Objective Q 1: Short questions answers

i. What are the components of data communication system? Components or Elements of a Data Communication

i. Message

ii. Sender

iii. Receiver

iv. Medium (Communication Channel)

v. Encoder & Decoder

Message: The message is the information or data that is communicated. It may consist of

text, numbers, images, sound, video etc.

Sender: The computer or device that sends the data or messages is called sender. In data

communication system, computer is usually used as a transmitter. It is also called sender.

A sender may be computer, workstation, telephone, video camera etc.

Receiver: The device that receives the data or messages is called receiver. Receiver is also

known as sink. The receiver can be a computer, workstation, printer or a fax machine.

Communication channel: The path through which data is sent or transmitted from one

location to another is called communication channel. If the receiver and the sender are

within a building, a wire may be the communication channel. If they are located at

different locations, the channel may be the telephone lines, fibre optics, satellite or

microwaves.

Encoder: The computer works with digital signals. The communication channels usually

use analog signals. Therefore, to send data through a communication channel, the digital

signals are encoded (or converted) into analog signals or into a form which can be

transmitted through transmission medium. This is called encoding. The device that carries

out this function is called encoder.

Decoder The computer works with digital signals. The communication channels usually

use analog signals. Therefore, to receive data from a communication channel, the coded

analog signals or any other encoded form are converted back to digital signals. This is

called decoding. The device that carries out this function is called decoder.

ii. What is compression? Which layer is responsible for compression? Data compression is a reduction in the number of bits needed to represent data.

Compressing data can save storage capacity, speed up file transfer, and decrease costs

for storage hardware and network bandwidth.

Presentation layer is responsible for the compression.

iii. What is reflection?

Reflection occurs when an electromagnetic signal encounters a surface that is large relative to the wavelength of the signal. For example, suppose a ground-reflected wave near the mobile unit is received. Because the ground-reflected wave has a 180° phase shift after reflection, the ground wave and the line-of-sight (LOS) wave may tend to cancel, resulting in high signal loss.

iv. What type of address is used in Network and Data-link layer? IP address is used on Network layer and MAC address is used on Data-link Layer.

v. Define digital to analog conversion?

When data from one computer is sent to another via some analog carrier, it is first converted

into analog signals. Analog signals are modified to reflect digital data. An analog signal is

characterized by its amplitude, frequency, and phase. There are three kinds of digital-to-

analog conversions:

Amplitude Shift Keying

In this conversion technique, the amplitude of analog carrier signal is modified to reflect

binary data.

When binary data represents digit 1, the amplitude is held; otherwise it is set to 0. Both

frequency and phase remain same as in the original carrier signal.

Frequency Shift Keying

In this conversion technique, the frequency of the analog carrier signal is modified to

reflect binary data.

This technique uses two frequencies, f1 and f2. One of them, for example f1, is chosen

to represent binary digit 1 and the other one is used to represent binary digit 0. Both

amplitude and phase of the carrier wave are kept intact.

Phase Shift Keying

In this conversion scheme, the phase of the original carrier signal is altered to reflect the

binary data.

When a new binary symbol is encountered, the phase of the signal is altered. Amplitude

and frequency of the original carrier signal is kept intact.

vi. How does sky propagation differ from line-of-sight propagation?

Sky propagation is not limited to send signals to receivers, line-of-sight is dependent on direction, range and objects which may occur between sender and receiver. Sky propagation is not limited in sense of distance of source and destination and not restricted by being in range or in direction with antennas. In this case, signals are sent towards space and then signals have vast range to reach receivers back to the earth. We should consider this thing, Sky is the beyond the troposphere and ionosphere. When signals gone beyond these spheres so when satellite will reflect those signals back, they will have much vast access to receivers. On other hand line-of-sight propagation is limited because of earth curvature. If antennas (source and target) are not directional, not facing each other or something preventing to establish the connection so communication won’t be made.

vii. What is the goal of multiplexing? Goals of multiplexing is to carry more channels of information using one medium. Two

main kinds are multiplexing by chopping the amount of time used and the other is which

frequency is used. The second example is used in radio of course and the second is

typically used in digital applications. viii. Difference between half duplex and full duplex?

There are three modes of transmission simplex, half duplex, and full duplex. Transmission

mode describes the direction, of flow of signal between two connected devices. The main

difference between simplex, half duplex, and full duplex is that:

Simplex mode of transmission the communication is unidirectional.

Half-duplex mode of transmission the communication is two directional but the channel

is alternately used by the both the connected device.

