2.1 Basic Elements of Communication System

The history of communication started from telegraphy in early 1840s. After that, radio communication made easier during the time period of wars. After all, the use of copper cable, optical fiber and satellite has made communication faster, reliable and secured. Various forms of data such as text, number, audio, video etc. can be transmitted from one place to another place. Telephone, Television, Radio and Internet are very popular and widely used as means of communication.

Communication and networking are changing the way of our life, businesses and services, Communication refers to the exchange of information between sender and receiver with the help of electronic means of communication. The electronic means of communication may be wires, radio or satellite. The communication can be local or remote. The local communication occurs face to face whereas the remote communication occurs over distance.

Communication is defined as the exchange of electronic form of information between sender and receiver devices with the help of means of communication

Figure 2.1: Components of Commamication System

A communication system has five major basic components/elements: message, sender, receiver, transmission medium and protocol as shown in the figure below:

Message: The message is data or information to be communicated and it is in the form of text, number, picture, audio and video.
Sender: The sender device sends the messages to the receiver and it can be computer, workstation or any networking devices.
Receiver: The receiver device receives the messages from the medium and it can be computer, workstation or any networking devices.
Transmission Medium: The transmission medium is physical connection in which a message can travel from sender to receiver and vice versa. The medium can be wired or wireless.
Protocols: A protocol is a set of common rules that manages the data and communication. Without protocol two devices can be connected but not be communicated. It represents an aggreeement among the different communication devices in a network.

Computer Network

Computer network is defined as the interconnection of two or more computers or networking devices with the help of transmission media and set of protocols. The transmission media help us to transfer information from one computer to another computer and set of protocols that defines the common rules for data transmission among the various types of network devices.

Figure 2.2 : Computer Network

Purpose of Networking (Advantages)

Sharing Resources : Hardware resources such as processor, storage devices, printers, scanner etc can be shared among us using computer network. It helps to minimize the operational cost an organization. Similarly software can be shared among us within a network. It is possible to share data or information among multiple users at the same time.
Faster and Cheaper Communication : Communication in modern days has become very faster and cheaper to send information to a long distance through network. We can send letters, attach files, perform voice chat and video conference with almost free of cost with the help of network.
Centralized Control : All network resources such as computers, printers , file , database etc can be managed and controlled by a central connecting computer also known as server. So, any rules, privileges, security measures that are necessary in the network can be automatically implemented to the computers and other devices from the server. Therefore it is easy and convenient to manage and troubleshoot all the network resources through server rather than individual one.
Backpup and Recovery: Server is used to keep data as backup. It maintains backup of all individuals computers information. So, if any data or files are lsot in any computer , it is possible to restore them from the server.
Remote and Mobile Access: A remote user can access resource from the distance using computer network. It is also possible to access one computer resources from any other computers that are connected to the network.

Disadvanatges of Networking

Expensive : In order to install computer network, we require some extra cost to purchase networking devices such as hubs, switch, router, cables etc. Networking equipments and networking software are expensive too.
Security Problem : Network security is the most challenging job for network adminstrator in order to protect network resources from unauthorized users and physical destructions. Similarly, the hackers and viruses are the great threat to the computers in the networks.
Needs Technical Person : It is very difficult to install and operate good computer network. In order to manage computer network, the organization needs network specialist. So it is very expensive for an organization to hire network specialist.

2.2 Concepts of Communication System

Data communication is the transfer of data or inforamtion between a source and a receiver. The source transmits the data and the receivers it. The actual generation of the information is not part of Data communication nor is the resulting action of the information at the receiver. A data communication network is a collection of two or more computing devices that are interconnected to enable them to share data. Data communication networks deal with the transfer of data between two points. Data originates at the source and is finally delivered to the destination, which is also called a sink.

2.3 Block Diagram of Communication System/Model

The communication system consists of information transmission from one point to another. The original information signal is passes through various stages and many changes in its orientation and shape because of noise and attenuation. To provide safe and faster communication there are various stages with various elements which is described in block diagram. A communication model is necessary to enable us to introduce the main elements of a communication system as well as to define some of the terminology that are used in communication system. The block diagram of communication system is explained in figure 2.3 .

Communication system is the system of communicating from one point to another point. The elements of data communication are source, source encoder, transmitter, channel, receiver, source docoder and sink

Figure 2.3: Block Diagram of Commamication System

The source generates the information which comes from any source. Then source encoder translates source information into electrical form. A transmitter transfers encoded form of information into channel. Channel is the physical medium that connects the transmitter to the receiver. The channels are transmission medias that transform data into receiver. The receiver receives the incoming information and transfers into a source decoder. Finally, source decoder converts the electrical signal back to a form acceptable to the receiver and provide to the users.

2.4 Elements of Data Communication/Transmission

The elements of communication System are explained below:

Source : A source generates the information. The communication system is to pass information from one point to the other. The information comes from the information source where it is originated. The Information is in different form for communication, news, feeling, visual scene, and so on.
Source Encoder: A source encoder is a translator that converts the information into an electrical form called message signal. This encoded message then transferred to the transmitter.
Transmitter : It is a physical medium which connects the transmitter block with the receiver block. The use of transmitter is to collect the incoming message signal and modify it in a suitable form such that it can be transmitted through a channel to the receiving point. It is used to convert the message signal into a form acceptable to the channel.
Channel : The channel is the path or link that connects the transmitter and the receiver. It is the physical medium that connects the transmitter to the receiver. The physical mediums are copper wire, coaxial cable, fiber optic cable and so on. The type of channel depends on the purpose of the communication system.
Receiver : A receiver performs an inverse function of that of the transmitter to recover the message signal. The receiver receives the incoming modified version of the message signal from the channel. Then processes it to recreate the original form of the message signal. There are many receivers in communication system that depends on the processing required to recreate the original message signal.
Source Decoder : A source decoder converts the electrical signal back to a form acceptable to the receiver. This decoded message then transferred to the user.
Sink : A sink is the user of the information generated by the source. It is a final block in the communication system which receives the message signal.

