UNIT 3 - Signals, Multiplexing, and Switching.pptx

UNIT 3 - Signals, Multiplexing, and Switching.pptx

1. UNIT-3: Signals, Multiplexing, and Switching

2. Overview Slide 2

3. Learning Objectives Slide 3

4. Slide 4 Analog and Digital Data

5. Analog & Digital Signals An electromagnetic signal can be either analog or digital . A signal that passes through and includes a wide range of varying values of intensity over a period of time is referred to as analog signal . In contrast, a signal that has only a finite range of values (generally 0 and 1) is referred to as digital signal. Either of the analog or digital signals can be used to transmit either analog or digital data. Slide 5

6. Periodic & Non-Periodic Signals Both analog and digital signals can be either periodic or non-periodic. A periodic signal exhibits a specific signal pattern that repeats over time [Figures (a) and 2(b)]. Sine waves and square waves are the most common examples of periodic analog and digital signals, respectively. On the other hand, a non-periodic (or aperiodic) signal does not repeat any specific signal pattern over time [Figures (c) and (d)]. Usually, in data communications, periodic analog and non-periodic digital signals are used. Slide 6

7. Sine Wave In data communications, we commonly use periodic analog signals. They can be classified as simple or composite. A simple periodic analog signal, a sine wave, cannot be decomposed into simpler signals. The sine wave is the most fundamental form of a periodic analog signal. A sine wave can be represented by three parameters: period, peak amplitude, and phase. These three parameters fully describe a sine wave. The frequency is not an independent parameter: It is the inverse of the period. Slide 7

8. Peak Amplitude, Period, Frequency, Phase, & Wavelength Peak Amplitude: The peak amplitude of a signal is the absolute value of its highest intensity. For electrical signals, peak amplitude is normally measured in volts. Period and Frequency: The period (T) refers to the amount of time, in seconds, a signal needs to complete one cycle. The frequency (f) , measured in hertz (Hz), refers to the number of periods in 1 s. Period and frequency are inverses of each other, in other words (f = 1/ T). Phase: The term phase describes the position of the waveform relative to time 0. It indicates the status of the first cycle. Phase is measured in degrees or radians (360° is 2π rad). Wavelength: The wavelength is the distance a simple signal can travel in one period. If we represent wavelength by λ, propagation speed by c, and frequency by f, we get λ = c / f = c × T Slide 8

9. Time and Frequency Domains A signal can be represented in two ways: time-domain and frequency-domain. In time-domain representation , the signal is represented as a function of time. The time-domain plot of a signal depicts the changes in the amplitude of a signal with time. In frequency-domain representation , a signal is represented as a function of frequency. I t depicts only the peak amplitude of the signal and frequency. n addition, the complete sine wave is represented just by a spike. Slide 9

10. Composite Signals, & Bandwidth Composite Signals: So far, we have focused on simple sine waves. Simple sine waves have many applications in daily life, such as sending energy from one place to another. However, if we had only one single sine wave to convey a conversation over the phone, it would make no sense and carry no information. We would just hear a buzz. We need to send a composite signal to communicate data. A composite signal is made up of many simple sine waves. Bandwidth: The range of frequencies contained in a composite signal is its bandwidth. The bandwidth is the difference between the lowest and highest frequencies in the signal. For example, if a composite signal contains frequencies between 1000 and 5000, its bandwidth is 5000 – 1000, or 4000. The bandwidth of a composite signal is the difference between the highest and the lowest frequencies contained in that signal. Slide 10

11. Transmission of Digital Signals The fundamental question is, how can we send a digital signal from point A to point B? We can transmit a digital signal by using one of two different approaches : baseband transmission or broadband transmission. Baseband transmission means sending a digital signal over a channel without changing it to an analog signal. Broadband transmission or modulation means changing the digital signal to an analog signal for transmission.. Slide 11

12. Difference B/W Baseband, & Broadband Transmission Slide 12

13. Causes of Transmission Impairments 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: 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. To compensate for this loss, we need amplification. Distortion: means that the signal changes its form or shape. Distortion can occur in a composite signal made up of different frequencies. Each signal component has its own propagation speed through a medium and, therefore, its own delay in arriving at the destination. Differences in delay may create a difference in phase if the delay is not the same as the period duration. Slide 13

14. Causes of Transmission Impairments… Noise: is another cause of impairment. Several types of noise , such as thermal noise, induced noise, crosstalk, and impulse noise, may corrupt the signal. Thermal noise: is the random motion of electrons in a wire, which creates an extra signal not originally sent by the transmitter. Induced noise: comes from sources such as motors and appliances. These devices act as a sending antenna, and the transmission medium acts as the receiving antenna. Crosstalk: is the effect of one wire on the other. One wire acts as a sending antenna and the other as the receiving antenna. Impulse noise: is a spike (a signal with high energy and very short duration) that comes from power lines, lightning, and so on. Slide 14

