2nd PUC Computer Science Chapter 10 Computer Networks Notes

10.1 Introduction to Computer Networks

We live in a connected world where information is produced, exchanged and traced across the globe in real time. Digital devices are interconnected in different ways.

Examples of Networks

• Social network
• Mobile network
• Network of computers
• Airlines, railway, banks and hospitals networks

A basic network may connect a few computers placed in a room. The network size can vary from small to large depending on the number of computers connected.

Hosts / Nodes

A computer network can include different types of hosts, also called nodes, such as:

• Server
• Desktop
• Laptop
• Cellular phones

Networks also include networking devices such as switch, router and modem. These devices are used to connect multiple computers in different settings.

Devices in a network can be connected through wired media such as cables or wireless media such as air.

A node can be a modem, hub, bridge, switch, router, digital telephone handset, printer, computer or server.

10.2 Evolution of Networking

In the 1960s, a research project was commissioned by the Advanced Research Projects Agency Network (ARPANET) in the U.S. Department of Defence to connect academic and research institutions located at different places for scientific collaboration.

The first message was communicated between the University of California, Los Angeles (UCLA) and Stanford Research Institute (SRI).

Gradually, more organisations joined ARPANET and many independent smaller networks were formed.

10.3 Types of Networks

Computer networks range from handheld devices such as mobile phones or tablets connected through Wi-Fi or Bluetooth within a room to millions of computers spread across the globe.

Based on the geographical area covered and data transfer rate, computer networks are broadly categorised as:

• PAN — Personal Area Network
• LAN — Local Area Network
• MAN — Metropolitan Area Network
• WAN — Wide Area Network

10.3.1 Personal Area Network (PAN)

• Devices lie within an approximate range of 10 metres.
• A PAN may be wired or wireless.
• A mobile phone connected to a laptop through USB forms a wired PAN.
• Two smartphones communicating through Bluetooth form a wireless PAN or WPAN.

10.3.2 Local Area Network (LAN)

The geographical area covered by a LAN can range from:

• A single room
• A floor
• An office having one or more buildings in the same premise
• Laboratory
• School
• College
• University campus

Connectivity can be through wires, Ethernet cables, fibre optics or Wi-Fi.

• LAN is comparatively secure because only authentic users in the network can access other computers or shared resources.
• Users can print documents using a connected printer.
• Documents and software can be uploaded/downloaded to and from the local server.
• LAN provides short-range communication with high-speed data transfer rates.
• LAN can be extended up to 1 km.
• Data transfer usually varies from 10 Mbps called Ethernet to 1000 Mbps called Gigabit Ethernet.

10.3.3 Metropolitan Area Network (MAN)

• Data transfer rate in MAN is in Mbps.
• It is considerably less as compared to LAN.
• Cable TV network and cable-based broadband internet services are examples of MAN.
• It can be extended up to 30–40 km.
• Many LANs can be connected together to form a MAN.

10.3.4 Wide Area Network (WAN)

• A WAN can be formed by connecting LANs through wired or wireless media.
• Large business, educational and government organisations connect branches at different locations through WAN.
• The Internet is the largest WAN.
• It connects billions of computers, smartphones and millions of LANs from different continents.

10.4 Network Devices

Different devices are required to communicate data through different transmission media and to configure networks with different functionality.

The chapter discusses Modem, Ethernet Card, RJ45, Repeater, Hub, Switch, Router and Gateway.

10.4.1 Modem

Computers store and process data as 0s and 1s. During transmission, digital data are converted into an analog signal and the medium carries the signal to the receiver.

• The modem at the sender’s end acts as a modulator.
• It converts digital data into analog signals.
• The modem at the receiver’s end acts as a demodulator.
• It converts analog signals into digital data.

10.4.2 Ethernet Card

• It acts as an interface between the computer and the network.
• It is a circuit board mounted on the motherboard of a computer.
• An Ethernet cable connects the computer to the network through NIC.
• Ethernet cards can support data transfer between 10 Mbps and 1 Gbps (1000 Mbps).
• Each NIC has a MAC address, which helps uniquely identify the computer on the network.

10.4.3 RJ45

• It is a standard networking interface.
• It can be seen at the end of network cables.
• It is a small plastic plug that fits into RJ-45 jacks of Ethernet cards.

Internet Service Provider (ISP)

10.4.4 Repeater

Data are carried in the form of signals over a cable. These signals can travel a specified distance, usually about 100 m. Beyond this limit, signals lose their strength and become weak.

10.4.5 Hub

10.4.6 Switch

• When data arrives, the switch extracts the destination address from the data packet.
• It looks up the destination address in a table.
• It sends signals only to selected devices instead of sending them to all.
• It can forward multiple packets at the same time.
• A switch does not forward noisy or corrupted signals.
• It drops such signals and asks the sender to resend them.
• Ethernet switches are common in homes and offices to connect multiple devices and create LANs or access the Internet.

