Unit I – Introduction – Express Learning: Data Communications and Computer Networks


Overview of Data Communications and Networking

1. What is the difference between communication and transmission?

Ans: Both communication and transmission deal with the exchange of information. However, a few differences between them are listed in Table 1.1.

Table 1.1 Differences Between Communication and Transmission

  Communication          Transmission
• It refers to exchange of meaningful information between two communicating devices. • It refers to the physical movement of information.
• It is a two-way scheme. • It is a one-way scheme.

2. What is meant by data communication? What are the characteristics of an efficient data communication system?

Ans: Data communication refers to the exchange of data between two devices through some form of wired or wireless transmission medium. It includes the transfer of data, the method of transfer and the preservation of the data during the transfer process. To initiate data communication, the communicating devices should be a part of a data communication system that is formed by the collection of physical equipments (hardware) and programs (software). The characteristics of an efficient data communication system are as follows:

Reliable Delivery: Data sent from a source across the communication system must be delivered only to the intended destination.
Accuracy: Data must be delivered at the destination without any alteration. If the data is altered or changed during its transmission, it may become unusable.
Timely Delivery: Data must be delivered in time without much time lags; otherwise, it may be useless for the receiver. As in case of video and audio transmissions, timely delivery means delivering data at the same time it is produced, in the same order in which it is produced and also without any delay.
Jitter: It refers to the differences in the delays experienced during the arrival of packets. It is uneven delay in the arrival time of audio or video data packets. These packets must be delivered at a constant rate; otherwise, the quality of the video will be poor.

3. What are the components of data communication system?

Ans: There are five basic components of a data communication system (Figure 1.1).

Message: It refers to the information that is to be communicated. It can be in the form of text, numbers, images, audio or video.
Sender: It refers to the device, such as a computer, video camera and workstation, which sends the message.
Receiver: It refers to the device, such as a computer, video camera and workstation, for which the message is intended.
Transmission Medium: It refers to the path which communicates the message from sender to receiver. It can be wired such as twisted-pair cable and coaxial cable or wireless such as satellite.
Protocol: It refers to a set of rules (agreed upon by the sender and the receiver) that coordinates the exchange of information. Both sender and receiver should follow the same protocol to communicate with each other. Without the protocol, the sender and the receiver cannot communicate. For example, consider two persons; one of which speaks only English while another speaks only Hindi. Now, these persons can communicate with each other only if they use a translator (protocol) that converts the messages in English to Hindi and vice versa.

Figure 1.1 Components of a Data Communication System

4. What are the different forms of data representation? Explain in detail any two coding schemes used for data representation.

Ans: Data can be available in various forms such as text, numbers, images, audio and video. In networking, data has to be transmitted from source to destination in binary form. Thus, information such as alphabets (a–z, A–Z), numbers (0, 1, 2,…9), special symbols (!, @, #, $ etc.) and images (in the form of pixels/picture elements) has to be converted into sequences of bits, that is, 0 and 1. Audio and video data have to be converted from analog to digital signal for transmission with the help of different encoding schemes (explained in Chapter 03). There are various coding schemes including Unicode, ASCII and EBCDIC which are used these days to represent the data. Here, we discuss only ASCII and EBCDIC.

American Standard Code for Information Interchange (ASCII)

The standard binary code for the alphanumeric characters is ASCII. This code was originally designed as a seven-bit code. In addition, ASCII is often used with a parity bit which is the eighth bit. This bit is used at the most significant bit (MSB) position and is used for detecting errors during transmission. ASCII is commonly used in the transmission of data through data communication and is used almost exclusively to represent the data internally in the microcomputers.

In ASCII, uppercase letters are assigned codes beginning with hexadecimal value 41 and continuing sequentially through hexadecimal value 5A and lowercase letters are assigned hexadecimal values of 61 through 7A. The decimal values 1–9 are assigned the zone code 011 in ASCII. Table 1.2 of ASCII-7 coding chart shows uppercase and lowercase alphabetic characters, numeric digits 0–9 and special characters. The standard ASCII-7 code defines 128 character codes (0–127), of which, the first 32 are control codes (non-printable) and other are printable characters.

