Chapter 2 Theory XI Majeed Network

8/9/2019 Chapter 2 Theory XI Majeed Network

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Describe "Information Networks" in your own words. Information Networks

Information networks are the computer networks that communicate with public telecommunicationnetworks. In information networks, two important technologies: computing and telecommunications worktogether. Computer networks were originally used for research experiments between the computerscientists, telecommunication engineers, and other researchers. These scientific and engineering

experiments have brought a huge change in our society. Information network has brought many usefulbenefits. It also has created some problems as well. The most popular information network is the Internetor World Wide Web. It exchanges information between Internet users. We can say that a network is alarge circulatory system, through which information continuously flows. This circulation or distribution of information may flow rapidly or slowly.

What is a Network? Also write the uses of network. Network

A network (or computer network) is a way to connect computers together so that they can communicatewith each other and share information.

Today, most of the organizations, research centers etc. have developed their own computer networks. They may include business, scientific or technological research, educational institutes or governmentoffices etc. World community has come closer to each other due to the computer networks. Today, youcan access information all over the world sitting on your table using the Internet.Networks are divided into the following types

Local Area Network (LAN) Wide Area Network (WAN) Metropolitan Network (MAN)

Uses of Networks

Computer network can be used for various purposes. The main uses of computer network are;

It allows the multiple users to access and share programs and data simultaneously. It allows multiple users to share peripherals devices, such as printers and hard disks. It allows to send e-mails along with files. It allows the users to communicate via teleconferencing and video conferencing.

What is local area network (LAN)? Also describe the components of LAN.

Local Area Network (LAN)

A Local Area Network (LAN) is a network system in which computers are interconnected in a limitedgeographical area, such as network of computers in college computer laboratory or network of computers in office building etc.In LAN, computers are connected together through communications media so that each computer canshare the resources of others. A LAN covers short distance, such as one office or building or a group of buildings that are closer to each other.

Components of LAN

A local area network has a standard set of components used to connect computers and to share theinformation between them. These are described below.

(i) Communication Media

Data travels from one node to another through communication media. LAN does not use telephone linefor this purpose. In low cost LANs, the nodes are connected with twisted wire pairs, but many LANs usecoaxial cable or fiber optical cable, which are both more expensive and faster. Some LANs use wirelesstransmission media in which data is communicated through the air or space, using radio signals orinfrared signals, instead of -*f cables. Wireless networks are easy to set up and configure, but they have


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slower transmission rates and limited distance between nodes.(ii) Network-Interface Card (NIC)A network interface card (NIC) is used to connect a computer to other computers in the local areanetwork. It is a circuit board that is installed inside the computer's internal expansion slots.(iii) BridgeA bridge is an electronic device that connects two similar networks and controls the data flow between

them. A bridge recognizes the messages on a network and passes on those addressed to nodes in othernetwork.(iv) GatewayA gateway is also an electronic device or system (collection of hardware and software resources) thatconnects two networks and translates information from one to the other. It enables a computer or nodeto communicate with a computer on another different network.(v) RouterRouter is a communication device that connects two or more networks lo called router and transmitsdata to the correct destination on the network. It uses the routing protocols.

Describe the Ethernet protocol used in LAN. Ethernet

Ethernet is the most commonly used protocol in LAN. It uses a high-speed network cable and bustopology, so it is relatively simple and cheaper. Since all the nodes (or computers) use the same cable tosend and receive data and two or morecomputers communicate with each other at the same time. Therefore, this type of network must follow aset of rules to communicate the computers -with each other, otherwise it may cause loss of data ormessages. Before transmitting the data, a node must find out if the cable is in use. If so, the node mustwait. When the cable is free, the node must begin transmitting immediately. This process is also knownas CSMA/CD (Carrier Sense Multiple Access with Collision Detection).If by chance, two nodes transmit data at the same time, the messages collide. When a collision occurs, aspecial message is sent out over the network to indicate that it is jammed (i.e. stopped due to anydefect). Each node stops transmitting, waits for a random period of time, and then starts transmittingprocess again. Since, the wait period for each node is random, they may be transmitting at the sametime again but it does not happen always.

