Tiêu đề Unit-2-The-Physical-Layer.pdf ✅

Prepared By: Krishna Bhandari www.bcanotesnepal.com Unit 2: The Physical Layer Functions of Physical Layer: The physical layer coordinates the functions required to carry a bit stream over a physical medium. It deals with the mechanical and electrical specifications of the interface and transmission medium. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to occur. Figure below shows the position of the physical layer with respect to the transmission medium and the data link layer. Specific responsibilities of phycial layer include: • Physical characteristics of interfaces and medium • Representation of bits • Data rate • Synchronization of bits • Line configuration • Physical topology • Transmission mode Network Devices (Connecting Devices): We divide connecting devices into five different categories based on the layer in which they operate in a network, as shown in Figure below.
Prepared By: Krishna Bhandari www.bcanotesnepal.com The five categories contain devices which can be defined as: 1. Those which operate below the physical layer such as a passive hub. 2. Those which operate at the physical layer (a repeater or an active hub). 3. Those which operate at the physical and data link layers (a bridge or a two-layer switch). 4. Those which operate at the physical, data link, and network layers (a router or a three- layer switch). 5. Those which can operate at all five layers (a gateway). Data and Signals: Signal: A signal is an electrical or electromagnetic current that is used for carrying data from one device or network to another. It is the key component behind data communication and networking. Signals can be periodic and nonperiodic. A periodic signal repeats the pattern over identical periods. A nonperiodic signal changes without repeating a pattern or cycle over time. A signal can be either analog or digital. Analog Signal: Analog signal is a continuous wave that keeps on changing over a time period. In other words, an analog signal is a continuous wave denoted by a sine wave and may vary in signal strength (amplitude) or frequency (waves per unit time). Analog signals can be classified as simple or composite. A simple analog signal or sine wave cannot be further decomposed into simpler signals. A composite analog signal is composed of multiple sine waves. Digital Signal: Digital signals also carry information like analog signals but is somewhat is different from analog signals. Digital signal is noncontinuous, discrete time signal. Digital signal carries information or data in the binary
Prepared By: Krishna Bhandari www.bcanotesnepal.com form i.e. a digital signal represent information in the form of bits (0s and 1s). Digital signals are easier to transmit and are more reliable when compared to analog signals. Key Differences Between Analog and Digital Signal: • An analog signal represents a continuous wave that keeps changing over a time period. On the other hand, a digital signal represents a noncontinuous wave that carries information in a binary format and has discrete values. • An analog signal is always represented by the continuous sine wave whereas, a digital signal is represented by square waves. • While talking of analog signal, we describe the behavior of the wave in respect of amplitude, period or frequency, and phase of the wave. On the other hand, while talking of discrete signals we describe the behavior of the wave in respect of bit rate and bit interval. • The range of an analog signal is not fixed whereas the range of the digital signal is finite and which can be 0 or 1. • An analog signal is more prone to distortion in response to noise, but a digital signal has immunity in response to noise hence it rarely faces any distortion. • An analog signal transmits data in the form of wave whereas, a digital signal transmits the data in the binary form i.e. in the form of bits. • The best example of an analog signal is a human voice, and the best example of a digital signal is the transmission of data in a computer. Analog Data: The term analog data refers to information that is continuous; For example, an analog clock that has hour, minute, and second hands gives information in a continuous form; the movements of the hands are continuous. 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: Digital data refers to information that has discrete states. For example, a digital clock that reports the hours and the minutes will change suddenly from 8:05 to 8:06. Digital data takes on discrete values. For example, data are stored in computer memory in the form of 0s and 1s. They can be converted to a digital signal or modulated into an analog signal for transmission across a medium.
Prepared By: Krishna Bhandari www.bcanotesnepal.com Transmission Impairment: Signals travel through transmission media, which are not perfect. The imperfection causes signal impairment. This means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. What is sent is not what is received. Three causes of impairment are attenuation, distortion, and noise. Attenuation Attenuation means a loss of energy. When a signal, simple or composite, travels through a medium, it loses some of its energy in overcoming the resistance of the medium. That is why a wire carrying electric signals gets warm, if not hot, after a while. Some of the electrical energy in the signal is converted to heat. To compensate for this loss, amplifiers are used to amplify the signal. Attenuation is measured in terms of Decibels. The decibel (dB) measures the relative strengths of two signals or one signal at two different points. Note that the decibel is negative if a signal is attenuated and positive if a signal is amplified. dB=10log10 P2/P1 Variables P1 and P2 are the powers of a signal at points 1 and 2, respectively. Distortion: Distortion means that the signal changes its form or shape. Distortion can occur in a composite signal made of different frequencies. Each signal component has its own propagation speed through a medium