Full duplex mode of transmission, the communication is bi-directional, and the channel is used by both the connected device simultaneously.

ix. Define two main categories of network?

Common types of area networks are:

LAN - Local Area Network.

WAN - Wide Area Network.

WLAN - Wireless Local Area Network.

MAN - Metropolitan Area Network.

SAN - Storage Area Network, System Area Network, Server Area Network, or sometimes Small Area Network.

LAN and WAN are the two primary and best-known categories of area networks, while the others have emerged with technology advances

LAN: Local Area Network A LAN connects network devices over a relatively short distance. A networked office building, school, or home usually contains a single LAN, though sometimes one building will contain a few small LANs (perhaps one per room), and occasionally a LAN will span a group of nearby buildings. In TCP/IP networking, a LAN is often but not always implemented as a single IP subnet. In addition to operating in a limited space, LANs are also typically owned, controlled, and managed by a single person or organization. They also tend to use certain connectivity technologies, primarily Ethernet and Token Ring. WAN: Wide Area Network As the term implies, a WAN spans a large physical distance. The Internet is the largest WAN, spanning the Earth. A WAN is a geographically-dispersed collection of LANs. A network device called a router connects LANs to a WAN. In IP networking, the router maintains both a LAN address and a WAN address. A WAN differs from a LAN in several important ways. Most WANs (like the Internet) are not owned by any one organization but rather exist under collective or distributed ownership and management. WANs tend to use technology like ATM, Frame Relay and X.25 for connectivity over the longer distances.

x. Which connectors are used in Fiber-optic cable?

Fiber optic cable connectors are hardware installed on fiber cable ends to provide cable attachment to a transmitter, receiver or other cable. In order for information to be transmitted efficiently, the fiber cores must be properly aligned. They are usually devices that can be connected and disconnected repeatedly.

There are many types of fiber optic cable connectors:

ST Connectors: Slotted bayonet type connector with long ferrule. Common connector for multimode fibers.

FC Connectors: Screw on type connector. Popular with single mode fibers. SC Connectors: Push/pull connector that can also be used with duplex fiber construction.

LC Connectors: Much like the ST connector but with a ferrule that is half the size. MT-RJ Connectors: Connector configured for duplex fibers with both fibers in one ferrule. MU Connectors: Much like the SC connector but with a ferrule about half the size.

xi. What are the two types of line configuration?

Line Configuration in Computer Networks A Network is nothing but a connection made through connection links between two or more devices. Devices can be a computer, printer or any other device that is capable to send and receive data. There are two ways to connect the devices:

i. Point-to-Point connection ii. Multipoint connection

Point-To-Point Connection

It is a protocol which is used as a communication link between two devices. It is simple to establish. The most common example for Point-to-Point connection (PPP) is a computer connected by telephone line. We can connect the two devices by means of a pair of wires or using a microwave or satellite link.

Example: Point-to-Point connection between remote control and Television for changing the channels.

Multipoint Connection It is also called Multi-drop configuration. In this connection two or more devices share a single link. There are two kinds of Multipoint Connections:

If the links are used simultaneously between many devices, then it is spatially shared line configuration.

If user takes turns while using the link, then it is time shared (temporal) line configuration.

xii. Define infrared waves? Give an example?

Infrared radiation (IR), sometimes referred to simply as infrared, is a region of the electromagnetic radiation spectrum where wavelengths range from about 700 nanometers (nm) to 1 millimeter (mm). Infrared waves are longer than those of visible light, but shorter than those of radio waves. Correspondingly, the frequencies of IR are higher than those of microwaves, but lower than those of visible light, ranging from about 300 GHz to 400 THz. Some examples of radiating infrared waves are burning charcoal, heat from an electric heater, fire or a radiator emitting warmth. Infrared radiation is a type of electromagnetic radiation that objects emit when they are hot but not quite hot enough to emit visible light.

xiii. What is handoff? In cellular telecommunications, the terms handover or handoff refer to the process of

transferring an ongoing call or data session from one channel connected to the

core network to another channel. xiv. What is the difference between switch and router?

A switch is used to segment a LAN into separate networks. If the traffic on you LAN were flooding the network with data then you would install a switch to segment the traffic. Routers are used to connect a WAN or WANS to your LAN or connect dissimilar networks together. Routers can be used to filter and isolate traffic or segment network traffic like switches. The differences of a router and switch can be confusing. A router can search among multiple active paths and determine which the best path is at that particular moment. In contrast, a switch can recognize only one path between networks.