Table 1.2 ASCII-7 Coding Chart

Extended Binary Coded Decimal Interchange Code (EBCDIC)

EBCDIC uses eight bits for each character; therefore, it is possible to represent 256 different characters or bit combinations (Table 1.3). This provides a unique code for each decimal value from 0 to 9 (for a total of 10), each uppercase and lowercase letters (for a total of 52), and for a variety of special characters. Since it is an eight-bit code, each group of the eight bits makes up one alphabetic, numeric, or special character and is called a byte.

In EBCDIC, the bit pattern 1100 is the zone combination (zone and digit) used for the alphabetic characters A through I, 1101 is used for the characters J through R and 1110 is the zone combination used for characters S through Z. The bit pattern 1111 is the zone combination used when representing decimal digits. Other zone combinations are used when forming special characters.

The concepts and advantages of ASCII are identical to those of EBCDIC. The important difference between EBCDIC and ASCII coding systems lies in the eight-bit combinations assigned to represent the various alphabetic, numeric and special characters. While using ASCII eight-bit code, we notice that the selection of bit patterns used in the positions differs from those used in EBCDIC.

Table 1.3 EBCDIC Coding Chart

5. Explain different modes of data transmission between two devices.

Ans: There are three types of transmission modes: simplex, half-duplex and full-duplex (Figure 1.2).

Simplex: This mode of transmission is unidirectional. The information flows in one direction across the circuit, with no capability to support response in the other direction. Only one of the communicating devices transmits information; however, the other can only receive it. Television transmission can be considered as an example of simplex mode of transmission where the satellite only transmits the data to the television, vice versa is not possible.

Figure 1.2 Data Transmission Modes

Half-Duplex: In this transmission mode, each communicating device can receive and transmit information, but not at the same time. When one device is sending, the other can only receive at that point of time. In half-duplex transmission mode, the entire capacity of the transmission medium is taken over by the device, which is transmitting at that moment. Radio wireless set is an example of half-duplex transmission mode where one party speaks and the other party listens.
Full-Duplex: This transmission mode allows both the communicating devices to transmit and receive data simultaneously. A full-duplex mode can be compared to a two-way road with traffic flowing in both directions. A standard voice telephone call is a full-duplex call because both parties can talk at the same time and be heard.

6. Define computer network. What are the different criteria that a network should meet?

Ans: A computer network refers to a collection of two or more computers (nodes) which are connected together to share information and resources. Nodes are connected if they are capable of exchanging information with each other. To be able to provide effective communication, a network must meet a certain number of criteria, some of which are as follows:

Performance: Performance of a network can be determined by considering some factors such as transit time, response time, throughput and delay. The amount of time taken by a message to travel from one device to another is known as transit time and the time elapsed between the user initiates a request and the system starts responding to this request is called the response time. The amount of work done in a unit of time is known as throughput. To achieve greater performance, we need to improve throughput and reduce the transit time, response time and delay. However, increasing the throughput by sending more data to the network often leads to traffic congestion in the network and thus, increases the delay. Some other factors that affect the performance of a network are the type of transmission medium, the total number of users connected to the network and the efficiency of connected hardware and software.
Reliability: An efficient network must be reliable and robust. Reliability of a network is determined by the factors such as how frequently the failure is occurring and how much time is being spent in recovering from a link failure.
Security: A network must also provide security by protecting important data from damage and unauthorized access. In addition, there must be procedures and policies to handle theft and recovery of data.

7. What are the various advantages of a computer network?

Ans: Today, computer networks are being used in every facet of life, as they provide the following advantages.