2.5 Simplex, Half Duplex and Full Duplex Communication Mode

Data communication can be classified into three types: simplex, half-duplex and full- duplex on the basis of direction of communication flow.

Direction of communication refers to how data is transmitted between sender and receiver. It is also known as mode of transmission.

Simplex The flow of data signal in simplex mode of communication is unidirectional. Only one of two devices on a link can transmit data and the other can receive. It is just like one way street. Example: communication between keyboard and computer, radio and TV broadcasting etc. are simplex mode of communication.

Half Duplex The flow of data signal in half duplex mode of communication is bidirectional. But, both devices cannot receive and transmit data at the same time. When one device is sending, the other can only receive and vice versa. Example: communication between walkie-talkie.

Figure 2.4.: Direction of communication flow

Full Duplex The flow of data signal in full duplex mode of communication is bidirectional. Both device can receive and transmit data at the same time. It is just like two way street with traffic flowing in both direction at the same time. Example: communication in telephone, cell phones etc.

2.6 Concept of LAN, MAN and WAN

On the basis of size or geographical area that means how much area it covers, computer networks are classified into three categories: local area network (LAN), metropolitan area network ( MAN) and wide area network (WAN). The LAN and WAN are defined below.

LAN is the smallest size network that covers small areas whereas ; WAN is largest network that covers large area.

Local Area Network

A local area network (LAN) is privately owned small size network. It spans only in small geographical area such as within a room, office, buildings or up to few kilometers (2KM or 3KM). It connects the network resources such as computers, faxes, printers, and various networking devices. Moreover, user also can share the resources such as data, file, software, etc in the network. Common example of LAN is network in our college or office.

LAN is simpler, cheaper and highly secured network. Generally, LAN uses only one type of transmission medium such as twisted pair cable/coaxial cables/optical fiber cable etc. Two common types of LAN protocols are Ethernet and Token Passing. The most common topologies for LAN are bus, ring and star. It has high speed data transfer rate 100Mbps or 1000Mbps (Megabits Per Second) and low amount of error rate 1000 times lower than WAN.

Features of LAN

LAN is small privately owned network that covers only few kilometers (up to 3KM).
It has higher speed data transfer rate maximum up to 1000Mbps i.e. 1Gbps.
It is highly secured network and it has least error rate than others.
It is least expensive type of network because it requires least expensive devices such as NIC, Hub, Bridge, Modem and Switches, etc.
It uses same type of transmission media such as twisted pair cable, coaxial cable, or wireless.
The most common type of LAN protocols are Ethernet or Token passing.
Common LAN topologies are bus, ring, star, tree, mesh, and hybrid.

Metropolitan Area Network

A metropolitan area network (MAN) can be either public or privately owned network. Its size is bigger than LAN and smaller than WAN. It spans within one metropolitan city or larger geographical area. It can connect large number of computers and heterogeneous multiple LANS within a city maximum up to 100KM. Branch offices are connected to head office through MANs. Common examples of MAN are Cable TV network, Internet Service Provider (ISP) in a city etc.

Figure 2.6: Metropolitan Area network

MAN may use wired or wireless transmission media such as coaxial cable, optical fiber or microware. It is more expensive, more complex and less secured network than LAN. It uses different set of protocols uch as Frame Relay, ATM, X.25, etc. The most common topology for MAN is Mesh or hybrid. It has lower data transfer rate 10Mbps (Megabits Per Second) than LAN and low amount of error rate 100 times lower than WAN.

Features of MAN

MAN is complex and heterogeneous type of network and it may be either public of privately own.
The size of MAN is bigger than LAN and it covers maximum up to 100KM.
It is less secured network and higher error rate than LAN.
It is more expensive than LAN because it requires repeater, router, gateway, a microwave station etc.
It uses different types of media such as coaxial cable, optical fiber, microwave etc.
The most common MAN protocols are ATM, Frame Relay, X.25.
It uses mesh and hybrid topologies.
It can connect 100s of LANS and 1000s of individual computers.

Wide Area Network

A wide area network (WAN) is basically public type heterogeneous network. It is the largest sized network and connects millions of computers, thousands of LANS, hundreds of MANS around the countries, continents and even the whole world. Example of WAN is Internet where we can share information around the world. Communication is done through public channels such as telephone line, optical fiber and microwave, satellite and VSAT (Very Small Aperture Terminal). It is the most expensive and most complex type of network that transfers text, graphics, audio, video, etc around the world. It uses hundreds of different protocols such as TCP/IP, ATM, Frame Relay, X.25 etc. It is capable to connect different types of computers, transmission media, topologies, etc. It has least speed data transfer rate 64Kbps (Kilobits Per Second) to 10Mbps or higher and very high amount of error rate 1000 times higher than that in LAN.

Figure 2.6 Wide area network

Features of WAN

WAN is basically public network that covers millions of kilometers around the world. It has low speed data transfer 64Kbps to 10Mbps or more.
It is less secured network and it has highest error rate.
It is the most expensive type of network because it requires large amount of expensive devices such as router, gateway, microwave station, satellite, etc.
It uses various types of transmission media such as telephone line, optical fiber, radio wave, microwave, satellite , etc.
It uses hundreds of different protocols such as TCP/IP, FIP, HTTP, ATM, Frame Relay, X.25 etc.
It uses mesh and hybrid topologies.
It can connect unlimited LANS and MANS.

