Data Transmission on Physical Media PDF

Data Transmission on Physical Media CHAPTER - 2 DATA TRANSMISSION ON PHYSICAL MEDIA 2.0 INTRODUCTION The following most essential topics covered for “Data Transmission on a Physical media” are Data & Signal, Baseband and Broadband...

Data Transmission on Physical Media CHAPTER - 2 DATA TRANSMISSION ON PHYSICAL MEDIA 2.0 INTRODUCTION The following most essential topics covered for “Data Transmission on a Physical media” are Data & Signal, Baseband and Broadband transmission, Impairments, Band width ,Data rate and Baud rate Encoding Transmission Media Categories 2.1 DATA & SIGNAL Data can be analog or digital. Analog data are continuous and take continuous values. Digital data have discrete states and take discrete values. Signals can be analog or digital. Analog signals can have an infinite number of values in a range; digital signals can have only a limited number of values. In data communications, we commonly use periodic analog signals and non-periodic (a periodic) digital 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. The bandwidth of a composite signal is the difference between the Highest and the lowest frequencies contained in that signal. A digital signal can have more than two levels, we can send more than 1 bit for each level. Figure 2.1 shows two signals, one with two levels and the other with four. Fig. 2.1(a) & 2.1 (b) Encoded digital signal We send 1 bit per level in figure 2.1(a) and 2 bits per level in figure 2.1 (b). In general, if a signal has L levels, each level needs log2 L bits. E.g. If a digital has eight (8) levels, we need three (3) bits per level Number of bits per level= log2 8 = 3 IRISET 21 TA2 – Data Communication & Networking Data Transmission on Physical Media Digital signal (whether periodic or non-periodic) is a composite analog signal with frequencies between zero and infinity. Digital signal is transmitted either baseband or broadband transmission method. Baseband Transmission: In Baseband, data is sent as digital signals through the media as a single channel that uses the entire bandwidth of the media. The signal is delivered as a pulse of electricity or light depending on the type of cabling being used. Baseband communication is also bi-directional, which means that the same channel can be used to send and receive signals. In Baseband frequency-division multiplexing is not possible. In baseband transmission, the required bandwidth is proportional to the bit rate, if we need to send bits faster, we need more bandwidth. Fig. 2.2 Baseband and Broadband Transmission Broadband Transmission: In Broadband information is sent in the form of an analog signal, which flows as electromagnetic waves or optical waves. Each transmission is assigned to a portion of the bandwidth; hence multiple transmissions are possible at the same time. Broadband communication is unidirectional, so in order to send and receive, two pathways are needed. This can be accomplished either by assigning a frequency for sending and assigning a frequency for receiving along the same cable or by using two cables, one for sending and one for receiving. In broadband frequency-division multiplexing is possible. Difference between Baseband and Broadband is illustrated below in fig 2.3 Fig. 2.3, Difference between Baseband and Broadband Transmission IRISET 22 TA2 – Data Communication & Networking Data Transmission on Physical Media Transmission Impairment Signals travel through transmission media is 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. Ref fig 2.4 Fig. 2.4, Signal impairment. Attenuation: Attenuation means loss of energy. When a simple or composite signal travels through a medium, it loses some of its energy in overcoming the resistance of the medium. To compensate this loss amplifiers are used to amplify the signal. The decibel (dB) measures the relative strengths of two signals or one signal at two different points. The decibel value is negative if a signal is attenuated & positive if a signal is amplified. dB = 10 log 10 P2 / P1 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; own delay in arriving at the destination causes phase difference at the receiver. Noise: Noise causes signal impairment. Several types of noise, like thermal noise, induces noise, crosstalk noise & impulse noise corrupts the signal Signal-to-Noise