Sharing Information: This is one of the most important advantages of a computer network. In the absence of a network, transferring information from one computer to another requires the use of a compact disk, floppy disk, printer, etc. However, if the communicating systems are geographically apart, sharing information becomes even harder. Computer networks solve this problem, as computers connected to a network can share information as if they are in the same building even when they are located geographically apart. For example, when we connect to the Internet and open a website on our computer, we can access information that is not stored in our own computers. In such a networked system, all information is stored on a central and powerful computer known as a server. All other computers in the network can easily access information from the server as if it was located on their own computer.
Sharing Hardware Resources: A network facilitates sharing of hardware resources in an effective and user-friendly manner. When computers are connected to a network, they can share peripherals such as printer and hard disk drives, with any other computer. For example, in an office having five to ten computers and one printer, in the absence of network, only the computer that is connected to the printer can be used to print data. If others have to access the printer, then they would first need to transfer their data over to the computer connected to the printer. Contrastive to this, in a networked environment, the printer can be shared on the network and every computer on the network can easily access the printer without having the need to transfer data.
Sharing Software Resources: Software resources are the programs or applications that are used by computers to perform any useful function or to carry out daily basis task. In an environment where networking is not available, users will have to install and configure any applications that they need individually. However, if the computers are connected via a network, the required software or application can be installed and configured centrally on a server and shared by all. This saves the valuable time and disk space.
Preserve Information: In addition to sharing information, a networked environment helps to preserve information as well. It is difficult to maintain regular backups of data on a number of standalone computers and without backup, important data can be lost in case of some accident or failure of computer. However, in a networked environment, a copy of the important data can be kept on the server as well as on other connected computers on the network. In this case, failure of one computer will not result in loss of information, as the data can still be accessed from other computers whenever required.
Communication: Computer networks have revolutionized the way people communicate. Rather than exchanging memos and directives on paper, which involves a lot of printing costs and delays, network users can instantly send messages to others and even check whether or not their messages have been received.

8. What are the various applications of a computer network?

Ans: Nowadays, computer networks have become an essential part of industry, entertainment world, business as well as our daily lives. Some of the applications of a computer network in different fields are as follows:

Business Applications: There is a need of effective resource sharing in companies for the exchange of ideas. This can be achieved by connecting a number of computers with each other. It allows transferring of business information effectively without using paper. For example, an employee of one department can access the required information about another department using network.
Marketing and Sales: Marketing firms utilize networks for conducting surveys to gather and analyze data from the customers. This helps them to understand the requirements of a customer and use this information in the development of the product. Sales professionals can use various applications such as online shopping, teleshopping and online reservation for airlines, hotel rooms etc. in order to increase the revenue of their organization.
Financial Services: Computer networks play a major role in providing financial services to people across the globe. For example, the financial application such as electronic fund transfer helps the user to transfer money without going into a bank. Some other financial applications that are entirely dependent on the use of networks include ATM, foreign exchange and investment services, credit history searches and many more.
Directory and Information Services: Directory services permit a large number of files to be stored a central location thereby speeding up the worldwide search operations. Information services of Internet such as bulletin boards and data banks provide a vast amount of information to the users within seconds.
Manufacturing: Computer networks are widely being used in manufacturing. For example, the applications such as computer-aided design (CAD) and computer-assisted manufacturing (CAM) use network services to help design and manufacture the products.
E-mail Services: This is one of the most widely used applications of network. With the help of computer networks, one can send e-mails across the world within a few seconds and without using paper.
Mobile Applications: With the help of mobile applications such as cellular phones and wireless phones, people wishing to communicate are not bound by the limitation of being connected by fixed physical connections. Cellular networks allow people to communicate with each other even while travelling across large distances.
Conferencing: With the help of networking, conferencing (teleconferencing or videoconferencing) can be conducted that allows remotely located participants to communicate with each other as if they are present in the same room.

9. Discuss two types of computer network architectures.

Ans: The computer network architecture specifies how the physical and logical components of a computer network are assembled and connected with each other to facilitate information exchange and resource sharing. The two major types of network architectures are client/server and peer-to-peer architectures.