2.7 Transmission Media

Transmission media refer to the physical connection through which data are transmitted between sender and receiver devices. Like human can communicate through telephone wires or sound waves in the air, computers can communicate through cables, light, radio wave and microwaves. Transminsion media are classified into two categories: bounded (guided or wired) and unbounded (unguided or wireless).

Bounded Media

Bounded transmission media use "cabling" system that guides the data signals along a specific path. The most common guided media are: twisted pair cable, coaxial cable and optical fibre cable

Transmission media are defined as the means of communication which help to transfer data among network devices.

Transmission Media 1. Bounded Media (a) Twisted Pair Cable (b) Coaxial Cable (c) Optical Cable 2. Unbounded Media (a) Radio Waves (b) Micro Waves (c) Satellite

Twisted Pair Cable

It is made from a pair of copper wires twisted to each other and finally surrounded by outer insulating jacket. One wire of the pair is used for receiving data signal and the other wire is used for transmitting data. The wires are twisted in order to reduce unwanted noise and interference from external sources. It is used for both analog and digital transmission. It is available in different categories such as: category 4, category 5, category 6 and category 7 depend on the bandwidth of the cables. The category 1, category 2 and category 3 are not used these days. It is available in two forms: unshielded twisted pair (UTP) and shielded twisted pair (STP).

Figure 2.8: Twisted pair cable

Unshielded Twisted Pair (UTP): UTP cable is commonly used in telephone system. It transmits both data and voice. It consists of two conductors (copper), each with its own colour plastic insulation. UTP cable has lower bandwidth maximum up to 10Mbps and it may interfere by the external sources. But it is less expensive and easy to use than any other cables. Shielded Twisted Pair (STP): STP cable is commonly used in LAN for digital data transmission. It has metal foil or braided mesh covering each pair of insulated conductors. It prevents the penetration of electromagnetic noise from external sources. It has more bandwidth from 100Mbps up to 1000Mbps.

Advantages of Twisted Pair Cable

It is inexpensive and suitable for digital data transmission.
It is easier for clamping connector and flexible for wiring purpose.
It has higher data transfer rate up to Gbps (Gigabits Per Second)

Disadvantages of Twisted Pair Cable

It is not possible to transmit data for long distance (maximum up to 200).
It emits electromagnetic interference. It is not suitable for analog data such as video transmission.

Coaxial Cable

Coaxial cable is commonly used in cable TV in our home. It consists of two conductors. The inner copper conductor is surrounded by an insulator over which a sleeve of copper mesh is woven. This copper mesh is again surrounded by PVC jacket. It spans longer distance at higher speed than unshielded twisted pair cable. It is availed in two forms thinnet coaxial cable and thicknet coaxial cable. Thinnet coaxial cable: It is also known as 10Base2 which refers to the specifications for thinnet coaxial cable carrying 10 Mbps signals maximum up to 200 meters. This cable is popular in bus topology network. Thicknet coaxial cable: It is also known as 100Base5 which refers to the specifications for thicknet coaxial cable carrying 100Mbps signal up to 500 meter. This cable is popular in Cable TV network.

Advantages of Coaxial Cable

It has higher bandwidth than unshielded twisted pair cable.
It is less effective by electromagnetic interference.
It can support multiple channels in a medium.

Disadvantages of Coaxial Cable

It is expensive than unshielded twisted pair.
It is inflexible and not suitable for digital data transmissions.
It is easily tapped.

Optical Fiber cable

Optical fiber cable is made up of glass or plastic that transmits signals in the form of light. It is made up of a light conducting glass or plastic core through which light propagates. The core is surrounded by a glass cladding with lower index of refraction than the core to keep the light source in the core. Next a thin plastic jacket is used to protect the cladding. Fibers are typically grouped in bundle, protected by an outer sheath.

Figure 2.10: Optical Fiber Cable

Advantages of Optical Fiber Cable

They are suitable for long data transmission with unlimited bandwidth more than Gbps than any other cable.
They are much thinner and lighter than the others.
Data is transmitted in the form of light, so it is difficult to tap from the middle of cable.
They are not affected by outer interference such as electromagnetic interference.

Disadvantages of Optical Fiber Cable

They are not so flexible than other cables, so it is difficult to bend.
They are expensive than the others.
It is very difficult to connect two fibre optic cables.

Unbounded Media

Transmission media which do not use any physical connection between two communicating devices are called Unbounded Media or Unguided Media or Wireless Media. Such type of media use radio waves, microwaves and satellite. Wireless media such as Infrared, Bluetooth and WIFI (Wireless Fidelity) are also common for short range data transmission.

Radio Wave

A low range electromagnetic wave is called radio frequency. Frequencies below 1000MHz are radio frequencies and the above frequencies are microwave. It is suitable for transmitting radio signals around earth surface. Common examples of radio frequencies are: AM or FM.

Figure 2.11: Radio Wave

Advantages of Radio Wave

It is inexpensive and easy to produce.
It covers larger geographical area.

Disadvantages of Radio Wave

The data transfer rate is slower than that in wired media.
The quality of data transmission is very poor.

Microwave

High frequency electromagnetic wave (more than 1GHz) is known as microwave. It cannot bend and pass obstacles like hill or building so it requires line of sight transmission. The line of sight due to the earth's curvature is only 50 kilometre to the horizon. Therefore, the microwave must be placed within the distance . Specially, microwave is used in WAN or MAN communication, satellite communication and telephone communication.