Ratio (SNR): It is ratio of what is wanted (Signal) to what is not wanted (noise). A high SNR means the signal is less corrupted by noise; a low SNR means the signal is more corrupted by noise. SNR = average signal power / average noise power Because SNR is the ratio of powers, it is often described in decibel units called SNRdB SNRdB = 10 log 10 SNR Data rate: In data communications it is very important how fast we send data in bits per second (bps) over a channel. This data rate depends on 1. The bandwidth available 2. The level of the signal we use 3. The quality of the channel (the level of the noise) IRISET 23 TA2 – Data Communication & Networking Data Transmission on Physical Media Data rate is calculated, using two methods 1. Nyquist bit rate (noiseless channel) 2. Shannon capacity (noisy channel) Nyquist bit rate (noiseless channel): For a noiseless channel, the nyquist bit rate defines the theoretical maximum bit rate is Bit Rate = 2 x bandwidth x log 2 L ‘bandwidth’ is the bandwidth of the channel ‘L’ is the number of signal levels used to represent data On a given specific bandwidth we can increase the bit rate by increasing the number of signal levels. But practically there is a limit, it will burden the receiver. Hence increasing the levels of a signal may reduce the reliability of the system. Shannon capacity (noisy channel): Practically we cannot have a noiseless channel, hence as per the Shannon capacity the theoretical maximum bit rate of a noisy channel is Bit Rate = bandwidth x log 2 (1 + SNR) ‘bandwidth’ is the bandwidth of the channel ‘SNR’ is the signal-to-noise ratio Whatever may be the no. of signal levels, we cannot achieve a data rate higher than the capacity of the channel and it defines the characteristics of the channel, not the method of transmission. Bandwidth: In networking, we use the term bandwidth in two contexts. The first, bandwidth in hertz, refers to the range of frequencies in a composite signal or the range of frequencies that a channel can pass. The second, bandwidth in bits per second, refers to the speed of bit transmission in a channel or link. An increase in bandwidth in hertz means an increase in bandwidth in bits per second. This relationship depends on whether baseband transmission or broadband (modulation) transmission. Baud Rate: Baud rate refers to the signal (symbol) rate, how many signal changes are transmitted per second. One goal of the data communications is to increase the data rate while decreasing the signal rate. Increasing the data rate increases the speed of transmission; decreasing the signal rate decreases the bandwidth requirement. Bit rate is the number of bits per second. Baud rate is the number of signal elements per second. In the analog transmission of digital data, the baud rate is less than or equal to bit rate. BPS = Baud per second x the number of Bits per Baud The relationship between the data rate (bit rate) and the signal rate (baud rate) is S = N x 1 / r baud IRISET 24 TA2 – Data Communication & Networking Data Transmission on Physical Media Where ‘S’ is the baud rate, ‘N’ is the bit rate and ‘r’ is the ratio of number of data elements carried in one signal element. r = log 2 L, Where ‘L’ is the no. of signal elements. Throughput: It is a measure of how fast we can actually send data through a network; throughput is also measured as bits per second (bps) as that of bandwidth. But both are not same, throughput is always less than the bandwidth. Bandwidth is a potential measurement of a link; the throughput is an actual measurement of a link. 2.2 ENCODING Data (or) Signal encoding can be of four types:  DIGITAL-TO-ANALOG CONVERSION  ANALOG TO ANALOG CONVERSION  ANALOG-TO-DIGITAL CONVERSION  DIGITAL-TO-DIGITAL CONVERSION 2.2.1 DIGITAL-TO-ANALOG CONVERSION Digital-to-analog conversion is the process of changing one of the characteristics of an analog signal based on the information in digital data. (Refer Fig. 2.5) Fig. 2.5 Digital-to-Analog conversion There can be different types of DIGITAL TO ANALOG conversion techniques as shown below fig 2.6 Fig. 2.6 Types of Digital-to-Analog conversion techniques This sort of conversion will enable digital data to be carried over a ‘Long Distance Communication Link‘. Example: Modem is one such device that employs this conversion technique to interface a Digital source and