Describe the "Token Ring" protocol used in LAN. Token Ring

A token is a special electronic signal. It consists of a series of bits, which functions like a ticket. Thecomputer with the token can transmit data over the network. Only one token exists per network. Acomputer network in which each node is connected to two adjacent nodes or neighbors form a closedring or loop is called ring network topology. The token ring protocol is used in this type of network. Thetoken ring is closely associated with IBM, which works on the concept of a ring network and a token. Themethod of controlling access to the shared network cable is called token passing.Only one token is available on the network. When a node on the network wishes to transmit data, it firstcaptures (or takes) the token, and then it can transmit data. When the node has sent its message, itreleases the token back to the network. Since only one token is circulating around the network, only onedevice is able to access the network at a time. Thus no collision occurs. The main disadvantage of thisscheme of data exchange is that, it has very slow data transfer rate.Describe the "ARCnet" protocol used in LAN. ARCnet

ARCnet stands for Attached Resource Computer network. It uses either twisted pair wire or coaxial cable,and the star topology with hub attached to the network.

The original ARCnet protocol was very slow, but it became popular because it was inexpensive, reliable,and easy to set up and to expand. Today, transmission rateof fast ARCnet is improved which is about 100 M bits per second. Similarly, ARCnet also includes thecapability to use optic fiber cable.

Describe the Wide Area Network (WAN). Wide Area Network (WAN)

A Wide Area Network is a network system that covers a large (or wide) ' geographical area such as


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different cities of country or different countries of the world. In WAN, usually the personal computers asremote terminals are connected to the larger computer such as mainframe or mainframe, to send andreceive data. Since the larger computers are designed to be accessed by terminals, so PC cancommunicate with larger computers only if the PC follows to be a terminal. This is made possible byusing a software on the PCs, known as terminal emulation software, which makes capable of a PC toperform a function as terminal to communicate with larger computer. The larger computer then

considers the PC or workstation as just another input/output device or terminal. The larger computer to which the terminal (or PC) is attached is called Host computer. If a PC is used asterminal, then the user can upload or download data files to and from the Host through file transfersoftware.* Download a file means to retrieve the file from Host computer and store into the user computer whichis used as terminal.* Upload a file means to send a file from user's computer and store on the Host computer.In WAN, ordinary telephone lines, microwave or satellite links etc. are used as transmission media. .

Typically, a WAN consists of two or more LANs connected together across a wide geographical area. TheInternet is the worlds largest WAN because it connects many thousands of computers and LANs aroundthe world and making it as web.

Differentiate between LAN & WAN

LAN & WANThe main difference between LAN and WAN is:

S.No

Local Area Network Wide Area Network

1 Covers small geographical area. Covers large or wide geographical area

2Normally computers are directlyconnected through physical cablealthough wires LAN are also available

No physical cable or wire is used and data issent and received transmission. Throughmicrowave system or satellite.

3 Ethernet card is used for datatransmissionModem card is used for data transmission.

4 Data transmission speed is very high Data transmission speed is slow.

5 Its installation and configurationcost is less than WANIts installation and configuration cost isgreater than LAN.

.What are the network standards? Also describe their types. Network Standards

The standards are the precise documents containing technical and physical specifications about thenetwork being designed. Normally those standards are taken into consideration and are worldwideacceptable.By following the network standards, the networks can be reliable and efficient. Normally, two types of standards are followed. These are:

De Facto Standard

De facto means "by tradition" or "by facts". These standards were developed without any formal planand came to existence because of historical developments. These standards are most commonly used bythe organizations worldwide.