Figure 2.12: Microwave Tower

Advantages of Micro wave

It has a high bandwidth than radio wave.
The quality of data transmission is better than radio wave.

Disadvantages of Micro wave

It cannot bend and pass obstacles so requires line of sight for data transmission.
It does not cover very large space because of earth's curvature.

Satellite

Satellite is set in the geostationary orbits of the earth, which is 22000 miles above from the earth surface. It continuously rotates the earth in the same orbit so that it appears stationary from the earth. The communication is done through uplink and downlink from the earth's stations. Transmitting a signal from ground to satellite is called uplink; the reverse is called downlink. The data transfer rate of uplink is lower than that of the downlink. Communication satellites are used for telecommunication, radio, television transmission, mobile communication, global positioninig system (GPS), etc.

Figure 2.13: Satellite

Advantages of Satellite

It covers all geographical area of earth.
It has higher bandwidth than radio or microwave data transmission.

Disadvantages of Satellite

It is very expensive for installation and for maintenance.
It has signals experience propagation delay.

2.8 Transmission Impairments Terminology

In communication system, analog signals travel through transmission media. This tends to fade the quality of analog signal. That means that the signal at the beginning of the medium is not the same as the signal at the end of the medium. This is signal impairment.

So, transmission impairments means the signals that are transmitted at the beginning of the medium are not the same as the signals that are received at the end of the medium that is what is sent is not what is received. These impairments tend to deteriorate the quality of analog and digital signals.

Jitter It is the disturbance in the normal sequence of sending data packets. It is also called fluctuation in delay as packets are being transferred across a network. The standard jitter measurement is in milliseconds (ms). Actually, it is the variation in the time between data packets arriving which is caused by network congestion or route changes. The longer data packets take to transmit, the more jitter affects audio quality.

Echo Actually, echo is a sound that is repeated because the sound waves are reflected back. So, In telecommunications, it is a sound that is a copy of another sound and that is produced when sound waves bounce off a surface. Local echo is where the local sending equipment displays the outgoing sent data.

Crosstalk Crosstalk is a feature of copper cables only. It is a form of interference in which signals in one cable induce electromagnetic interference (EMI) in an adjacent cable. The twisting in twisted-pair cabling reduces the amount of crosstalk whereas the fiber-optic cables do not experience crosstalk.

Distortion Distortion is changes in the form or shape of the signal. In communications and electronics, it is the alteration of the waveform of an information-bearing signal such as an audio signal representing sound or a video signal representing images. It is used to describe an interruption of transmitting signals that cause an unclear reception. It is more common in sound generation, video, display signals and data cables.

Noise Noise is random or unwanted signal that mixes up with the original signal. It is the disturbance that corrupts the quality of the signals. The noise causes signal loss or connection poor or data loss. Some common noises are audio and video noise, electrical signal noise, cross talk noise and wireless signal noise. 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.

Bandwidth Bandwidth is a common term in networking. It refers the amount of data that can be transferred from one point to another within a network in a specific amount of time. In computing, bandwidth is the maximum rate of data transfer across a given path. Bandwidth may be characterized as network bandwidth, data bandwidth, or digital bandwidth. Number of receivers It is concerned with the number of users or destination points where data are received.

2.9 Basic Concepts of Network Architecture

The network architecture refers to various services provided by the network such as how data is transmitted from one computer to another, how services can be implemented and how they are interconnected using various protocols. On the basis of architecture, it is classified into two categories: peer to peer network (point to point) and client server network (broadcast).

Network architecture refers to the various services provided by the network and it also deals with how data is transmitted from one computer to others.

Client Server Network Architecture In this architecture, there are two types of computers, one is server and the other is client. Server is the main computer in a network that controls, manages and provides various services to the clients such as print server, file server , database server, mail server and web server. Server is a very high performance computer and it is expensive too. A client is normal workstation that is connected to server. A client requests for services to the server and the server responses for the respective services. Network resources are centralized to the server so all the network activities such as data storage, data processing, data transfer etc. are carried out via server. It is based on request response mechanism.

Figure 2.14: Client Server Architecture

As it is based on broadcast architecture, so message is transmitted from one client to others via server. Server is responsible for broadcasting the messages to the entire network.

Suppose client 1 wants to send some message to Client 4, then firstly message is sent to the server. The server broadcasts the message along with source IP address and destination IP address to all the clients which are connected to the server. After receiving the message all the clients check the destination IP address of the message. If the destination IP address of the message and IP address of the client is same, then the message is accepted otherwise the message is discarded. Finally Client 4 receives the message. Similarly acknowledge message is transmitted from receiver client 4 to the sender client 1.

Advantages of Client Server Network

It is easier for centralized administration, controlling and monitoring the entine network from server.
It provides centralized backup and recovery features so it is considered as more secure and reliable architecture.
It is easier for managing the large size network (more than 10 computers)

Disadvantages of Client Server Network

Resources are centralized to the server, so if any problem occurs in the server the entire network will down.
There may be maximum data traffic at server so there may be chances af data collision.
It is more expensive due to dedicated server and additional network utilites and network operating system such as MS Windows Server, UNIX or Linux etc

Peer to Peer Network Architecture In this architecture, each workstation can have equal capabilities and responsibilities in a network. There is no server, each workstation acts like server as well as client. Message is transmitted from one workstation to another workstation in one to one basis by making peer. Network resources are distributed throughout the network and each workstation has responsibility to store, process and share data to others. It is simple and inexpensive architecture than client server architecture. It is suitable for small sized private owned networks such as personal network in home, school, cyber cafe.