10.4.7 Router

• A router connects a local area network to the Internet.
• Compared to a hub or switch, a router has advanced capabilities.
• It can analyse data carried over a network.
• It can decide or alter how data is packaged.
• It can send data to another network of a different type.
• If a different network cannot handle bigger packets, the data can be repackaged as smaller packets and sent by the router.
• A router can be wired or wireless.
• A wireless router can provide Wi-Fi access to smartphones and other devices.
• Home Wi-Fi routers may perform the dual task of a router and a modem/switch.

10.4.8 Gateway

• It serves as the entry and exit point of a network.
• Data coming into or going out of a network must first pass through the gateway to use routing paths.
• It maintains information about internal connection paths and identified paths of remote networks.
• A packet going from one network to a foreign network passes to the gateway.
• The gateway routes it to the destination using the best possible route.
• For simple Internet connectivity at homes, the gateway is usually the Internet Service Provider.
• Generally, a router is configured to work as a gateway.
• A gateway can be implemented completely in software, hardware, or a combination of both.
• A firewall is usually integrated with a gateway because it is placed at the edge of a network.

10.5 Networking Topologies

Common network topologies are:

• Mesh
• Ring
• Bus
• Star
• Tree or Hybrid

10.5.1 Mesh Topology

In mesh topology, each communicating device is connected with every other device in the network.

• It can handle large amounts of traffic.
• Multiple nodes can transmit data simultaneously.
• It is more reliable because failure of one node does not break transmission between other nodes.
• It is more secure compared to other topologies because each cable between two nodes carries different data.
• Wiring is complex.
• Cabling cost is high.
• There are many redundant or unutilised connections.

10.5.2 Ring Topology

In ring topology, each node is connected to two other devices, one on either side. The connected nodes form a ring.

• The link in a ring topology is unidirectional.
• Data can be transmitted in one direction only: clockwise or counterclockwise.

10.5.3 Bus Topology

In bus topology, each communicating device connects to a transmission medium called bus.

• Data sent from a node are passed onto the bus.
• Data are transmitted along the length of the bus in both directions.
• Any node connected to the bus can receive the data.
• A single backbone wire is shared among the nodes.
• It is cheaper and easier to maintain.
• Bus topology is considered less secure and less reliable.

10.5.4 Star Topology

In star topology, each communicating device is connected to a central node, which is a networking device such as a hub or switch.

• It is considered very effective, efficient and fast.
• Each device is directly connected with the central device.
• Disturbance in one device does not affect the rest of the network.
• Failure of the central networking device may cause failure of the complete network.
• The central node can be a broadcasting device or a unicast device.
• A broadcasting device transmits data to all nodes.
• A unicast device identifies the destination and forwards data only to that node.

10.5.5 Tree or Hybrid Topology

• Such topologies are usually realised in WANs where multiple LANs are connected.
• The LANs may be in the form of ring, bus or star.
• The textbook figure shows a hybrid topology connecting four star topologies in a bus.
• Data transmitted from the source first reaches the centralised device.
• From there, data passes through every branch.
• Each branch can have links for more nodes.

10.6 Identifying Nodes in a Networked Communication

Each node in a network should be uniquely identified so that a network device can identify the sender and receiver and decide a routing path to transmit data.

10.6.1 MAC Address

• The MAC address is engraved on the NIC at the time of manufacturing.
• It is a permanent address and cannot be changed under any circumstances, as stated in the textbook.
• The machine on which the NIC is attached can be physically identified on the network using its MAC address.
• Each MAC address is a 12-digit hexadecimal number.
• It is 48 bits in length.
• The first six digits (24 bits) contain the manufacturer’s ID called Organisational Unique Identifier (OUI).
• The later six digits (24 bits) represent the serial number assigned to the card by the manufacturer.

10.6.2 IP Address

• IP addresses are assigned to nodes in a network using Internet Protocol for communication.
• If a computer’s IP address is known, communication with that computer is possible from anywhere in the world.
• Unlike a MAC address, an IP address can change if a node is removed from one network and connected to another network.

IPv4

• IPv4 is a 32-bit numeric address.
• It is written as four numbers separated by periods.
• Each number is the decimal representation of an 8-bit binary number.
• Each number can have a value from 0 to 255.

192.168.0.178

IPv6

With increasing numbers of devices connected to the Internet, it was realised that the 32-bit IP address would not be sufficient because it offers just under 4.3 billion unique addresses.

• A 128-bit IP address called IPv6 was proposed.
• An IPv6 address is represented by eight groups of hexadecimal numbers.
• The groups are separated by colons.

2001:CDBA:0000:0000:0000:0000:3257:9652

10.7 Internet, Web and the Internet of Things

Smart electronic appliances such as TV, AC, refrigerator, fan and light can also communicate through a network. Other examples mentioned in the chapter include drones, vehicles, door locks and security cameras.

