Data Communication Class 12 Notes: CBSE Computer Science

These are the Data Communication class 12 notes for Chapter 11 of the NCERT Class 12 Computer Science textbook, following the 2026-27 session.

The page condenses the whole chapter — the five components of communication, bandwidth and data transfer rate, communication modes, switching, transmission media, mobile generations and protocols — into definitions, tables and stepwise calculations you can revise from alone, with textbook page numbers cited so you can verify anything against the book.

Use the chapter map below to locate a weak area, then jump to it from the table of contents. If you have only a few minutes, read the Key Terms table, the worked examples and the Chapter at a Glance table at the end — those three carry most of what the chapter expects you to recall.

Chapter Map: What These Notes Cover

Every topic in this chapter builds on the earlier ones: components of communication → measuring the channel → modes → switching → media → wireless technologies → mobile generations → protocols. The map below shows where each topic sits in the textbook (pages 203–222) so you can revise a weak area directly.

The official textbook PDF is available on the official NCERT portal if you want to check any detail against the source.

For the full set of subject pages, browse the Class 12 notes collection, and the Class 12 Computer Science notes hub for the rest of this subject. Neighbouring networking ideas are explained in the Computer Networks notes, and the Security Aspects notes pair naturally with the protocol discussion later in this chapter.

You can also reach every subject from the main CBSE notes page.

What Is Data Communication? The Five Components

Data can be any text, image, audio, video or multimedia file. Communication is the act of sending or receiving data. So data communication is the exchange of data between two or more networked or connected devices that are capable of sending and receiving over a communication medium (NCERT, p. 203).

Personal computers, mobile phones and laptops are the usual examples of such devices.

Figure 11.1 shows the simplest version of this idea: a computer, a printer, a server and a switch are connected through media so that information can travel from one device to another. Whenever such devices communicate, five aspects matter — sender, receiver, message, communication media and protocols (NCERT, p. 204).

Figure 11.2 anchors these five pieces in one diagram — for any communication you meet, you should be able to point to its sender, receiver, message, media and protocols.

In computer communication the sender and receiver are together known as nodes (NCERT, p. 204).

Memory device — the letter-post chain. Posting a letter uses all five components: you are the sender, the letter is the message, the postal route is the communication media, the reader is the receiver, and the postal rulebook (address format, stamp, postage rate) is the protocol. Every network communication in this chapter fits the same five slots.

Key Terms and Definitions from This Chapter

This is the table to scan the night before a test. Every term is defined in the chapter.

Bandwidth and Data Transfer Rate: Formulas That Matter

In data communication the transmission medium is also known as the channel. A channel’s capacity is the maximum amount of signals or traffic it can carry, and it is measured in two ways: bandwidth and data transfer rate (NCERT, p. 205).

Bandwidth — a range of frequencies

Bandwidth is the range of frequencies available for transmission through a channel. It is the difference between the maximum and minimum frequency contained in the composite signal, measured in Hertz (Hz) (NCERT, p. 205). The wider the range, the more data the channel can carry — which is why higher bandwidth supports higher data transfer rates.

\[ \text{Bandwidth} = f_{\text{max}} – f_{\text{min}} \]

Frequency conversions use 1000, not 1024:

Data transfer rate — bits per second

Data transfer rate (also called bit rate) is the number of bits transmitted between source and destination in one second, measured in bits per second (bps). One signal carries one or more bits over the channel (NCERT, p. 205).

MBps ≠ Mbps. MBps is megabytes per second; Mbps is megabits per second. Since 1 byte = 8 bits, a value in MBps is 8 times smaller than the same value in Mbps (NCERT, p. 205).

Worked Examples: Calculating Data Rate and Bandwidth

Every calculation in this chapter is the same pair of operations — multiply to get total bits, then divide by time, or subtract the two frequencies. The textbook’s own example uploads 10 pages of 1600 characters at 8 bits each in 20 seconds and obtains 6400 bps = 6.25 Kbps (NCERT, p. 206). The examples below use fresh numbers so you can practise the identical method.

Example 1 — required data rate of a channel

Example 2 — bandwidth of a composite signal

Example 3 — converting Gbps to bits per second

Practical use — estimating transfer time

Simplex, Half Duplex and Full Duplex: The Three Communication Modes

Data communication between devices is classified into three modes based on direction and timing (NCERT, p. 206).

Simplex — one way

In simplex communication one device is always the sender and the other always the receiver; data flows only one way, and the devices use the entire capacity of the link. The chapter compares it to a one-way street.

Keyboards sending to a computer, audio sent to a speaker, and — with IoT — controlling home appliances such as fans and lights from outside the home are all simplex uses (NCERT, p. 206).