Figure 2.15 : Peer to Peer Network Architecture

Each workstation or computer connected in the network has unique address for its indentification. When a message is to be sent from one computer to another computer in the network, the address of source computer and destination computer are sent along with the message. Suppose computer A wants to send some message to computer D, then first the message is sent to computer B and computer B compares the destination address of the message with its address. If both the addresses are same then the message is accepted oherwise it is retransmitted to the next adjacent computer C and the process continues, finally computer D receives the message. Similarly acknowledge message is transmitted from receiver computer D to the sender computer A.

Advantages of Peer to Peer Network

It has simple architecture and easy to install.
It doesn't require additional servers and network devices so it is inexpensive architecture.
Each node has equal privileges to access the resources of its own and other nodes connected to the network.
It is suitable for small sized network which has maximum 10 nodes.

Disadvantages of Peer to Peer Network

Its performance becomes very poor for large size network (more than 10 nodes).
There is no central administration and monitoring mechanism for the network.
Network security problems due to equal privileges of the nodes.
There is no central backup and recovery mechanism, each node has responsibility to maintain its own backup.

2.10 Some Basic Terms and Tools

IP address Internet Protocol (IP) address is a unique address which identifies a device on the internel. It is the set of rules governing the format of data sent via the internet or local network. IP addresses are the identifier that allows information to be sent between devices on a network which contains location information and make devices accessible for communication. The internet needs a way to differentiate between different computers, routers, and websites.
IP address is an Internet Protocol address. An Internet Protocol is a set of rules that govern Internet activity and facilitate completion of a variety of actions on the World Wide Web. It is also known as logical address that defines the unique address for the given network.

It defines IP address as a 32-bit number and it is known as Internet Protocol Version 4 (IPV4). However, due to the enormous growth of the Internet a new addressing system called IPV6 (IPng) has been developed that uses 128 bits for the address. Example of IP address is: 172.16.254.1 (for IPv4), 2001: DB8:0:1234:0:5678:8:1 (for IPV6).

IP address is a string of numbers separated by periods. They are expressed as a set of four numbers for example 192.168.2.42. Each number in the set can range from 0 to 255 so that the full IP addressing range goes from 0.0.0.0 to 255.255.255.255.

Subnet mask A subnet is a logical subdivision of an IP network. The process of dividing a network into two or more networks is called subnetting. A subnet mask is a 32-bit number created by setting host bits to all 0s and setting network bits to all 1s. This is a way in which the subnet mask separates the IP address into the network and host addresses.
A subnet mask hides the network part of a system's IP address and leaves only the host part as the machine identifier. It uses the same format as an IPV4 address. Each section of the subnet mask can contain a number from 0 to 255, just like an IP address. For example, subnet mask for a Class C IP address is: 255.255.255.0.
There are five classes of subnetworks: Class A, Class B, Class C, Class D, and Class E. Each class relates to a specific range of IP addresses. Classes A, B and C are used the most often by different networks. Subnet classes are made unique by the number of bits their IP addresses have dedicated to a network and the number of bits dedicated to hosts. They each have a default subnet mask. Classes can be identified by the number in the first octet of their address. Class A: First Octet Value 0-127 Class B: First Octet Value 128-191 Class C: First Octet Value 192-223 Class D: First Octet Value 224- 239 Class E: First Octet Value 240-255

Gateway A gateway is a node in a computer network that can be used to join one device to another. It is used to communicate and send data back and forth. The gateway is the device that routes traffic from a workstation to the outside network. The gateway in the Internet Service Provider gives us access to the entire Internet. The gateway may be the stopping point or controls the traffic of Internet Service Provider (ISP). If you have connected a wireless network at home, the gateway is the modem. Gateways are distiet from routers or switches in that they communicate using more than one protocol to connect a bunch of networks and can operate at any of the seven layers of the Open Systems Interconnection(OSI) model.

MAC address A MAC (Media Access Control) address is a media access control address. It is a unique identifier assigned to a network interface controller for use as a network address in communications within a network segment.
It is usually assigned by the manufacturer of a Network Interface Controller (NIC), and it is stored in the hardware. The NIC is a computer circuit card that allows a computer to connect to a network. The MAC address is formed in accordance to the rules of the three numbering name spaces. These are managed by the Institute of Electrical and Electronic Engineers (IEEE).
It is a 48 or 64-bit address associated with a network adapter. MAC addresses are linked to the hardware of network adapters. For this reason, the MAC address is sometimes called the hardware address or the physical address. The format is six sets of two digit or characters which is separated by hyphens. MAC addresses are expressed in hexadecimal notation in the following format: 01-23-45-67-89-AB, in the case of a 48-bit address, or 01-23-45-67-89-AB-CD-EF , in the case of a 64-bit address. MAC addresses are most often assigned by the manufacturer of a network interface card (NIC) and are stored in its hardware, the card's read-only memory, or some other firmware mechanism.

Internet Internet is a global computer network providing a variety of information and communication facilities that consists of interconnected networks using standardized communication protocols. It is a global network of billions of computers and othe electronic devices. It is used to access almost any information and communicate with anybody in the world.
An Internet consists of two major components: network protocols and hardware. The protocols are the set of rules in order to complete tasks such as the TCP/IP. Without this common collection of rules, machines do not able to communicate. The hardware device are the devices in which data can be transferred in the internet. Internet works by using packet routing network that follows Internet Protocol (IP) and Transport Control Protocol (TCP). TCP and IP work together to ensure that data transmission across the internet consistent and reliable. Using IP Address, the IP system receives further instructions on how the data should be transferred. The TCP works with IP to ensure transfer of data dependable and reliable. This helps to make sure that no packets are lost.