Routers

Switch

xv. Why are protocol is needed?

Network protocols are sets of rules for exchanging information. This exchange usually

occurs much like a dialog between two computers. The exchange often begins with the

client sending a signal to the server, providing key information about what kind of data is

being requested.

Without a set of rules, computers would not have the capability of "talking" to each other

across the Internet. Certain protocols help computers identify themselves on the Internet. xvi. How does single bit error differ from burst error?

When data is being transmitted from one machine to another, it may be possible that data become corrupted on its, way. Some of the bits may be altered, damaged or lost during transmission. Such a condition is known as error.

The error may occur because of noise on line, attenuation and delay distortion. For reliable communication, it is important that errors are detected and corrected.

Type of Errors There are two main types of errors in transmissions:

1. Single bit error

2. Burst error

Single bit error: It means only one bit of data unit is changed from 1 to 0 or from 0 to 1 as shown in fig.

Single bit error can happen in parallel transmission where all the data bits are transmitted using separate wires. Single bit errors are the least likely type of error in serial transmission.

Burst Error: It means two or more bits in data unit are changed from 1 to 0 from 0 to 1 as shown in fig.

In burst error, it is not necessary that only consecutive bits are changed. The length of burst error is measured from first changed bit to last changed bit. As shown in fig. length of burst error is 8, although some bits are unchanged in between.

Burst error is most likely to occur in a serial transmission. The noise occurring for a longer duration affects multiple bits. The number of bits affected depends on the data rate & duration of noise. For e.g. if data rate is 1 kbps, a noise of 1/100 second can affect 10 bits.

Subjective Q 2. Explain OSI model with functionality of each layer.

OSI (Open Systems Interconnection) is a reference model for how applications

communicate over a network. A reference model is a conceptual framework for

understanding relationships. The purpose of the OSI reference model is to guide vendors

and developers so the digital communication products and software programs they

create can interoperate, and to facilitate a clear framework that describes the functions

of a networking or telecommunication system.

Most vendors involved in telecommunications make an attempt to describe their products and services in relation to the OSI model. And although it is useful for guiding discussion and evaluation, OSI is rarely actually implemented as-is. That's because few network products or standard tools keep related functions together in well-defined layers, as is the case in the OSI model. The TCP/IP protocol suite, which defines the internet, does not map cleanly to the OSI model.

OSI model layers The main concept of OSI is that the process of communication between two endpoints in a network can be divided into seven distinct groups of related functions, or layers. Each communicating user or program is on a device that can provide those seven layers of function. In this architecture, each layer serves the layer above it and, in turn, is served by the layer below it. So, in a given message between users, there will be a flow of data down through the layers in the source computer, across the network, and then up through the layers in the receiving computer.

The seven layers of function are provided by a combination of applications, operating systems, network card device drivers and networking hardware that enable a system to transmit a signal over a network Ethernet or fiber optic cable or through Wi-Fi or other wireless protocols.

The seven Open Systems Interconnection layers are:

Layer 7: The application layer. This is the layer at which communication partners are

identified. Is there someone to talk to? Network capacity is assessed, Will the network let me talk to them right now? And where the data or application is presented in a visual form the user can understand. This layer is not the application itself, it is the set of services an application should be able to make use of directly, although some applications may perform application-layer functions.

Layer 6: The presentation layer. This layer is usually part of an operating system

(OS) and converts incoming and outgoing data from one presentation format to another. For example, from clear text to encrypted text at one end and back to clear text at the other.

Layer 5: The session layer. This layer sets up, coordinates and terminates

conversations. Its services include authentication and reconnection after an interruption. On the internet, Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) provide these services for most applications.

Layer 4: The transport layer. This layer manages packetization of data, then the

delivery of the packets, including checking for errors in the data once it arrives. On the internet, TCP and UDP provide these services for most applications as well.

Layer 3: The network layer. This layer handles addressing and routing the data --

sending it in the right direction to the right destination on outgoing transmissions and receiving incoming transmissions at the packet level. IP is the network layer for the internet.

Layer 2: The data-link layer. This layer sets up links across the physical network,

putting packets into network frames. This layer has two sub-layers: the logical link control layer and the media access control layer (MAC). MAC layer types include Ethernet and 802.11 wireless specifications.

Layer 1: The physical layer. This layer conveys the bit stream across the network

either electrically, mechanically or through radio waves. The physical layer covers a variety of devices and mediums, among them cabling, connectors, receivers, transceivers and repeaters.

Q 3. Discussed the steps involved in a typical call originated from a mobile user to a fixed subscriber.