The Internet architecture is continuously evolving. Computers may be connected to a modem through a cable or wirelessly using Wi-Fi.

A local Internet Service Provider connects to a national network. Many ISPs connect together to form a regional network, regional networks connect to form a national network, and country-wise networks form the Internet backbone.

The Internet is used for education, creativity, entertainment, socialisation and e-commerce.

10.7.1 The World Wide Web (WWW)

Earlier, to access files residing in different computers, a person had to log in individually to each computer through the Internet. Files in different computers could also be in different formats, making it difficult to understand each other’s files and documents.

Sir Tim Berners-Lee, a British computer scientist, invented the World Wide Web in 1990 by defining three fundamental technologies that led to the creation of the web.

Three Fundamental Web Technologies

Internet vs World Wide Web

10.8 Domain Name System

The Internet contains millions of websites. Each website is stored on a server connected to the Internet, which means each server has an IP address.

To access a website, we need its IP address. However, remembering the IP addresses of different websites is difficult because they are numerical or strings.

The textbook compares this concept to a phonebook: instead of remembering a person’s phone number, a name is assigned to the number.

Table 10.1 — Examples of Domain Names and Their Mapped IP Addresses

10.8.1 DNS Server

A browser needs to find the IP address corresponding to the domain name entered by the user.

How DNS Resolution Works

1. The user enters a URL in a web browser.
2. The HTTP protocol approaches a DNS server.
3. The DNS server provides the IP address corresponding to the domain name.
4. After obtaining the IP address, HTTP retrieves the information.
5. The information is loaded in the browser.

DNS Hierarchy

DNS servers are placed in hierarchical order.

• At the top level, there are 13 root servers.
• Below the root servers are DNS servers at different levels.
• A DNS server may contain the IP address corresponding to a domain.
• It may also contain the IP address of another DNS server where the domain entry can be searched.

Chapter Summary

• A computer network is an interconnection among two or more computers or computing devices.
• A computer network allows computers to share data and resources.
• Networking devices connect multiple computers in different settings.
• A node is a device that can receive, create, store or send data through network routes.
• Networks are broadly categorised into PAN, LAN, MAN and WAN based on geographical area and data transfer rate.
• LAN covers a limited area and provides high-speed communication.
• MAN is an extended form of LAN covering a city or town.
• WAN connects LANs and MANs over large geographical locations.
• The Internet is the largest WAN.
• Modem stands for MOdulator DEModulator.
• Ethernet card is also known as Network Interface Card or NIC.
• Each NIC has a MAC address.
• Repeater regenerates weakened signals.
• A switch connects multiple devices and forwards data to selected devices.
• A router receives, analyses and transmits data to other networks.
• A gateway acts as the entry and exit point of a network.
• Topology means the arrangement of computers and peripherals in a network.
• Common topologies are Mesh, Ring, Bus, Star and Tree/Hybrid.
• MAC is the physical or hardware address associated with a NIC.
• IP address uniquely identifies a node in an IP network and can change when a node changes networks.
• IPv4 is 32-bit, while IPv6 is 128-bit.
• The Internet is the global network of computing devices.
• WWW is a collection of interlinked webpages and web resources accessible over the Internet.
• Sir Tim Berners-Lee invented the World Wide Web in 1990.
• HTML, URI/URL and HTTP are the three fundamental web technologies discussed in the chapter.
• Domain names or hostnames correspond to IP addresses.
• Domain name resolution converts a domain name into its corresponding IP address.
• A DNS server performs domain name resolution.

Questions & Answers

Important Questions

1 Mark

1. Expand ARPANET.
2. Expand MAC.
3. Expand ISP.
4. Expand URI.
5. What is a network?
6. What is a node?
7. What is RJ45?
8. What does DNS stand for?
9. Who invented the World Wide Web?
10. What is a gateway?

2 Marks

1. Mention any two advantages of using a network of computing devices.
2. What is a modem? Why is it used?
3. What is a repeater?
4. What is a switch?
5. What is an Ethernet card?
6. What is an IP address?
7. What is a domain name?
8. What is domain name resolution?

3 Marks

1. Differentiate between LAN and WAN.
2. Define topology and name the popular network topologies.
3. Explain PAN, LAN and MAN.
4. How is tree topology different from bus topology?
5. How is IP address different from MAC address?
6. Explain the difference between Internet and WWW.

4/5 Marks

1. Explain the different networking devices.
2. Explain the different types of network topologies.
3. Draw a network layout of star topology and bus topology connecting five computers.
4. Explain MAC address and IP address.
5. Explain DNS and DNS server.
6. Explain the difference between World Wide Web and Internet with suitable examples.

⚡ Quick Revision

Number of wires = n(n−1)/2