Client/Server Architecture

In this architecture, each computer is either a client or a server. To complete a particular task, there exists a centralized powerful host computer known as server and a user's individual workstation known as client (Figure 1.3). The client requests for services (file sharing, resource sharing etc.) from the server and the server responds by providing that service. The servers provide access to resources, while the clients have access to the resources available only on the servers. In addition, no clients can communicate directly with each other in this architecture. A typical example of client/server architecture is accessing a website (server) from home with the help of a browser (client). When a client makes a request for an object to the server, then the server responds by sending the object to the client. In addition, it must be noticed that two browsers accessing the same website, never communicate with each other.

Figure 1.3 Client/Server Architecture

An advantage of client/server architecture is that the IP address of the server is always fixed and the server is always available on the network for clients. However, the disadvantage of this architecture is that with time as the number of clients starts to increase, the number of requests to the server also increases rapidly. In this scenario, we might need more than one server to serve larger number of requests.

Peer-to-Peer (P2P) Architecture

This architecture does not rely on dedicated servers for communication; instead, it uses direct connections between clients (peers) (Figure 1.4). A pure P2P architecture does not have the notion of clients or servers, but only equal peer nodes that simultaneously function as both “clients” and “servers” to the other nodes on the network. That is, every node is able to initiate or complete any supported transaction (file transfer) with the other connected node and every node can directly communicate with another.

Figure 1.4 Point-to-Point Architecture

The upper limit of the number of nodes that can function as both clients and servers on a P2P network is between 10 and 25. If there are more nodes, then a P2P machine can be used as a dedicated server with additional high-performance hardware. An advantage of P2P architecture is that it is very scalable, that is, millions of nodes can be connected to the network to contribute to resources irrespective of the differences in their local configuration, processing speed, network and storage capacity. However, the highly decentralized approach of P2P architecture can be tough to manage. For example, during file sharing with other remote clients, the only client having a specific file might log off from the network. Examples of P2P networks are file-sharing applications such as Morpheus and Kaaza.

10. Differentiate between P2P and client/server architecture.

Ans: Both P2P and client/server networks have associated advantages and disadvantages. These advantages and disadvantages form a part of distinction between the two. These differences are listed in Table 1.4.

Table 1.4 Differences Between P2P Architecture and Client/Server Architecture

Basis   P2P architecture   Client/server architecture
Centralized   There is no central repository for files and applications in this architecture.   Resources and data security are controlled through the server.
Maintenance   It incurs low maintenance cost.   A large network requires extra staff to ensure efficient operation.
Installation   It can be easily installed.   It requires experts for proper installation of the network.
Cost   It does not require a dedicated server; thus, it is not much expensive.   It is expensive, as it requires a dedicated server.
Security   Lack of proper security policies is the biggest drawback.   It provides high level of security.
Dependence   All nodes are independent of each other. Failure occurring in one node does not affect the functioning of other nodes in the network.   When server goes down, it affects functioning of the entire network.

11. Distinguish between point-to-point and multipoint connections? Give suitable diagrams.

Ans: In order to communicate with each other, two or more devices must be connected using a link. A link is a physical path using which data can be transferred between two or more devices. The two possible types of connections between two or more devices are point-to-point and multipoint connections.

Point-to-Point: In a point-to-point connection, there is a dedicated link between the communicating devices (Figure 1.5). The link is totally reserved for transmission specifically for those two devices. Most point-to-point connections are established with cable or wire though satellite or microwave links are also possible. For example, operating any device such as television using a remote control establishes a point-to-point connection between the device and the remote control.

Figure 1.5 Point-to-Point Connection

Multipoint: In a multipoint (multidrop) connection, a single link is shared between two or more devices, that is, there is no dedicated link between the communicating devices (Figure 1.6). If the shared link can be utilized by many devices simultaneously, it is known as a spatially shared connection whereas if devices need to take turns to utilize the link, it is known as a timeshared connection.

