Data Communication and Networking Physical Layer and Media
Objectives We will start our discussion from the Bottom Most Layer and it is called the Physical Layer. It is the Layer that actually interacts with the Transmission Media, The physical part of the Network that connects the Network Components together. Examples are HUB and Repeater. You must Know HUB is the Device that is used for the Interconnections and Repeater repeats the signal as the signal strength gets weaker and weaker.
Objectives Continued Physical Layer has to perform complex tasks. One major task is to provide services for the Data Link Layer. One of the services that is provided by the Physical Layer is to convert the stream of 0’s and 1’s into signals so the transmission should be ready to go. If you have a physical medium data will travel in the form of signals. 0 and 1 in the form of signal will be voltage level if the medium is cable.
Data and Signals You must know the difference between Analog and Digital Signals. Periodic and Non Periodic Signals. Representation of Analog and Digital Signals. How to calculate Peak Amplitude? Example is: The power we use at home has a frequency of 60 Hz. How can you determine the period of this wave? The answer to the above example is x 10 Power 3 milliseconds.
Transmission Impairment Signal Travels through transmission media which are not perfect. This imperfection cause signal impairments, that means that the signal at the beginning of the medium is not same as the signal received at the end of the medium. It will be more worst if the medium is wireless. What information you have sent is not received and the factors can be attenuation, distortion and noise.
Diagrammatic Representation of Impairment
Attenuation It means Loss of Energy. Whenever a signal travel through medium it losses some of its energy in overcoming the resistance of the medium. That is the reason a wire carrying electric signal gets warm. To compensate this loss amplifiers are used to amplify the signal.
Decibel To demonstrate whether a signal has lost or gained strength, Engineers use the unit of decibel. The dB means the relative strengths of two signals. Or it can be one signal at two different points. Note that dB is negative if the signal is attenuated and positive if the signal is amplified. NB: Power is square of voltage.
Examples Suppose a signal travels through a transmission medium and its power is reduced to one half, how will you calculate the power loss? A signal travels through an amplifier, and its power is increased 10 times. How can you calculate the amplification? In first example the answer will be negative because it is attenuation and the second example answer will be Positive because of the amplification.
Why Decibels One of the reasons why engineers use dB to measure the changes in strength of the signal is that dB numbers can be added or subtracted, when we measure several points. Have a look in the example below and you will find how easy it is to find the attenuation or amplification of the overall system.
Distortion Distortion means if the signal changes its form or shape. If you have a composite signal made of different frequencies than it will cause distortion. Every signal has its own propagation speed through a medium and therefore it will cause its own delay while receiving at the final destination.
Noise There are so many types of Noise. It can be thermal, induced noise, cross talk, and impulse noise. Thermal noise is the random motion of electrons in a wire which creates an extra signals not created by a transmitter. Induced noise comes from sources such as motors and appliances. These devices act as a sending antenna. Cross talk is the effect of one wire over another. One wire act as a sending antenna and other as receiving. Impulse noise is a spike (A Signal with High Energy in a Very short Time) it comes from power lines, lightning etc.
Representation of Noise
Signal-to-Noise Ratio Assignment 2: You need to find out what is meant by theoretical bit rate limit and how it is calculated? This assignment has to be submitted till 19 th of January 2013 before 12 am. Address is If you fail to do so you will not have any We need to know the ratio of the signal power to noise power. The signal to noise ratio is defined as; SNR= Average Signal Power/ Average Noise Power
SNR (Continued) We need to consider the average because the signal power and noise power keeps on changing with the time. SNR is to find the actual signal power not the noisy signal power. Because SNR is the unit of two powers and we have discussed we calculate power in dB’s
Example The power of a signal is 10 million watts and the power of the noise is 1 micro watt. What are the values of SNR and SNR in dB’s?
Data Rate Limits A very important consideration in data communications is how fast we can send the data in bits per second over a channel. Data rate depends on three factors; 1. The available bandwidth 2. The level of signals we use 3. The quality of the channel (The Level of Noise) Two theoretical formulas were developed to calculate the data rate. One by ‘Nyquist’ for a noiseless channel and another by ‘Shannon’ for a noisy channel
Noise Less Channel: Nyquist Bit Rate For a noiseless channel the formula defines the theoretical maximum bit rate. Bit rate = 2 x bandwidth x Log2 L In the above formula the bandwidth is the bandwidth of the channel, L is the number of signal levels used to represent data, and the bit rate is the bit rate in bits per second. Example: Does the Nyquist theorem bit rate agree with the intuitive bit rate described in baseband transmission? Solution Chapter 3 Data and Signals
Challenging aspect Quiz No 1: You all know now the formula, if we have given with a specific bandwidth, we can have any bit rate? Justify your answers is that true or false? (2 marks) Understanding the question is part of your studies so please don’t ask me to clarify the question, its all there for you. You have 10 minutes from now, be quick! NB: You must understand all the examples of Chapter 3.
Noisy Channel Shannon Capacity Can you name any channel or think about any channel that is a noise free channel. In 1944 Shannon introduced a formula to determine the theoretical highest data rate for a noisy channel. Shannon capacity = bandwidth x log 2 (1 + SNR). You must know the difference of Log 10 and Log 2 and how it is converted. In the examination you MUST bring the calculators with you.
Explanation Can you think why in the Shannon Capacity theorem there are no signal levels. The answer is that you can never increase practically the higher data rate than the capacity of the channel. If the channel has the capacity of say 10 MB/ s how can you have fast data rate, never ever. So that clearly reflects that in reality there is no such channel that is noiseless.