De Jure Standard


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De Jure means "according to law or regulation". These standards have been properly approved by thenetworks governing body. Few of these governing bodies are:* American National Standard Institute (ANSI)* The Institute of Electrical and Electronics Engineers (IEEE)* The International Standard Organization (ISO)*& The International Telecommunications Union-Telecommunication Standards Sector (ITU-T, formally

CCITT)* The Electronic Industries Association (EIA)* Telcordia

Describe the network models.

The most popular network models are described below.

Client/Server Network

In client/server network (or arrangement), the clients are all computers or nodes on the network andserver is a central computer that controls the network. In addition to control the network, the server alsoprovides a centralized storage area for programs and data. It has hard disks that hold the shared datafile or database. Under the client/server arrangement, processing is usually done by the server, and onlythe results are sent to the clients or nodes. Sometimes, the server and the client share processing.

The client/server network has the following advantage.* It reduces the volume of data traffic on the network.* It allows faster response at each node.* It allows using less expensive computers as clients because most of the work is done by server.

Peer-to-Peer Network

In peer-to-peer arrangement, all nodes (or computers) on the network have equal status. No one hascontrol over others. Each computer stores files on its own storage devices and has its own peripheraldevices. The users can share each other's data and devices (or resources) as and when needed. Themain disadvantage of peer-to-per network is that it becomes slow under heavy use.Hybrid Network

The hybrid network has combined features of both client/server and peer-to-peer networks (orarrangements). It has a server and the users can share the data 'and software. Similarly, each node canstore its own files, programs, and has its own peripheral devices. The users can also share each other'sdata and devices. These are the main advantage of hybrid arrangement.What is meant by network topologies? Also write names of different LAN topologies.Network Topologies

In networking, the term topology is the way of connecting nodes (or devices.) on a network. In otherwords, a topology represents the shape of network. Network topology can be divided into the followingbasic types.(a) Bus topology(b) Star topology(c) Ring topology(d) Tree topology(e) Mesh topology\ Two or more of the above mentioned basic types of topologies may becombined together to form more complex shape of networks.

Describe in detail the "Bus" topology. Bus Topology

In bus network, all nodes (or devices) are connected to a common communication medium or centralsingle cable. This single cable is called bus. The devices or nodes are attached with the central cable (orbus) through interface connector. The central cable is the backbone cable, which functions as sharedcommunication medium. In bus network, when a computer or node sends a message to anothercomputer, it also attaches the- address of the destination computer or node. A node sends a broadcast


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message onto the central cable or wire that all other nodes or devices can see, but only the destinationnode actually accepts and processes the message.In LAN, Ethernet bus topology is mostly used. It is relatively easy to install and does not require muchcabling as compare to others. The bus network is best if the number of nodes (or devices) are limited, i.e. if it supports few dozen of nodes. If more than a fewdozen of nodes (or computers) are added to a bus, the performance problems are created. Similarly,

entire network fails if backbone cable fails.Figure: Bus Topology

Describe the "Ring" topology. Ring Topology

In a ring network, each node is connected to two adjacent nodes or neighbors for communicationpurpose and forms a closed ring or loop. In this way the last node connects to the first node to completethe ring. In this network, all messages travel through a ring in one direction (clockwise or anti-clockwise)from node to node. A failure in any cable or node breaks the loop. It means that entire network fails.

Token passing method is used in this topology.Figure: Ring Topology

Describe the "Star" topology. Star Topology

In a star network, each node or device is directly connected to a central connection point known as 'Hub'. The central connection may be an actual Hub or a switch. Typically, the nodes or devices are connectedto the Hub with unshielded twisted pair (UTP) Ethernet. Star topology is most commonly used in LAN.Many home networks use the star topology.

Figure: Star TopologyAdvantages

The main advantages of star topology are:* It is easy to install and to maintain.* If any node or cable fails, other nodes are not affected.

Disadvantages

The main disadvantages of star topology are:* A star network generally requires more cable as compared to bus topology. So it becomes morecostly.* This type of network depends upon the central Hub. If Hub fails, the entire network is failed.Briefly describe the "Tree" Topology. Tree Topology

In tree network, the nodes are connected to each other in such a way that forms a tree like structure. Typically to form a tree network, multiple star topologies are combined together onto a bus. In thissimplest form, only Hub devices connect directly to the tree bus. Each Hub functions as the root of a treeof devices. This type of network has combined features of bus and star topology.