Intranet An intranet is the part of internet. It is a private enterprise network which is designed to support an organization's employees to communicate, collaborate and perform their roles. It serves a broad range of purposes and uses. Intranet refers to an internal network that utilizes the tools and techniques of the Internet to provide services limited to local network users. An intranet is a company wide network making it possible to share documents, databases, and applications. Intranet is a private network group and owned by a particular person or organization. So, only those persons, who are authorized and authenticated users of that organization can use this net.
Some intranets are limited to a specific local area network and some can be accessed from remote locations over the internet. Local intranets are more secure because they can only be accessed from within the network. Intranets can be used for many different purposes. The main objective is to facilitate internal communication. A business may create an intranet to allow employees to securely share messages and files with each other.
Most intranet solutions provide a web-based interface for users to access. This interface provides information and tools for employees and team members. It includes calendars, project timelines, task lists, confidential files, and a messaging tool for communicating with other users. Some examples of intranet services are Microsoft SharePoint, Huddle, Igloo, and Jostle. Some services are open source and free of charge, most intranet solutions require a monthly fee. The cost is usually related to the number of users within the intranet.

2.11 Network Tool: Packet Tracer, Remote Login

Packet Tracer

Packet Tracer is a tool built by Cisco System. This tool provides a network simulation to practice simple and complex networks. It allows users to simulate the configuration of Cisco routers and switches using a simulated command line interface. It uses drag and drop user interface that allows users to add and remove simulated network devices as they want.

The main purpose of Cisco Packet Tracer is to help students learn the principles of networking with hands-on experience as well as develop Cisco technology specific skills. Students complete assignments using this tool, working on their own teams. Packet Tracer provides simulation, visualization, authoring, assessment, and collaboration capabilities and facilitates the teaching and learning of complex technology concept. Packet Tracer allows to simulate highly complex learning environments and allows to see how packets move between different devices. Also, Engineers who would like to organize any change in the production network prefer to use Cisco Packet Tracer to first test the required changes and proceed to deploy if and only if everything is working as expected.

Installation of packet Tracker

To download Packet Tracer, go to https ://www.netacad.com and log in with your Cisco Networking Academy credentials then, click on the Packet Tracer graphic and download the package appropriate for your operating system. Windows Installation in Windows is simple and straightforward; the setup comes in a single file named Packet-tracer Setup6.0.1.exe. Open this file to begin the setup wizard, accept the license agreement, choose a location, and start the installation. Linux Linux users with an Ubuntu/Debian distribution should download the file for Ubuntu, and those using Fedora/Redhat/CentOS must download the file for Fedora. Grant executable permission to this file by using chmod and execute it to begin the installation. chmod +x PacketTracer601_i386_installer-rpm.bin ./PacketTracer601_i386_installer-rpm.bin

Remote Login

A login that allows a user terminal to connect to a host computer via a network or direct telecommunications link, and to interact with that host computer as if the user terminal were directly connected to that host computer. Remote login works exactly the same way as desktop sharing. In desktop sharing, there are two separate parties. They are the host computer and the remote user. The host computer allows a remote user to view the contents of the host computer's desktop over the Internet to share the desktop. The host computer can also hand over keyboard and mouse controls to the remote user.

2.18 Remote Login

The remote login requires three components: downloaded software, internet connection and secure desktop sharing network. So, remote login will only work if the host computer is powered on, connected to the Internet and running the desktop sharing software. Each time you open and run the desktop sharing software on the host computer, the software starts a new session. Each session has a particular ID and/or password that's required to remotely log in to the host computer. Once the session has been established, most desktop sharing software quietly runs in the background of the host computer until a remote login request is made.

2.12 Network Connecting Devices

Network connecting devices enable to interconnect the computers in a network. It also provides the route for data transfer. The most commonly used network connecting devices are: modem, NIC, bridge, router, gateway, repeater, Bluetooth, IR, Wi-Fi, switch and hub.

NIC

A network interface card (NIC) is an interface for connecting the computer to the network devices through communication media. It is also known as network adapter. It is basically a circuit board that is commonly inbuilt inside the main board of computer. It works on layer two of OSI layer. Each network interface card is uniquely identified by a special number called MAC (Media Access Control) address. MAC addresses are distributed by the standard authorities.

Modem

The word modem stands for modulator and demodulator. It is an electronic device which translates data from digital to analog and vice-versa. Modulation is the conversion of digital data into analog data and the reverse process is called demodulation. Modem is used to connect internet via telephone line which uses analog signals whereas the computer uses digital signals. Mainly modem is classified into two types: internal modem and extemal mostem. The internal modem is placed in the slots of computer system and the exteat modem is placesd outside computer system.

Router

Router is hardware and software device. It is a highly intelligent and potocol sensitive linking device used to connect two different LANS or WANS. Moreover it also helps for data filtering, routing the packets and isolating the networks. It works up to layer three of OSI reference model. A router keeps track of the address of all the segments of a network and can even determine the best path for sending data.

Figure 2.21: Router

Switch

Switch is also a multiport network connecting device which helps to connect multiple computers to a server in a LAN. It is generally used in star topology. But it works on a layer two of OSI reference model. Hub works on half-duplex mode where as the switch works on full- duplex mode. It has capability for inspecting address of the data packets and directly switch the source port to destination port. Therefore, its functionality is same as bridge so switch is also known as multiport bridge.

Figure 2.22. Switch

2.13 Network Topologies

Network topology refers to the physical layout of the network. It shows the geometrical representation of all the links and linking devices, also called nodes. Simply, it reflects the computers in the network and how they are interconnected to each other. The main objective of the network topology is to find out the most economical and efficient way of transmission channel. The most common LAN topologies are bus, star, ring, tree, mesh and hybrid.