Figure 11.3 shows the IoT version of simplex: a user controlling an appliance such as a fan or light remotely, with commands flowing in one direction only.

Half-duplex — two way, one direction at a time

Half-duplex is bidirectional, but both devices cannot send at the same instant — while one sends, the other receives, and vice versa. The chapter compares it to vehicles sharing a one-way narrow bridge: they cannot pass simultaneously. Essentially, it is a simplex channel whose direction of transmission can be switched.

A walkie-talkie is the classic example — pressing push-to-talk turns on the transmitter and turns off the receiver, so others can only listen (NCERT, pp. 206–207).

Full-duplex — both directions at once

Full-duplex communication lets both devices send and receive simultaneously, like a two-way road. It is the mode used by mobile phones and landline telephones. The link’s capacity is shared between the two directions — either using two physically separate simplex lines (one for sending, one for receiving) or sharing the capacity of a single channel between the signals (NCERT, p. 207).

Figure 11.5 shows data travelling in both directions between two devices at the same moment — the defining feature of full duplex.

VoIP and VoLTE (NCERT, p. 208). VoIP (Voice over Internet Protocol) is a communication methodology designed to deliver both voice and multimedia communications over the Internet Protocol. VoLTE (Voice over Long-Term Evolution) is a standard for high-speed wireless communication for mobile phones, including IoT and wearables.

Circuit Switching vs Packet Switching

Connecting every pair of devices with a dedicated link (mesh topology) or through a central device (star topology) is costly in large networks. Switching solves this by routing data through various nodes, forming a temporary route for the transmission (NCERT, p. 208). The two common techniques are circuit switching and packet switching.

Circuit switching: before communication starts, a dedicated path — a connected sequence of links between network nodes — is identified between sender and receiver. All packets follow this same pre-established path. The traditional telephone system worked this way: switching equipment found a physical path all the way from the caller’s telephone to the receiver’s (NCERT, p. 208).

Packet switching: the message is broken into smaller pieces called packets. Each packet has two parts — a header containing the destination address and other information, and the main message part. Packets travel independently and may take different routes depending on availability; at the destination they are reassembled into the complete message.

Unlike circuit switching, a channel is occupied only during the transmission of a packet and is then free for other parties (NCERT, p. 208).

Because of this flexibility, the Internet uses packet switching.

Guided Media: Twisted Pair, Coaxial Cable and Optical Fibre

A transmission medium is anything that carries signals between source and destination — the electric wire carrying current to a ceiling fan, or simply the air between two people talking, as in Figure 11.6. In data communication, transmission media are the links that carry messages between communicating devices, and they divide into guided and unguided (NCERT, p. 209).

Guided media (wired) are physical links made of wire or cable — metallic cable, fibre-optic cable and so on — through which signals propagate between nodes. Unguided media (wireless) send data through air as electromagnetic waves using an antenna (NCERT, p. 209). Figure 11.7 shows this classification tree.

Twisted pair cable

A twisted pair consists of two insulated copper wires twisted like a DNA helical structure. Several such pairs are combined and covered with a protective outer wrapping, and each pair acts as a single communication link. The twisted configuration minimises electrical interference from similar pairs close by.

Twisted pairs are inexpensive and are the most common medium in telephone lines and LANs; they come as UTP (Unshielded twisted-pair) and STP (Shielded twisted-pair) (NCERT, pp. 210–211).

Coaxial cable

Coaxial cable is better shielded and offers more bandwidth than twisted pair. Its construction, shown in Figure 11.10, runs from the inside out: a copper wire at the core → insulating material around it → an outer conductor (usually a copper mesh) → a plastic cover.

This shielded design lets the copper core transmit data quickly without interference from environmental factors, so coaxial cable carries signals of higher frequencies over a longer distance (NCERT, p. 211).

Optical fibre

Optical fibre carries data as light inside a thin strand of glass, using refraction to direct the light through the media. A thin transparent glass core is covered with a layer of less dense glass called cladding, and the whole arrangement is covered with an outer jacket of PVC or Teflon (NCERT, p. 212).

Fibre is lightweight, offers higher bandwidth (so higher data transfer rates), carries signals over longer distances and is immune to electromagnetic noise. It is used in backbone networks — the segments that interconnect different LANs or subnetworks and provide a path for information exchange between them (NCERT, p. 211).

The drawbacks: optical fibre is expensive and unidirectional, so two cables are required for full-duplex communication (NCERT, p. 212).