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Figure: Tree Topolog)>

Describe the "Mesh" Topology. Mesh Topology

In mesh network, each node is directly connected to all node on the network. This type of networkinvolves the concept of routes. In this type of network, each node may send message to destinationthrough multiple paths. It means that each node of Mesh network has several possible paths to send (orto receive), message, but in Bus, Star, Ring and Tree topologies each node has only one path. SomeWANs like the Internet employ mesh routing where reliability is important factor.Mesh topology has the following advantages:* It has multiple links, so if one route is blocked then other routes can be used for data communication.

* Troubleshooting of this topology is easy as compared to other networks.* Its performance is not affected with heavy load of data transmission.

Figure: Mesh Topology

What is meant by workgroup computing? Workgroup Computing

In a computer network, people can work together as a group even when they are thousands of milesaway from each other. This concept of working together is called workgroups.A group of people (teams of certain projects) can share work together by means of linked computers.

The phenomenon of sharing information by various members of workgroup is called Workgroup Computing. The workgroup computing is also known as collaborative computing., A technique to share information, in which many users or researchers can work on their projects bysharing the same domain of information online, is called groupware. A groupware also permits theindividuals to collaborate (work together) with their colleagues inside the organization by means of linked computer (computer network). At the same time, they can also link to other people (on thenetworks) outside their organization.

The information may reside on different types of databases. Similarly, different operating systems, dataformats, and hardware may be used on different networks. The users can communicate with each otherwithout facing any problem. In fact, it is due to communication technology that has brought a revolutionin this type of, computing. The concept of "Global Village" has become a reality now and the '. computerusers are "chatting" with each other as if they are sitting face to face.Describe the OSI Model. Also briefly describe all layers of OSI model. OSI Model

OSI stands for Open System Interconnection. OSI model is also known as OSI Reference Model. It is usedfor networking. It was created by the International Standard Organization (ISO) to provide a logicalframework for how data communication processes should take place across networks.

There are 7 layers in the OSI model. Each layer is responsible for a particular aspect of datacommunication. For example, one layer may be responsible for establishing connections betweendevices, while another layers may be responsible for error checking during data transfer. The layers of the OSI model are divided into two groups: upper layers and lower layers.* The upper layers are related to user applications and how files are represented on the computers.* The lower layers concentrate on how the communication across a network actually occurs.


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Data Communication

What is data communication? Define the basic components of communicationnetwork.

Data Communication*Communication means the exchange of information or messages. When we talk with" each other, we areexchanging information. For corhSliunication of information or messages, we use telephones, computers,and wireless devices that are linked in a network. Data communication is the exchange of data betweentwo devices via transmission media such as a wire cable. In other words, we can say that transfer of information or data from one location to another is called Datacommunication. Generally, the term data communication can be defined as the movement of encoded information by means of electrical transmission systems from one computer or device toanother through communication channels such as cable, wireless media etc.

Components of Data Communication

There are five components in data communication network. These are defined Message: The message is

the information or data that is to be transferred from one location to another. It may consist of text,numbers, pictures, sounds, videos or any combination of these.Sender: A device that sends messages or data is called sender. It may be computer, workstation,telephone, video camera and so on.Receiver :

A device that receives messages or data is called receiver. It may be computer, workstation, telephone,television and so on.Encoder:

The encoder converts digital signals to a form, which can be transmitted through transmission medium.Decoder: , o The decoder converts signals from encoded form into digital forms that areunderstandable for receiver.

What is meant by "signals" used in data communication? Also describe the types of signalssuch as Analog and

digital.Signals

The data is transmitted from are place to another in the form of electromagnetic or light waves throughcommunication medium. The electromagnetic or light waves representing data are called signals. Datacommunication signals can be in analog or digital form.