Figure 1.6 Multipoint Connection

12. What do you understand by the term network topology?

Ans: The term topology refers to the way a network is laid out, either physically or logically. A topology can be considered as the network's shape. It is the geometric representation of the relationship of all the links. There are five basic topologies: bus, ring, star, tree and mesh.

13. Discuss bus and mesh topology. Compare them.

Ans: Bus and mesh topology are detailed as under.

Bus Topology

Bus topology uses a common bus or backbone (a single cable) to connect all devices with terminators at both ends. The backbone acts as a shared communication medium and each node (file server, workstations and peripherals) is attached to it with an interface connector. Whenever a message is to be transmitted on the network, it is passed back and forth along the cable, past the stations (computers) and between the two terminators, from one end of the network to the other. As the message passes each station, the station checks the message's destination address. If the address in the message matches the station's address, the station receives the message. If the addresses do not match, the bus carries the message to the next station and so on. Figure 1.7 illustrates how devices such as file servers, workstations and printers are connected to the linear cable or the backbone.

Figure 1.7 Bus Topology

Mesh Topology

In a mesh topology, every node has a dedicated point-to-point link to every other node (Figure 1.8). Messages sent on a mesh network can take any of several possible paths from source to destination. A fully connected mesh network has n(n – 1)/2 physical links to link n devices. For example, if an organization has five nodes and wants to implement a mesh topology, then 5(5–1)/2, that is, 10 links are required. In addition, to accommodate those links, every device on the network must have n – 1 communication (input/output) ports.

A comparison of bus topology and mesh topology is given in Table 1.5.

Figure 1.8 Mesh Topology

Table 1.5 Comparison of Bus and Mesh Topology

  Bus topology  Mesh topology
• It uses a single cable to connect all devices with terminators at both ends. • It has a dedicated point-to-point link to every other node.
• It requires least amount of cabling. • It requires more amount of cabling.
• It incurs less cost. • It is expensive.
• Entire network shuts down if there is failure in backbone (cable). • If one link becomes unusable, it does not disable entire system.
• It is easy to install and reconfigure. • It is difficult to install and reconfigure.
• An example of this topology is cable TV network. • An example of this topology is mobile ad hoc network (MANet).

14. Explain ring, star and tree topologies along with their advantages and disadvantages.

       Ans: The ring, star and tree topology are detailed as under.

Ring Topology

In this topology, computers are placed on a circle of cable without any terminated ends since there are no unconnected ends (Figure 1.9). Every node has exactly two neighbours for communication purposes. All messages travel through a ring in the same direction (clockwise or counterclockwise) until it reaches its destination. Each node in the ring incorporates a repeater. When a node receives a signal intended for another device, its repeater regenerates the bits and passes them along the wire.

Figure 1.9 Ring Topology

The advantages of ring topology are as follows:

It is easy to install and reconfigure.
Every computer is given equal access to the ring. Hence, no single computer can monopolize the network.

The disadvantages of ring topology are as follows:

Failure in any cable or node breaks the loop and can take down the entire network.
Maximum ring length and number of nodes are limited.

Star Topology

In this topology, devices are not directly linked to each other; however, they are connected via a centralized network component known as hub or concentrator (Figure 1.10). The hub acts as a central controller and if a node wants to send data to another node, it boosts up the message and sends the message to the intended node. This topology commonly uses twisted-pair cable; however, coaxial cable or fibre-optic cable can also be used.

Figure 1.10 Star Topology

The advantages of star topology are as follows:

It is easy to install and wire.
The network is not disrupted even if a node fails or is removed from the network.
Fault detection and removal of faulty parts are easier in star topology.

The disadvantages of star topology are as follows:

It requires a longer length of cable.
If the hub fails, nodes attached to it are disabled.
The cost of the hub makes the network expensive when compared to bus and ring topologies.

