencies between 1000 and 5000, its bandwidth is , or 4000.

Periodic analog signals: Bandwidth The range of frequencies contained in a composite signal is its bandwidth. The bandwidth is normally a difference between two numbers. For example, if a composite signal contains frequencies between 1000 and 5000, its bandwidth is , or 4000.

Periodic analog signals: Bandwidth