Analog Signals


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The analog signals are continuous electrical signals in the form of wave. This wave is called a carrierwave. The analog signal has two characteristics. These are:Frequency:

The number of times a wave repeats during a specific time of interval is called frequency.Amplitude:

The height of a wave with a given period of time is called amplitude.

Digital Signals

The digital signals are on-off electrical pulses in discontinuous or discrete form. Most of the computersare digital and data is represented inside these computers in the form of binary numbers (or in the formof discrete set of values). It means that computers accept and process data in the form of digital signals.

Describe different types of data.

Types of Data

There are different types of data representation in data communication. These are text, numbers,images, audio, and video.

(i) Numeric

Numeric data consists of digits from 0 to 9, +(positive) or -(negative) signs and a decimal points .It canbe of integer type or real type data Integer data: It consists of positive or negative whole valuesincluding 0.For example,

420, 302, + 62, -26 etc.

Real data: It consists of values that have decimal point. For example, 15.4, .006, 4.07, -6.27 etc.

(ii) TextIt consists of words, sentences, paragraphs etc. For example, 'Pakistan', "I Love Pakistan", and thisparagraph written about "Text" etc. The text is stored as ASCII codes.(Hi) ImageIt consists of charts, graphs, pictures and freehand drawings. The data is sent as contiguous bits. Thesebits are packed into bytes (8-bits = 1 byte).

Describe the popular coding schemes to represent data. BCD Code

BCD

BCD stands for Binary Coded Decimal. It was 4-bits code used in early computers. It was used only torepresent and process the numeric data. Today, this coding system is not used in modern computers.

EBCDIC Code

EBCDIC stands for Extended Binary Coded Decimal Interchange Code. It is 8-bit code. In this binarycoding system 256 (2 ) different characters can be represented inside the computer. It is used by IBM(International Business Machine) computers. It provides an efficient way to communicate data betweenhosts, which use EBCDIC coding scheme for data representation. For transmission, the data isrepresented as 8-bit EBCDIC characters.

ASCII Code

ASCII stands for American Standard Code for Information Interchange. It was developed by American


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National Standards Institute (ANSI) and is a standard code to represent alphanumeric data. The firstASCII code was 7-bit code. Then ASCII 7-bit code system was extended to 8-bit code. The 7-bit codesystem can represent 128 characters while 8-bit code system can represent 256 characters. Today, ASCII8-bit code system is used in most of the computers.

Unicode Code

Unicode stands for Universal Code. It is a 16-bit code and can represent 65536 (216 = 65536) charactersor symbols. It is developed by following the ASCII coding scheme. The first 256 codes in Unicode areidentical to the 256 codes used by ASCII system.

Describe the modes of Data communication. Modes of Data Communication

The way in which data is transmitted from one place to another is called data communication mode (ordata transmission mode). When a person is giving a lecture, information is conveyed in one direction.Similarly during a conversation between two persons, spoken messages are exchanged in bothdirections. These messages may be exchanged alternatively or simultaneously. Similarly, when data istransmittedbetween two devices or equipments, three ways for transmitting data can be used.

Sometimes, communication modes are also referred to as directional modes in data transmission. Thecommunication modes are:

(i) Simplex Communication Mode

In simplex communication mode, the data communication can take place in only one direction. In thismode, a terminal can only send data and cannot receive it or it can only receive data but cannot senddata. Today, this mode of data communication is not popular, because most of the moderncommunications require two-way exchange of data. However, this mode of communication is used inbusiness field at certain point-of-sale terminals in which sales data is entered without a correspondingreply. The other examples of simplex communication modes are Radio and T.V transmissions.

(ii) Half-Duplex Communication Mode

In half-duplex communication mode, the data communication can take place in both directions, but onlyin one direction at a time. In this mode, data is sent and received alternatively. It is like a one-lane bridgewhere two-way traffic must give way in order to cross the other.