Tree Topology

A tree topology combines characteristics of linear bus and star topologies (Figure 1.11). It consists of groups of star-configured workstations connected to a bus backbone cable. Not every node plugs directly to the central hub. The majority of nodes connect to a secondary hub that, in turn, is connected to the central hub. Each secondary hub in this topology functions as the originating point of a branch to which other nodes connect. A tree topology is best suited when the network is widely spread and partitioned into many branches.

Figure 1.11 Tree Topology

The advantages of tree topology are as follows:

The distance a signal can travel increases, as the signal passes through a chain of hubs.
It allows isolating and prioritizing communications from different nodes.
It allows for easy expansion of an existing network, which enables organizations to configure a network to meet their needs.

The disadvantages of tree topology are as follows:

If the backbone line breaks, the entire segment goes down.
It is more difficult to configure and wire than other topologies.

15. What are the different categories of a network?

Ans: There are no generally accepted criteria to classify the computer networks; however, two dimensions are considered more important, which are scale and transmission technology. On the basis of scale, computer networks can be classified into three types: local area network (LAN), metropolitan area network (MAN) and wide area network (WAN). On the basis of transmission technology, computer networks can be categorized as point-to-point networks and broadcast networks.

16. Explain the categories of networks based on scale.

Ans: A network can be as few as several personal computers on a small network or as large as the Internet, a worldwide network of computers. Today, when talking about networks, we are generally referring to three primary categories: LAN, MAN, and WAN.

Local Area Network

A LAN is a computer network that covers only a small geographical area (usually within a square mile or less) such as an office, home or building (Figure 1.12). In a LAN, connected computers have a network operating system installed onto them. One computer is designated as the file server, which stores all the software that controls the network. It also stores the software that can be shared by the computers attached to the network. Other computers connected to the file server are called workstations. The workstations can be less powerful than the file server and they may have additional software on their hard drives. On most LANs, cables are used to connect the computers. Generally, LAN offers a bandwidth of 10–100 Mbps.

Figure 1.12 Local Area Network

Figure 1.13 Metropolitan Area Network

Metropolitan Area Network

A MAN is a network of computers spread over a “metropolitan” area such as a city and its suburbs (Figure 1.13). As the name suggests, this sort of network is usually reserved for metropolitan areas where the city bridges its LANs with a series of backbones, making one large network for the entire city. It may be a single network such as a cable television network or it may be a means of connecting a number of LANs. Note that MAN may be operated by one organization (a corporate with several offices in one city) or be shared and used by several organizations in the same city.

Wide Area Network

A WAN is a system of interconnecting many computers over a large geographical area such as cities, states, countries or even the whole world (Figure 1.14). These kinds of networks use telephone lines, satellite links and other long-range communication technologies to connect. Such networks are designed to serve an area of hundreds or thousands of miles such as public and private packet switching networks and national telephone networks. For example, a company with offices in New Delhi, Chennai, and Mumbai may connect the LANs for each of those locations to each other through a WAN. Although a WAN may be owned or rented by private business, it is usually a public network designed to connect small and intermediate sized networks together. The largest WAN in existence is the Internet.

Figure 1.14 Wide Area Network

WAN offers many advantages to business organizations. Some of them are as follows:

It offers flexibility of location because not all the people using the same data have to work at the same site.
Communication between branch offices can be improved using e-mail and file sharing.
It facilitates a centralized company wide data backup system.
Companies located in a number of small and interrelated offices can store files centrally and access each other's information.

17. What are the two types of transmission technology available?

Ans: The two types of transmission technology that are available include broadcast networks and point-to-point networks.