Topology refers to the physical structure of a network that deals with how computers are interceonnected by using cabling system.

Bus Topology

Figure 2.23: Bus Topology

Computers are connected to a single continuous cable that is called 'bus'. It acts as backbone. A bus must be terminated on both sides to prevent signal bounce and computers are connected to the bus with the help of drop line and T-connecter (tap). A bus runs throughout the office to which all the computers are connected. Message originating from a computer is transmitted in both directions on the bus. It is based on client server network architecture.

Advantages of Bus Topology

It is simple and easy to setup and extend the network.
It is inexpensive topology because it requires less amount of cable and no additional network devices.
If any computer in the network downs, then it does not affect other computers.
It is more flexible because we can easily connect and disconnect any number of computers in the bus.

Disadvantages of Bus Topology

Data traffic is very high in bus so there may be chances of data collision.
The length of bus should be small otherwise the performance of the network goes down.
If there is problem in bus then the entire network goes down.
It is very difficult to find out the fault in the bus.

Star Topology

Figure 2.24 Star Topology

Computers in the network are connected to each other with the help of central connecting device hub or switch or server. It is based on client server architecture. The communication is done through the central server with the help of hub or switch in the entire network. The hub controls traffic on the network. It is the most popular and widely used topology for LAN.

Advantages of Star Topology

It is simple, reliable and easy to set up and re-configuration.
It is flexible to connect new computer and remove existing computer in the network.
It is very easy to find out fault because of the use of hub or switch.
If any computer in the network goes down, then other computers can continue the functions.

Disadvantages of Star Topology

It requires very large amount of cables.
It is expensive topology because of cables and networking device hub or switch.
If there is any problem in central device hub or switch then the entire network will be down.
The data traffic is high in central device hub so there may be chances of data collision.

Ring Topology

Computers are interconnected to each other by making a closed circular structure that means each computer is connected to other two adjacent computers in either side. It is based on peer to peer network architecture. Communication is done in single direction only. When one computer receives the message, then it compares the destination address of the message and its own address, if it matches then it accepts the message otherwise the message is retransmitted to the next computer in the network.

Figure 2.25 Ring Topology

Advantages of Ring Topology

It is simple and inexpensive topology.
There is less chance of data collision because of unidirectional data transmission.
There is no server so each computer has equal access facilities to the resources.
It's performance is better than bus topology for small size network.

Disadvantages of Ring Topology

It is not flexible topology so it is difficult for adding and removing new nodes.
It is not suitable for large size network (not more than 10 nodes).
If there is problem in any computer or connection then the entire network goes down.
It is very difficult to find out the errors in the network.

2.14 Basic Concept OSI Reference Model

International Standard Organization (ISO) is a multinational body committed for the worldwide standardization of products, services and businesses. ISO first introduced reference model for data communication in 1970s, called Open System Interconnection (OSI) model. The OSI model is a layered based system that allows communication between different types of computer system in the network. It is only a theoretical model which is not practically implemented till now. But, all data communication and network standards are based on this model. These layers are divided into three groups: upper layer, middle layer and lower layer. The physical, data link and network layers belong to lower layer, transport layer belongs to middle layer and the session, presentation and application layer belong to upper layer. The upper layers activities are performed by software

These layers are divided into three groups: Upper layer, middle layer and lower layer. The physical , data link and network layers belog to lower layer , transport layer belongs to middle layer and the session , presentation and application layer belog to upper layer. The upper layers activities are performed by software and the lower layers activities are performed by hardware.

An OSI reference model of network is ISO certified model that entirely describes how information is transmitted from an application of one computer to another application of other computer.

Figure 2.26 OSI Reference Model

1. Physical Layer : It is responsible for transmitting raw bits from one node to another over communication channel. So it defines the mechanical, electrical, functional procedural standards for the physical transmission of data over the communication medium.

2. Data Link Layer: It is responsible for moving frames from one node to another node. It provides different facilities such as physical addressing, data framing, fle control, error control, access control etc. It also helps to detect errors that may occur in the physical layer.

3. Network Layer: It is responsible for delivery of packets from source host to destination host. It provides different facilities such as logical addressing, routing, etc. It is the agent for establishing connection between heterogeneous networking systems. It also includes operational control procedures for inter-network communication as well as routing information through multiple networks.

4. Transport Layer: It is responsible for delivery of message from one application to another application. It provides different facilities such as service point addressing ,segmentation, reassembly, connection control, flow control, error control, etc. It divides the message into blocks and transports them.

5. Session Layer:The session layer is responsible for dialog control and synchronization between two devices. This layer provides the mechanism for organizing and structuring interaction between two applications or devices.

6. Presentation Layer: It is concerned with the syntax and semantics of the information transmitted. It is also responsible for representation of the data to the end user or application. It includes data connection and code translation (e.g. ASCII to EBCDIC).

7. Application Layer: It allows application to access network services. It is completely user-oriented layer. Some of its functions are file transfer, accessing remote file, database, e-mail etc.

2.15 Internet Protocol Addressing

The term protocol refers to the set of rules and procedures that govern the transmission of message over a physical networking medium. Protocol is a set of rules that defines communicating among computers in the network. Protocol has three major components syntax, semantics and timing. Syntax defines grammatical rules, semantic defines logical meaning and the timing defines the delivery time for the synchronization. Actually protocol is a software that must be installed on network components that need them Computer can communicate with each other only if they use a same protocol. Some standard protocols are TCP/IP, HTTP, FTP, SMTP, POP, CSMA/CD, TOKEN RINO. ETHERNET etc

The term protocol refers to the set of rules and procedures that govern u transmission of message over a physical networking medium.