In half-duplex mode, at a time only one end transmits data while other end receives. In addition, it ispossible to perform error detection and request the sender to re-transmit information. The Internetbrowsing is an example of half duplex. When we issue a request to download a web document, then thatdocument is downloaded and displayed before we issue another request.

(iii) Full-Duplex Communication Mode


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In full-duplex communication mode, the data communication can take place in both directionssimultaneously (i.e., at same time) on the same channel. It is the fastest directional mode of communication. Example of this mode is conversation of the persons through telephone. This type of communication is similar to automobile traffic on a two-lane rc^J. The telephone communication systemis an example of full-duplex communication mode.Full-duplex communication is made possible by devices called multiplexers. This type of communication

is limited to mainframe computers because expensive hardware is required for this directional mode.

Describe the Simplex Communication Mode.

See Q.7 part (i) Q.7.2 Describe the Half-Duplex Communication Mode.See Q.7 part (ii) Q.7.3 Describe the Full-Duplex Communication Mode.See Q.7 part (iii)

Describe the types of data transmission modes. Types of Data Transmission Modes

There are two types of data transmission modes. These are: (i) Parallel Transmission (ii) Serial Transmission(i) Parallel Transmission

In parallel transmission, bits of data flow concurrently through separate communication lines. Paralleltransmission is shown in figure below. The automobile traffic on a multi-lane highway is an example of parallel transmission. Inside the computer binary data flows from one unit to another using parallelmode. If the computer uses 32-bit internal structure, all the 32-bits of data are transferredsimultaneously on 32-lane connections. Similarly, parallel transmission is commonly used to transferdata from computer to printer. The printer is connected to the parallel port of computer and parallel

cable that has many wires is used to connect the printer to computer. It is very fast data transmissionmode.

(ii) Serial Transmission

In serial data transmission, bits of data flow in sequential order through single communication line. Serialdata transmission is shown in figure below. The flow of traffic on one-lane residential street is anexample of serial data transmission mode. Serial transmission is typically slower than paralleltransmission, because data is sent sequentially in a bit-by-bit fashion. Serial mouse uses serialtransmission mode in computer.

Figure(a): Parallel Transmission

Differentiate between synchronous and Asynchronous transmission. Synchronous


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Transmission

In synchronous transmission, large volumes of information can be transmitted at a time. In this type of transmission, data is transmitted block-by-block or word-byword simultaneously. Each block may containseveral bytes of data. In synchronous transmission, a special communication device known as'synchronized clock' is required to schedule the transmission of information. This special communication

device or equipment is expensive.Asynchronous TransmissionIn asynchronous transmission, data is transmitted one byte at a 'time'. This type of transmission is mostcommonly used by microcomputers. The data is transmitted character-by-character as the user types iton a keyboard.An asynchronous line that is idle (not being used) is identified with a value 1, a1 ^ known as 'Mark' state.

This value is used by the communication devices to find whether the line is idle or disconnected. When acharacter (or byte) is about to be transmitted, a start bit is sent. A start bit has a value of 0, also called aspace state. Thus, when the line switches from a value of 1 to a value of 0, the receiver is alerted that acharacter is coming.

What is bandwidth? Also differentiate between base band and broadband?

Bandwidth

Each type of communication media or channel has different transmission speed. The data transmissionrate over communication channel is measured in bandwidth. The bandwidth will be higher if more signalsare transmitted over the communication channel. Actually, the bandwidth measures the amount of information that can be transmitted through the media with a given period of time. For analog signalsbandwidth is represented in Hertz and for digital signals, it is represented in bit per second (bps).Baseboard

Base band is a communication technique in which digital signals are directly transmitted overtransmission line without changing into analog signals (i.e. without using modulation technique). In thiscommunication technique, there is no need to use any complex modem. The digital signals arecommonly called base band signals. The token ring and Ethernet use base band signals (or digitalsignals).