Figure shows the principal elements of a cellular system. In the approximate enter of each cell is a base station (BS). The BS includes an antenna, a controller, and a number of transceivers, for communicating on the channels assigned to that cell. The controller is used to handle the call process between the mobile unit and the rest of the network. At any time, a number of mobile user units may be active and moving about within a cell, communicating with the BS. Each BS is connected to a mobile telecommunications switching office (MTSO), with one MTSO serving multiple BSs. Typically, the link between an MTSO and a BS is by a wire line, although a wireless link is also possible. The MTSO connects calls between mobile units. The MTSO is also connected to the public telephone or telecommunications network and can make a connection between a fixed subscriber to the public network and a mobile subscriber to the cellular network. The MTSO assigns the voice channel to each call, performs handoffs, and monitors the call for billing information.

The use of a cellular system is fully automated and requires no action on the part of the user other than placing or answering a call. Two types of channels are available between the mobile unit and the base station (BS): control channels and traffic channels. Control channels are used to exchange information having to do with setting up and maintaining calls and with establishing a relationship between a mobile unit and the nearest BS. Traffic channels carry a voice or data connection between users. Figure 14.6 illustrates the steps in a typical call between two mobile users within an area controlled by a single MTSO.

Mobile unit initialization: When the mobile unit is turned on, it scans and selects the strongest setup control channel used for this system (Fig. a). Cells with different frequency bands repetitively broadcast on different setup channels. The receiver selects the strongest setup channel and monitors that channel. The effect of this procedure is that the mobile unit has automatically selected the BS antenna of the cell within which it will operate. Then a handshake takes place between the mobile unit and the MTSO controlling this cell, through the BS in this cell. The handshake is used to identify the user and register its location. As long as the mobile unit is on, this scanning procedure is repeated periodically to account for the motion of the unit. If the unit enters new cell, then a new BS is selected. In addition, the mobile unit is monitoring for pages, discussed subsequently. Mobile-originated call: A mobile unit originates a call by sending the number of the called unit on the preselected setup channel (Fig. b).The receiver at the mobile unit first checks that the setup channel is idle by examining information in the forward (from the BS) channel. When an idle is detected, the mobile may transmit on the corresponding reverse (to BS) channel. The BS sends the request to the MTSO.

Paging: The MTSO then attempts to complete the connection to the called unit. The MTSO sends a paging message to certain BSs depending on the called mobile number (Fig. C). Each BS transmits the paging signal on its own assigned setup channel. Call accepted: The called mobile unit recognizes its number on the setup channel being monitored and responds to that BS, which sends the response to the MTSO. The MTSO sets up a circuit between the calling and called BSs. At the same time, the MTSO selects an available traffic channel within each BS’s cell and notifies each BS, which in turn notifies its mobile unit (Fig D).The two mobile unit’s tune to their respective assigned channels.

Ongoing call: While the connection is maintained, the two mobile units exchange voice or data

signals, going through their respective BSs and the MTSO (Figure 14.6e).

Handoff: If a mobile unit moves out of range of one cell and into the range of another during a

connection, the traffic channel has to change to one assigned to the BS in the new cell (Figure 14.6f).The system makes this change without either interrupting the call or alerting the user. Other functions performed by the system but not illustrated in Figure 14.6 include the following:

Call blocking: During the mobile-initiated call stage, if all the traffic channels assigned to the

nearest BS are busy, then the mobile unit makes a preconfigured number of repeated attempts. After a certain number of failed tries, a busy tone is returned to the user.

Call termination: When one of the two users hangs up, the MTSO is informed and the traffic

channels at the two BSs are released.

Call drop: During a connection, because of interference or weak signal spots in certain areas, if

the BS cannot maintain the minimum required signal strength for a certain period of time, the traffic channel to the user is dropped and the MTSO is informed.

Calls to/from fixed and remote mobile subscriber: The MTSO connects to the public

switched telephone network. Thus, the MTSO can set up a connection between a mobile user in its area and a fixed subscriber via the telephone network. Further, the MTSO can connect to a remote MTSO via the telephone network or via dedicated lines and set up a connection between a mobile user in its area and a remote mobile user.