In broadcast networks, a single communication channel is shared by all the machines of that network. When a short message, let us say a packet is sent by any machine, it is received by all the other machines on that network. This packet contains an address field which stores the address of the intended recipient. Once a machine receives a packet, it checks the address field. If the address mentioned in the address field of packet is matched with the address of the recipient machine, it is processed; otherwise, the packet is ignored. In broadcast systems, there is a special code in the address field of the packet which is intended for all the destinations. When a packet with this code is transmitted, it is supposed to be received and processed by all the machines on that network. This mode of operation is called broadcasting. Some of the networks also support transmission to a subset of machines what is called multicasting.

In point-to-point networks, there could be various intermediate machines (such as switching devices called nodes) between the pair of end points called stations. Thus, there could be various possible routes of different lengths for a packet to travel from the source to the destination. Various routing algorithms are considered and finally, one of them is chosen for the packets to travel from the source to the destination.

Generally, for small geographically localized network (such as LAN), broadcasting is considered favourable while larger networks (such as WAN) use point-to-point networks. If there is specifically one sender and one receiver in any point-to-point network, it is sometimes considered as unicasting.

18. Define Internet and write a brief history on Internet.

Ans: The word Internet is derived from two words: Interconnection and Networks. Also known as “the Net”, Internet is a worldwide system of computer networks, that is, a network of networks, which allows the participants (users) to share information. It consists of thousands of separately administered networks of various sizes and types. Each of these networks comprises tens of thousands of computers. Moreover, the total number of users of the Internet is known to be in millions. This high level of connectivity encourages an unparalleled degree of communication, resource sharing and information access.

The foundation of Internet was laid in 1969 by the Department of Defense (DOD) of United States of America. They wanted to create a computer network that could continue to function in the event of a disaster, such as a nuclear war. Even if a part of the network was damaged or destroyed, the rest of the system would continue to work. That network was known as ARPANET (Advanced Research Projects Agency Network), which linked US scientific and academic researchers. It was the forerunner of today's Internet. Later in 1980, another agency, the National Science Foundation (NSF) created a new network of computers based on ARPANET, called NSFNET, which turned out to be more efficient and capable. Initially, NSFNET was designed to link five super computers situated at the major universities of NSF and allowed only academic research. Over the time, this network expanded to include sites for business, universities, government etc. and finally becoming a network consisting of millions of computers, now known as the Internet. Now, it is probably the most powerful and important technological advancement since the introduction of the desktop computer. With the advancement of Internet, the quality, quantity and variety of information also grew. Today, the Internet is a repository of every type of information. Nowadays, an Internet user can get all sorts of information ranging from how to add to the design of a functional spaceship to how to choose a product for personal use.

19. Define protocol. Describe the key elements of protocols.

Ans: Protocol refers to a set of rules that coordinates the exchange of information between the sender and receiver. It determines what is to be communicated, and how and when it is to be communicated. Both sender and receiver should follow the same protocol to communicate data. Without the protocol, the sender and receiver cannot communicate with each other. The main elements of a protocol are as follows:

Syntax: It refers to the format of the data that is to be transmitted.
Semantics: It refers to the meaning and interpretation of each bit of data.
Timing: It defines when the data should be sent and with what speed.

20. What do you understand by the standards? Explain briefly.

Ans: Standards are crucial for establishing an open and worldwide marketplace for the manufacturers, vendors, suppliers, government agencies and other service providers to provide them worldwide interconnectivity. Standards define some common set of rules and guidelines which ensure universal interoperability of data and communication technology and processes. The standards for data communication are classified into two categories, which are de facto and de jure standards.

De facto: This is a Latin word which means “from the fact” or “by convention”. This category includes standards that have not been approved formally by some authority but have been accepted as standards because of their widespread use. For instance, the UNIX operating system has largely been used in computer science departments of most universities and thus has been adopted as the standard. Similarly, many manufacturers prefer to copy IBM machines while developing PCs for small office and home computers and thus, IBM PCs are the de facto standard. The de facto standards are usually founded by the manufacturers in an attempt to specify the functionality of some new technology or product.
De jure: This is also a Latin word which means “by law” or “by regulation”. Unlike de facto standards, the de jure standards are formally declared as legal standards by recognized standardization authorities. The international standardization authorities governing de jure standards may be established by treaty among governments of different nations or may comprise volunteers from different standard organizations without nay treaty.