Broadband

Broadband is another communication technique in which large amount of data such as voice and video,is transmitted over long distances simultaneously by modulating each signal onto a different frequency.In "broadband transmission, FDM (Frequency Division Multiplexing) technique is used, in which multiplesignals (or several streams of data) can be transmitted simultaneously.Broadband is the bandwidth used for direct communication between very high-speed computer such asmainframe computer. This bandwidth includes microwaves, satellites, fiber optics media etc.

What is communication media or channel? Communication Media

A path through which data is communicated from one place to another is r ferred to as communicationmedia or channel. The twisted pair wire, coaxial cable, fi;>er optic cable, microwave, satellite etc. areexamples of communication channels.

The communication media is divided into two types,4(i) Guided Media

In guided communication media, communication devices are directly linked with each other via cables orphysical media such as twisted pair wire, coaxial cable, or fiber optic cable for transmission of data. Thedata signals are bounded to a cabling media. Therefore, guided media is also called bounded media.


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(ii) Unguided Media

In unguided communication media, data is communicated between communication devices in the formof wave. Unguided media provides means to transmit data signals but does not guide them along aspecific path. The data signals are not bounded to a cabling media. Therefore, unguided media is alsocalled unbounded media. The atmosphere and the space are examples of unguided media through which

microwaves and satellites are used for communication.Descrioe the "Twisted pair" communication media. Twisted Pair

Twisted pair is one of the most commonly used communication media and is used in telephone lines tocarry voice and data signals. It consists of a pair of thin diameter copper wires that are covered byinsulating material such as plastic and are twisted together to form a cable. The wires are twisted aroundeach other to minimize (or reduce) interference from other twisted pairs in the cable. The bandwidth of twisted pair is less than coaxial cable or optical fiber. The twisted pair has been the standardcommunication channel for voice and data communication, but now its use is reducing because todaymore reliable communication media are available such as . coaxial cable, fiber optic cable, microwaveand satellite.

Describe the "C axial Cable" communication media. Coaxial Cable

Coaxial .able consists of a single copper wire surrounded by insulating material. Coaxial cable containsfrom four to twenty-two coaxial units called tubes. Each coaxial tube consists of 0.1 inch copper innerconductor which is kept centered within a 0.375-inch cylindrical copper outer conductor by polyethylene

insulating disks spaced about 1 inch apart. The outer conductor is formed into a cylinder around thedisks.Coaxial cable can be used for telephone lines for voice and data transmission with very high frequency.

The bandwidth of coaxial cable is 80 times greater than twisted pair media. Coaxial cable is also widelyused in local area network (LAN). Becav ^ of its strength, coaxial cables are often used for long distancetelepK~>ne lines that are carried under water. Coaxial cable creates less distortion ant has lessinterference of external electromagnetic waves. Therefore coaxial cable has low transmission error rates.

Describe the "Fiber optic cable" communication media. Fiber Optic Cable

In twisted-pair cable and coaxial cable, data is transmitted in the form of electric frequencies. The fiberoptic cable uses light to transmit data. A fiber optic cable consists of tubes of glass (or thin glass fibers)through which data is transmitted as pulses of light. A typical optical fiber consists of a very narrowstrand or fiber of glass called the core ( It is as thin as a human hair). The core is surrounded by aconcentric layer of glass called Cladding. The diameter of a core is 62.5 microns (1micron = 10 6 meters). The diameter of cladding is about 125 microns, l! . cladding is coc ted withinsulating material such as plastic, which is called the JacketAn important characteristic (or property) of fiber optic is refraction. Refraction is the characteristic of amaterial to either pass or reflect light. When light passer through a medium, it bends as it passes from


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one medium to the other. The major advantages of fiber-optical media are:* Its transmission capacity is about 26,000 times greater than twisted pair media.^r It has high level of security.* It is not affected by electromagnetic waves.* It is more reliable and has lower data transmission errors.

What is microwave data transmission media? MicrowaveIn microwave transmission, data is transmitted through air instead of through cables or wires.Microwaves are high frequency radio waves that can only be traveled in straight lines. Microwavetransmission is limited to a particular city or geographical area.