15. Signal-to-Noise Ratio (SNR) To find the theoretical bit-rate limit, we need to know the ratio of the signal power to the noise power. The signal-to-noise ratio is defined as. SNR = (average signal power) / (average noise power) We need to consider the average signal power and the average noise power because these may change with time. Because SNR is the ratio of two powers, it is often described in decibel units, SNRdB, as: SNRdB = 10 log10 SNR Decibel (dB) is a measure of comparative strength of two signals or one signal at two different points. It is used by engineers to determine whether the signal has lost or gained strength. The positive value of a signal indicates gain in strength while negative value indicates that the signal is attenuated. dB = 10 log10(P2/P1), Where P 1 and P2 are the powers of signal at two different points or the powers of two different signals. Slide 15

16. Performance Parameters One important issue in networking is the performance of the network—how good is it? Bandwidth: One characteristic that measures network performance is bandwidth. Bandwidth in hertz is the range of frequencies involved. Bandwidth in bits per second can be defined as the number of bits a channel can pass. Throughput: The throughput is a measure of how fast we can actually send data through a network. The bandwidth is a potential measurement of a link; the throughput is an actual measurement of how fast we can send data. For example, we may have a link with a bandwidth of 1 Mbps, but the devices connected to the end of the link may handle only 200 kbps. This means that we cannot send more than 200 kbps through this link. Latency (Delay): The latency or delay defines how long it takes for an entire message to completely arrive at the destination from the time the first bit is sent out from the source. We say that normally there are four types of delay: propagation delay, transmission delay, queuing delay, and processing delay. Latency or Total Delay = propagation delay + transmission delay + queuing delay + processing delay Slide 16

17. Performance Parameters… The time taken by a bit to travel from the source to destination is referred to as the propagation time . It can be calculated using the following formula. Propagation time = distance/propagation speed As the propagation speed increases, propagation time decreases. The measures the time between the transmission of first bit from the sender’s end and the arrival of last transmission time bit of the message at the destination. Transmission time = message size/bandwidth Greater is the message size is, the more will be the transmission time. Whenever a message being transmitted arrives at some intermediate device, such as router, it is kept in a queue (if device is not free) maintained by the device. The device processes the queued messages one by one. The time a message spends in the queue of some intermediate or end device before being processed is referred to as the queuing time. It depends on the traffic load in the network. More the load of traffic is, the more is the queuing time. Slide 17

18. Multiplexing Multiplexing is a technique used for transmitting several signals simultaneously over a single communication link. In a multiplexed system, several devices share a communication link called common medium. Each part of the communication link being used for carrying transmission between an individual pair of input and an output line is referred to as a channel. At the sender’s end, the N input lines are combined into a single stream by a communication device, called multiplexer (MUX). At the receiver’s end, another communication device, called demultiplexer (DEMUX), completes the communication process by separating multiplexed signals from a communication link and distributing them to corresponding N output lines. Multiplexing can be used in situations where the signals to be transmitted through the transmission medium have lower bandwidth than that of the medium. This is because in such situations, it is possible to combine the several low-bandwidth signals and transmitting them simultaneously through the transmission medium of larger bandwidth. Slide 18

19. Multiplexing Types: Frequency-Division Multiplexing (FDM) is an analog technique that can be applied when the bandwidth of a link (in hertz) is greater than the combined bandwidths of the signals to be transmitted. In FDM, signals generated by each sending device modulate different carrier frequencies. These modulated signals are then combined into a single composite signal that can be transported by the link. Carrier frequencies are separated by sufficient bandwidth to accommodate the modulated signal. These bandwidth ranges can be thought of as channels through which the various signals travel. Channels can be separated by strips of unused bandwidth— guard bands to prevent signals from overlapping. In addition, carrier frequencies must not interfere with the original data frequencies. Some common applications of FDM include radio broadcasting and TV networks. Slide 19

20. Multiplexing Types: Wave-Division Multiplexing WDM is an analog multiplexing technique designed to utilize the high data rate capability of fiber-optic cables. Using WDM, we can merge many signals into one and hence, utilize the available bandwidth efficiently. Conceptually, FDM and WDM are same; that is, both combine several signals of different frequencies into one, but the major difference is that the latter involves fiber-optic cables and optical signals as well as the signals are of very high frequency. In WDM, the multiplexing and demultiplexing are done with the help of a prism, which bends the light beams by different amounts depending on their angle of incidence and wavelength. An application of WDM is the SONET network. Slide 20