an Analog Media. IRISET 25 TA2 – Data Communication & Networking Data Transmission on Physical Media 2.2.2 ANALOG TO ANALOG CONVERSION Analog-to-analog conversion is the representation of analog information by an analog signal. One may ask why we need to modulate an analog signal; it is already analog. Modulation is needed if the medium is band pass in nature or if only a band pass channel is available to us. There can be different types of ANALOG TO ANALOG conversion techniques as shown below fig 2.7 Fig. 2.7 Types of Analog-to-Analog conversion techniques 2.2.3 ANALOG-TO-DIGITAL CONVERSION A Digital signal is superior to an analog signal. The tendency today is to change an analog signal to digital data before transmission. One such technique is Pulse Code Modulation, is shown (Fig. 2.8), schematically, below. Fig. 2.8 Analog to Digital conversion Example: Transmission of Voice, Video, Telemetry 2.2.4 DIGITAL-TO-DIGITAL CONVERSION In this section, we see how we can represent digital data by using digital signals (Fig 2.9) The conversion involves three techniques: line coding, block coding, and scrambling. Line coding is always needed; block coding and scrambling may or may not be needed always. Fig. 2.9 Digital to Digital conversion IRISET 26 TA2 – Data Communication & Networking Data Transmission on Physical Media 2.3 Transmission Media Categories There are 2 basic categories of Transmission Media as shown in fig 2.10 a. Un-Guided Transmission Media b. Guided Transmission Media Fig. 2.10 Transmission Media Categories 2.3.1 Un-guided Transmission Media Wireless communication is the base of un - guided media. It transports electromagnetic waves without using a physical conductor. The wireless media available for transmitting network packets are 3 types a. Radio waves – used for Wireless LANs b. Microwaves – used for terrestrial and satellite communication. c. Infrared waves – used for controlling devices like remote controls. 2.3.2 Guided Transmission Media There 4 basic types of Guided Media: a. Open Wire c. Optical Fiber b. Coaxial Cable d. Twisted Pair Media versus Bandwidth Comparison of usable bandwidth between the different guided transmission media is shown in table 2.1 Cable Type Bandwidth Open Wire 0 - 5 MHz Coaxial Cable 0 - 600 MHz Optical Fiber Cable 0 - 10 GHz Twisted Pair Cable 0 - 100 MHz Table. 2.1 Comparison of bandwidth i. Open wire: This open wire is not used in Data communications. ii. Coaxial Cables: Two types of coaxial cable are used in data communications. Thick net (RG-8 and RG-11 coaxial cable) and Thin net (RG-58 coaxial cable). Thick net is a heavy- gauge coaxial cable that is fairly inflexible and requires special equipment (over and above a simple network card) to connect the computer to the network backbone. These co-axial cables are now not used much. IRISET 27 TA2 – Data Communication & Networking Data Transmission on Physical Media iii. Optical Fiber Cables: Fiber-optic cable is a high-speed alternative to copper wire and is often employed as the backbone of larger corporate networks. However, the drop in the price of fiber-optic cable has started to make it a possibility for other LAN uses. Fiber-optic cable uses glass or plastic filaments to move data and provides greater bandwidth as well as longer cable runs (up to 2 kilometers, depending on the network architecture). iv. Twisted Pair cables: LAN cables are generically called twisted pair cables. There are two (2) types. One is UTP (Unshielded Twisted Pair) and the other is STP (Shielded Twisted Pair). UTP is predominantly used for indoor areas and whereas STP for outdoor & in special areas. These UTP cables are identified with a category rating. UTP comes in two forms SOLID or STRANDED. SOLID refers to the fact that each internal conductor is made up of a single (solid) wire, STRANDED means that each conductor is made up of multiple smaller wires. The only obvious benefit of using stranded cable (which is typically more expensive) is that it has a smaller 'bend- radius' (we can squeeze the cable round tighter corners with lower loss) or where we plug and unplug the cable frequently. All other things being equal the performance of both types of cable is the same. In general solid cable is used for backbone wiring and stranded for PC to wall plug cables. IRISET 28 TA2 – Data Communication & Networking