The data is transmitted and received through a microwave station. A microwave station contains anantenna, transmitter, receiver, and other equipments that are required for microwave transmission.Microwave antennas are placed on the high towers or buildings and these are placed within 20 to 30miles of each other. The data messages are relayed from one location to another using antennas. In thisway, data is transmitted over larger distances.

Microwave Relay Station

What is the function of communication satellite? Satellite

A communication satellite is a space station that receives microwave signals (or messages) from earthstations. Satellite transmission stations that can send and receive messages are known as earth stations.

The signals are transmitted frorh one earth station to the satellite. The satellite receives and amplifiesthe signals and sends them to another earth stations. In this way, data or messages are transferred fromone location to another. Satellites rotate approximately 23,300 miles above the earth in preciselocations.In the early years of satellite communication research, the moon was used to transmit data signals fromone place to another. The time taken by a data signal to reach to the moon and then back to the earthwas about 2 seconds. This time is called satellite delay and is still present today in all our satellitecommunications.

Different communication satellites are used to carry different kinds of information such as telephonecalls, television channels, Internet traffic, military communication, weather data, and even radio stationsuse them for broadcasting.

The data transmission speed of communication satellite is very fast. The major advantage of satellitetransmission is that large amount of data can be communicated at once. The satellite transmission alsohas some disadvantages. The main disadvantages are:* The bad weather can affect the quality of satellite transmission.* It has serious security problems, because it is easy to catch (or intercept) the transmission as it travelsthrough the air.


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What is Mobile communication? Mobile Communications

Mobile communication is a radio-based network that transmits data to and from the mobile computers. The data is communicated through radio signals from one location to another. The computers can beconnected to the network through wireless connections or through wires.

What is a Modem? Modem

Modem stands for modulate and demodulate. It is an electronic device that converts digital signals intoanalog signals and vice versa. Modems are used on "both ends of the computers for data communicationbetween computers through telephone line. We know that data is communicated through telephone line

in the form of analog signals. So data in the form of digital signals from one computer must be convertedinto analog form to send it over telephone line. This process to convert the digital signals into analogsignal is called Modulation. Similarly, to receive the data from another computer through telephone line,in the form of analog signals, it must be converted to digital form to store it into the computer. Thisprocess of converting the analog signals into digital form is called demodulation. Modem enables theusers to transmit data from one computer to another by using standard telephone line instead of specialcommunication lines such as fiber optic cable etc. The basic terms that measure the capabilities of amodem are:

Transmission Rate: It indicates how many bits per second a modem can transmit (or receive). Speed: Amodem converts the digital signals to analog form to transfer over the communication media. Thistechnique of encoding data from digital form to analog is called modulation. The speed of modem isdetermined by the modulation standard. The rate at which a modem converts the digital signals inanalog form (and vice versa) and transmits over communication media is referred to as modem speed. It

is measured in bits per second (bps). Today, the modems are available with speed of 9600 bps andabove.Describe different types of modems. Types of Modems

In terms of physical size and shape, modems can be divided into following categories.

(i) External Modem

External modem is an external unit of computer and is separated from system unit. It is connected to theserial port COM1 or COM2 of the computer by means of a cable. It is connected to the telephone linethrough telephone wall Jack by another cable. An external power is also supplied to it. External modem isvery easy to set up.


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(ii) Internal ModemAn internal modem is a circuit board (or modem card) that is installed into one of the expansion slotsinside the system unit of the computer. This modem cannot be moved easily from one PC to another. It isalso difficult to set up than other types of modem such as external modem.(iii) Wireless ModemsWireless modems transmit the data signals through the air instead of by using a cable. They sometimes

are called radio-frequency modems. This type of modem is designed to work with cellular technology,and wireless local area networks. Wireless modems are not yet perfected, but the technology is rapidlyimproving.

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Chapter 2 Theory XI Majeed Network