Q 4. What are the advantages and disadvantages of Fiber optic cable? Advantages of Fiber Optic Transmission

Optical fibers have largely replaced copper wire communications in core networks in the developed world, because of its advantages over electrical transmission. Here are the main advantages of fiber optic transmission. Extremely High Bandwidth: No other cable-based data transmission medium offers the bandwidth that fiber does. The volume of data that fiber optic cables transmit per unit time is far great than copper cables. Longer Distance: in fiber optic transmission, optical cables are capable of providing low power loss, which enables signals can be transmitted to a longer distance than copper cables. Resistance to Electromagnetic Interference: in practical cable deployment, it’s inevitable to meet environments like power substations, heating, ventilating and other industrial sources of interference. However, fiber has a very low rate of bit error (10 EXP-13), as a result of fiber being so resistant to electromagnetic interference. Fiber optic transmission is virtually noise free. Low Security Risk: the growth of the fiber optic communication market is mainly driven by increasing awareness about data security concerns and use of the alternative raw material. Data or signals are transmitted via light in fiber optic transmission. Therefore there is no way to detect the data being transmitted by "listening in" to the electromagnetic energy "leaking" through the cable, which ensures the absolute security of information. Small Size: fiber optic cable has a very small diameter. For instance, the cable diameter of a single OM3 multimode fiber is about 2mm, which is smaller than that of coaxial copper cable. Small size saves more space in fiber optic transmission.

core diameter of fiber optic cable

Light Weight: fiber optic cables are made of glass or plastic, and they are thinner than copper cables. These make them lighter and easy to install.

Easy to Accommodate Increasing Bandwidth: with the use of fiber optic cable, new equipment can be added to existing cable infrastructure. Because optical cable can provide vastly expanded capacity over the originally laid cable. And WDM (wavelength division multiplexing) technology, including CWDM and DWDM, enables fiber cables the ability to accommodate more bandwidth.

Disadvantages of Fiber Optic Transmission Though fiber optic transmission brings lots of convenience, its disadvantages also cannot be ignored. Fragility: usually optical fiber cables are made of glass, which lends to they are more fragile than electrical wires. In addition, glass can be affected by various chemicals including hydrogen gas (a

problem in underwater cables), making them need more cares when deployed underground. Difficult to install: it’s not easy to splice fiber optic cable. And if you bend them too much, they will break. And fiber cable is highly susceptible to becoming cut or damaged during installation or construction activities. All these make it difficult to install. Attenuation & Dispersion: as transmission distance getting longer, light will be attenuated and dispersed, which requires extra optical components like EDFA to be added. Cost Is Higher Than Copper Cable: despite the fact that fiber optic installation costs are dropping by as much as 60% a year, installing fiber optic cabling is still relatively higher than copper cables. Because copper cable installation does not need extra care like fiber cables? However, optical fiber is still moving into the local loop, and through technologies such as FTTx (fiber to the home, premises, etc.) and PONs (passive optical networks), enabling subscriber and end user broadband access.

Q 5. Discuss the radio waves and microwaves.

Radio Waves

Radio waves are the lowest-energy, lowest-frequency and longest-wavelength electromagnetic waves. They are produced when an alternating current flows in an aerial and they spread out and travel through the atmosphere. They are not strongly absorbed by the atmosphere. Another aerial is used as a detector and the waves produce an alternating current in it, with a frequency that matches that of the radio waves. Anyone with a receiver can tune it to this frequency to pick up the radio waves so they are suitable for broadcasting (for example, radio and TV programmers) to large numbers of people. An advantage is that this method of communicating does not require wires to transmit information. A disadvantage is that radio stations using similar transmission frequencies sometimes interfere. A common mistake is to think that we can hear radio waves. We cannot hear any electromagnetic radiation. The radiation is used to carry a signal that is converted into a sound wave by the receiver. Medium wavelength radio waves are reflected from the ionosphere, a layer of charged particles in the upper atmosphere, so they can be used for long distance communication. Digital radio has better-quality reception as it uses digital signals and so does not have problems of noise and interference. Microwaves

Microwaves are sometimes considered to be very short radio waves (highfrequency and high-energy radio waves). Some important properties of microwaves are:

i. They are reflected by metal surfaces. ii. They heat materials if they can make atoms or molecules in the material vibrate. The

amount of heating depends on the intensity of the microwave radiation, and the time that the material is exposed to the radiation.

iii. They pass through glass and plastics. iv. They pass through the atmosphere. v. They pass through the ionosphere without being reflected.

vi. They are absorbed by water molecules, how well depends on the frequency (energy) of the microwaves.

vii. Transmission is affected by wave effects such as reflection, refraction, diffraction and interference.