Types of Protocols

1. Transmission Control Protocol (TCP):TCP represents a transport layer protocol that provides end-to-end reliable transmission of data. It includes functions such as flow control, error control and exchange of status information. It is connection oriented protocol. Internet Protocol (IP): IP represents a network layer protocol that helps to route the messages between two networks. It is connectionless protocol. 3. Simple Mail Transfer Protocol (SMTP):It is an application layer protocol that helps to transmit e-mail messages to the mail-server. Post Office Protocol (POP): It is a simple protocol for transmitting e-mail messages. It is also an application layer protocol to receive e-mail from the mail-server. Its latest version is 3, so called POP3. File Transfer Protocol (FTP): FTP is one of the oldest application layer protocol that helps to transfer files between two computer systems. It is client server based protocol where an FIP client accesses an FTP server. An FTP client is simple application that can be installed in our computer. 6. Hyper Text Transfer Protocol (HTTP): HTTP is most commonly used application layer protocol for transmitting hyper text documents in the internet. It provides a platform for data communication in World Wide Web. 7. Hyper Text Transfer Protocols Secure (HTTPS): HTTPS is a combination of HTTP protocol and some security protocols to provide secure and encrypted communication between two networks. It is reliable and popular protocol for transmitting hyper text. TELNET: TELNET is remote log in protocol for executing commands on a remote host. It is based on client server mode and it uses TCP protocol for data transmission. A client provides a TELNET request to the TELNET server.

Some Related Terms

1. Server:It is the main computer in client-server or broadcasting network architecture which manages and controls all other computers in the network. So, it plays very important role in the network. It is called server as it provides services to other by allowing other components for sharing expensive resources such as storage device, printer etc. Generally, there are three categories of server. 67 Approved by Curriculum Development Centre, Nepal Computer Science XII File server: It is used to share storage space for file. Print server: It is used to handle printing works of all the workstation conrece in the network. Modem server: It is used to get connected to other network or simply to use a telephone. 2. Client: A device or application that makes use of the services provided by a serve A client may be a PC or a workstation on a network using services provided by the network server. Client requests information from the server. Workstation: In client-server networking architecture, generally the clients are callec workstation. So, they run under the control of server and workstations are alwajs seeking to share the resources of the network and they always receive services tram the server. 3. Ch2 OSI Reference Model: It is an ISO certified model that entirely describes how information is transmitted from an application of one computer to another application of other computer, Protocol: It is a set of common rules and procedures that govern the transmission of message over a physical networking medium. Examples of commonly used protocols are TCP/IP, HTTP, FTP, SMTP, POP, CSMA/CD, TOKEN RING, ETHERNET etc. EXERCISE Multiple Choice Questions (Tick the best options) What are the five elements of basic communication system? a. Data, Sender, Receiver, Medium and Message b. Message, Sender, Receiver, Rules and Protocols Message, Sender, Receiver, Medium and Protocols d. Message, Sender, Receiver, Software and Protocols 2. Select the perfect order of dada communication system or model. Source, Sink, Encoder, Decoder, Transmitter, Receiver and Channel Source, Encoder, Channel, Transmitter, Receiver, Decoder and Sink Source, Encoder, Transmitter, Receiver, Channel, Decoder and Sink 'P. Source, Encoder, Transmitter, Channel, Receiver, Decoder and Sink 3. Which of the following statement in not true? Computer resources such as hardware, software, data can be shared in a Network. Communication has become faster, reliable but expensive for data communications. All networks' components can be controlled by a single computer system. d. Network security is the most challenging job for network administrator. Give an example of full duplex mode of data communication. 4. a. Television b. FM Radio Walkie-talkie d. Internet What is the maximum length of typical type of LAN? 1 KM 5. b. 3 KM 10 KM 100KM What is the basic characteristic of WAN? Simple and Mostly Private. b. Complex and Mostly Private Simple and Mostly Public d. Complex and Mostly Public Approved by Curriculum Development Centre, Nepal Computer Science XII 7 In a digital data transminsion of computer networking, what is Mbps? Mega Bits Por Second G Moga Hands Per Socond h Mega llytes Por Second d. Madia Bits Per tBecond 8. In client server network architecture, which statement is not true? Secured and roliable network architecture Data traffic is very high because of broad architecture It is less expensive architecture d. Network controlling and management is nasier, 9. Which topology in more reliable and secured for a LAN? Star d. Mesh . Ring 10. What is the correct order of OSI reference model of a computer network from bot tom to top? a physical, datalink, notwork, transport, session, application and presentation b. physical, datalink, network, transport, presentation, session, and application G physical, datalink, transport, notwork, session, presentation and application d. physical, datalink, network, transport, sossion, presentation and application or tole Piu tele (pis tpis (ple lgie (pie y Solve the following short questions, 1. Describe computer network with any three advantages, Differentiate between LAN and WAN. 3. How do you define packet tracer? 4. Differentiate between Internet and Intranet. 5. Explain about Bandwidth and echo. Define the term remote login, Differentiate between star topology and ring topology. Differentiate between IP Address and MAC Address 9. Write the functions of modem and switch. 10. What is protocol? List any five communication protocols. C. Solve the following long questions. 1. What do you mean by data communication? Explain the data communication me el between source and sink using nuitable block diagram. What do you mean by transmission media? Explain the different types of trans sion media used in computer network? Define network architecture. Differentiate between client-server and peer-per 3. works, What do you mec an by OSI reference model? Explain the different layers of Cs reerence model. topologies with suitable diagram,

End of Ch-2 Data Communication & Networking

(XII) Ch-2 Data Communication and Networking