21. Write short notes on intranet and extranet.


Intranet: This is a private network that is set up within an organization and also controlled by the organization; nobody outside the organization is permitted to access the network. Intranet utilizes the same protocols as used for accessing the Internet through a web browser. The users of intranet can access the basic services of the Internet such as e-mail. The difference between an intranet and the Internet is that an intranet user can view only those websites that are owned and maintained by the organization hosting the intranet. On the other hand, an Internet user may visit any website without any permission.
Extranet: This is an extended intranet owned, operated and controlled by an organization. In addition to allow access to members of an organization, an extranet uses firewalls, access profiles and privacy protocols to allow access to users from outside the organization. In essence, an extranet is a private network that uses Internet protocols and public networks to securely share resources with customers, suppliers, vendors, partners or other businesses.

22. Represent the message 5A.dat in ASCII code. Assume parity bit position (eighth bit) as 0.

Ans: Using the ASCII coding chart shown in Table 1.2, 5A.dat will be coded as shown here.

23. Represent HelLO34 in EBCDIC code.

Ans: Using the EBCDIC coding chart shown in Table 1.3, HelLO34 will be coded as shown here.

24. What is hybrid topology?

(a) Draw a hybrid topology with a star backbone and three bus networks.

(b) Draw a hybrid topology with a star backbone and four ring networks.

Ans: Hybrid topology is the combination of two or more topologies such that the resultant network does not retain the characteristics of any of the basic topologies including star, bus, ring and tree. A hybrid topology is created when two different basic topologies are connected.

(a) A hybrid topology with star backbone and three bus networks is shown in Figure 1.15.

Figure 1.15 Hybrid Topology with a Star Backbone and Three Bus Networks

(b) A hybrid topology with star backbone and four ring topologies is shown in Figure 1.16.

Figure 1.16 Hybrid Topology with a Star Backbone and Four Ring Networks

25. Assume a network with n devices. Calculate how many links are required to set up this network with mesh, ring, bus and star topologies?

Ans: The number of links required to set up this network with the various topologies are listed in Table 1.6.

Table 1.6 Topology along with Number of Links

Topology Number of links
Mesh (n(n – 1))/2
Ring n
Bus (n drop lines + one line for bus)
Star n

Multiple Choice Questions

1. Which of these is the part of a data communication system?
(a) Sender
(b) Message
(c) Protocol
(d) All of these
2. ASCII stands for
(a) American standard code for information identification
(b) American standard code for information interchange
(c) American standard coding for information interchange
(d) American standard coding for information identification
3. EBCDIC uses__________bits for each character.
(a) Six
(b) Seven
(c) Eight
(d) Two
4. In_______transmission mode, the flow of information is bidirectional at the same time.
(a) Half-duplex
(b) Simplex
(c) Full-duplex
(d) None of these
5. The amount of time taken by a message to travel from one device to another is known as
(a) Delay
(b) Response time
(c) Transit time
(d) Throughput
6. In star topology, devices are connected via a centralized network component known as
(a) Node
(b) Client
(c) Bus
(d) Hub
7. The network topology which uses hierarchy of nodes is
(a) Ring
(b) Tree
(c) Bus
(d) Fully connected
8. A MAN is___________in size as compared to a LAN.
(a) Larger
(b) Smaller
(c) Equal
(d) None of these
9. Internet is a
(a) LAN
(b) WAN
(c) MAN
(d) Both LAN and MAN
10. Which of these is not the key element of a protocol?
(a) Syntax
(b) Standard
(c) Semantics
(d) Timing


1. (d)

2. (b)

3. (c)

4. (c)

5. (c)

6. (d)

7. (b)

8. (a)

9. (b)

10. (b)