Microwaves and water molecules

A microwave frequency (energy) can be selected which is strongly absorbed by water molecules, causing them to vibrate, and increasing their kinetic energy. This effect can be used to heat materials containing water, for example food. If the most strongly absorbed frequency (energy) is used in a microwave oven it only cooks the outside of the food because it is all absorbed before it penetrates the food. So the frequency (energy) used in a microwave oven is changed slightly to one that will penetrate about 1cm into the food.

Conduction and convection processes then spread the heat through the food.mAs our bodies contain water molecules in our cells, microwave oven radiation will heat up our cells and is very dangerous at high intensity because it will burn body tissue. The radiation is kept inside the oven by the reflecting metal case and metal grid in the door.

Q 6. Discuss the techniques used in serial transmission?

Serial transmission This data transmission method consists in sending succeeding bits through a single bus, between an emitter and a receiver. These bits thus reach the receiver one after the other, which can take time depending on the quantity of data to be sent. In order to know when the transmission starts and when it ends, the sent data is organized in threads. A thread is made of a header, of data to transmit, and of a trailer (indicating the end of the transmission). There are two serial transmission methods for these threads: Synchronous transmission The emitter and the receiver are adjusted on the same clock, thanks to a synchronization signal (a signal indicating the moment where the receiver can read data), for the reading frequency to match the sending frequency. Without this, the receiver is likely to read only

one out of two bits, for instance, if its reading frequency is twice as slow as the receiving frequency. For that, there are two solutions: either the signal is regular, in which case the receiver will synchronize its internal clock to the emitter’s frequency, or it is not. In the later case, the synchronization signal is sent through a second bus, whose goal is thus to synchronize transmissions, placed in parallel of the bus transmitting the data. In both cases, data can be sent in an uninterrupted flow, since the receiver adjusts itself depending on the synchronization signals. Asynchronous position With the asynchronous method, however, there has to be an inactivity interval in-between two data transmissions. The data sending frequency does not count in this case. Indeed, within the general thread, the transmission of one single character is launched by a “starting” signal (or bit). Once the 8 bits of the character are transmitted, there is an “ending” bit indicating the end of the character transmission. The problem with this method is that 20% of the bandwidth is used by these framing bits. It is mainly used for small threads with a moderate pace. This data division work, as well as the insertion of bits between a “starting” bit and an “ending” bit is made by the UART (Universal Asynchronous Receiver/Transmitter). To be able to send a character made of 8 bits, the emitter’s UART thus puts these between the framing bits. For each character, it thus has a shift register containing the character:

The UART starts by sending the starting bit, then shifts its register to the left.

Data integrity In these two cases, one also has to check that all of the data has been transmitted. There are two ways to do this. The first one is to have a cyclic redundancy check: it is a software tool using the hashing method and enabling to spot transmission errors. The second one is to include, for each character, a parity bit. A parity bit takes the value 0 if the total of the other bits is an even number, and the value 1 if it is odd. Main use Serial transmission is mainly used for long distance (telecom, audiovisual media…). It can be found in the following protocols: optic fiber, USB, Ethernet, Fiber Channel, Serial Attached SCSI, PCI, SATA…

Q 7. Discuss transmission impairment in details?

Data Communication | Transmission Impairment In communication system, analog signals travel through transmission media, which tends to deteriorate the quality of analog signal. This imperfection causes signal impairment. This means that received signal is not same as the signal that was send.

Causes of impairment –

Attenuation – It means loss of energy. The strength of signal decreases with increasing

distance which causes loss of energy in overcoming resistance of medium. This is also known as attenuated signal. Amplifiers are used to amplify the attenuated signal which gives the original signal back.

Attenuation is measured in decibels(dB). It measures the relative strengths of two signals or one signal at two different point. Attenuation (dB) = 10log10(P2/P1)

P1 is power at sending end and P2 is power at receiving end.

Distortion – It means change in the shape of signal. This is generally seen in composite signals with different frequencies. Each frequency component has its own propagation speed travelling through a medium. Every component arrive at different time which leads to delay distortion. Therefore, they have different phases at receiver end from what they had at senders end.

Noise – The random or unwanted signal that mixes up with the original signal is called noise. There are several types of noise such as induced noise, crosstalk noise, thermal noise and impulse noise which may corrupt the signal. Induced noise comes from sources such as motors and appliances. These devices act as sending antenna and transmission medium act as receiving antenna. Thermal noise is movement of electrons in wire which creates an extra signal. Crosstalk noise is when one wire affects the other wire. Impulse noise is a signal with high energy that comes from lightning or power lines

SNR = AVG SIGNAL POWER / AVG NOISE POWER