21. Multiplexing Types: Time-Division Multiplexing TDM is a digital multiplexing technique that allows the high bandwidth of a link to be shared amongst several signals. Unlike FDM and WDM, in which signals operate at the same time but with different frequencies, in TDM, signals operate at the same frequency but at different times. In other words, the link is time-shared instead of sharing parts of bandwidth among several signals. TDM can be divided into two different schemes, namely, synchronous TDM and statistical TDM. Slide 21

22. Multiplexing Types: Time-Division Multiplexing Types Synchronous TDM: In this technique, data flow of each input source is divided into units where a unit can be a bit, byte or a combination of bytes. Each input unit is allotted one input time slot. A cycle of input units from each input source is grouped into a frame. Each frame is divided into time slots and one slot is dedicated for a unit from each input source. Since each slot in the frame is pre-assigned to a fixed source, the empty slots are transmitted in case one or more sources do not have any data to send. As a result, the capacity of a channel is wasted. Statistical TDM: Also called asynchronous TDM or intelligent TDM, overcomes the disadvantage of synchronous TDM by assigning the slots in a frame dynamically to input sources. A slot is assigned to an input source only when the source has some data to send. Thus, no empty slots are transmitted which in turn result in an efficient utilization of bandwidth. Slide 22

23. Switching & Its Types On a network, switching means routing traffic by setting up temporary connections between two or more network points. This is done by devices located at different locations on the network, called switches (or exchanges). Switches create temporary connections amongst two or more devices connected to them. In a switched network, some switches are directly connected to the communicating devices, while others are used for routing or forwarding information. There are three different types of switching techniques; namely, circuit switching, packet switching and message switching. Slide 23

24. Circuit Switching When a device wants to communicate with another device, circuit switching technique creates a fixed bandwidth channel, called a circuit, between the source and the destination. This circuit is a physical path that is reserved exclusively for a particular information flow, and no other flow can use it. Other circuits are isolated from each other, and thus their environment is well controlled. For example, in Figure, if device A wants to communicate with device D, sets of resources (switches I, II and III) are allocated which act as a circuit for the communication to take place. The path taken by data between its source and destination is determined by the circuit on which it is flowing and does not change during the lifetime of the connection. The circuit is terminated when the connection is closed. Therefore, this method is called circuit switching. Slide 24

25. Packet Switching Packet switching introduces the idea of breaking data into packets, which are discrete units of potentially variable length blocks of data. Apart from data, these packets also contain a header with control information such as the destination address and the priority of the message. These packets are passed by the source point to their local packet switching exchange (PSE). When the PSE receives a packet, it inspects the destination address contained in the packet. Each PSE contains a navigation directory specifying the outgoing links to be used for each network address. On receipt of each packet, the PSE examines the packet header information and then either removes the header or forwards the packet to another system. Packet switching may be classified into connectionless packet switching, also known as datagram switching, and connection-oriented packet switching, also known as virtual circuit switching. Slide 25

26. Difference B/W Virtual-Circuit Networks, & Datagram Networks Slide 26

27. Message Switching A message is a unit of information which can be of varying length. This switching technique employs two mechanisms; they are store-and-forward mechanism and broadcast mechanism . In store-and-forward mechanism (Figure), a special device (usually, a computer system with large storage capacity) in the network receives the message from a communicating device and stores it into its memory. Then, it finds a free route and sends the stored information to the intended receiver. In such kind of switching, a message is always delivered to one intermediate device where it is stored and then rerouted to its destination. In broadcast switching mechanism , the message is broadcasted over a broadcast channel as shown in Figure, as the messages pass by the broadcast channel, every station connected to the channel checks the destination address of each message. A station accepts only the message that is addressed to it. Slide 27

28. Difference B/W Circuit, Packet, & Message Switching Slide 28

29. Review Questions What is the difference between analog & digital data? Distinguish between analog and digital signals. Differentiate between periodic and non-periodic signals. What is sine wave, frequency, period, phase, wavelength? Demonstrate the difference between time and frequency domains with diagrams. State baseband and broadband transmission types. What are composite signals? What is bandwidth? Explain briefly the performance parameters. State the differences between bandwidth, throughput, and latency. Discuss the causes of transmission impairments. Elaborate types of noise. What is SNR, and Decibel? What is multiplexing? Explain with diagram. Discuss the types of multiplexing in detail with diagrams. Explain frequency, wavelength, and time-division multiplexing types with diagrams. Discuss the types of time-division multiplexing in detail with diagrams. What is switching? Explain with diagram. Describe the types of switching in detail with diagrams. Elaborate circuit switching and packet switching with diagrams. What are types of packet switching? Differentiate between datagram networks and virtual-circuit networks. Provide any five points. Distinguish between circuit, packet, and message switching with diagrams. Slide 29