Unguided Media: Radio Waves, Microwaves and Infrared

Unguided (wireless) media transmit information as electromagnetic waves through air. The usable electromagnetic spectrum for wireless communication ranges from 3 KHz to 900 THz, and it is divided into four categories — radio waves, microwaves, infrared waves and visible (light) waves. Of these, three are useful for wireless communication (NCERT, p. 212).

Figure 11.12 arranges the four wave families in order of increasing frequency: radio waves at the low-frequency end, then microwaves, then infrared, with visible light at the top. Read the figure together with Table 11.1 in the chapter (NCERT, p. 213):

• Radio waves are omni-directional — they spread in all directions — and the 3–300 KHz portion can penetrate walls.
• Microwaves are unidirectional and cannot penetrate solid objects such as walls, hills or mountains, so they need line-of-sight propagation: both communicating antennas must face each other.
• Infrared waves are very high frequency, cannot penetrate walls, and serve only short-distance point-to-point links.

For longer distances, dish-shaped antennas are mounted on tall buildings so they stay in line of sight. Because waves gradually become weaker after travelling through air, repeaters are installed to regenerate the signals with the same energy (NCERT, p. 210).

Bluetooth, Wi-Fi and WiMax: Wireless Networks You Use Daily

Bluetooth — short range, personal area

Bluetooth connects devices such as mobile phones, mouse, headphones, keyboards and computers over a short distance — about 10 metres at 1–2 Mbps. Each device carries a low-cost transceiver chip that uses the unlicensed 2.4 GHz frequency band.

Devices within 10 metres build a personal area network called a piconet, working in a master-slave configuration: one master can communicate with up to 7 active slaves at the same time. Up to 255 devices can form the network; of these, 8 can communicate at once and the rest stay inactive, waiting for a response command from the master (NCERT, p. 214).

Wi-Fi — wireless LAN

Wireless LAN is the IEEE standard numbered 802.11, popularly known as Wi-Fi. A WLAN consists of communicating devices (laptops, mobile phones) plus access points (APs) installed in buildings or floors.

An access point is a device that creates a wireless LAN by connecting to a wired router, switch or hub; all wireless devices communicate or access the Internet through it (NCERT, p. 214).

Figure 11.13 shows an access point creating a WLAN that mobile devices join to reach the wired network. The chapter lists these benefits of WLAN (NCERT, pp. 214–215):

• wireless connections can extend or replace an existing wired infrastructure
• increased access for mobile devices
• easy access to the Internet in public places
• users can move around within the network area while staying connected

WiMax — metropolitan area

WiMax (Worldwide Interoperability for Microwave Access) also works in wireless networks, but unlike Wi-Fi it uses a larger spectrum, offers a higher data transfer rate and spans a larger area — which is why it is used in MAN (metropolitan area network) applications (NCERT, p. 215).

The chapter gives numeric data rates only for Bluetooth; Wi-Fi and WiMax are compared qualitatively.

Mobile Telecommunication Generations: 1G to 5G

Mobile network architecture has evolved in landmark steps called generations — 1G, 2G, 3G, 4G and 5G (NCERT, p. 215). Each generation added something the previous one could not do.

M2M (Machine-to-Machine) is direct communication between devices, wired or wireless — the idea that lets machines exchange data without human involvement (NCERT, p. 216). The chapter’s Think and Reflect box asks you to explore how 5G can impact society.

Protocols: Why Rules Matter and Which Ones to Know

A protocol is a set of standard rules that the communicating parties — the sender, the receiver and all intermediate devices — need to follow. Protocols exist because the communicating devices may sit on different networks, at different geographic locations, with different data transfer rates and data formats (NCERT, p. 216).

Flow control and access control

Flow control handles speed mismatch. Figure 11.14 shows computer A sending at 1024 Mbps while computer B can receive at only 512 Mbps; if B cannot tell A to slow down, some data is lost. Access control decides which node may use a shared link at a particular instant; without it, simultaneous transmissions collide and the data gets lost or corrupted (NCERT, p. 216).

Protocols also define (NCERT, p. 217):

• how computers identify one another on a network
• the form to which data should be converted for transit
• how a node decides whether received data is for it or must be forwarded to another node
• how to ensure all the data reaches the destination without loss
• how to rearrange and process the packets at the destination

The five protocols you must know

One TCP/IP point worth remembering: because the Internet has many redundant connection paths, two packets of the same message can take different routes depending on congestion, and they are reassembled into the original message at the receiver’s end (NCERT, p. 220). Notice how this connects back to packet switching.

Since protocols govern access and data flow, they are closely tied to security — the Security Aspects notes on this site cover the related threats and protections.

Common Mistakes to Avoid in Data Communication

How to Write Answers for the NCERT Exercise Questions

Match your answer structure to the question type, and the exercise (NCERT, pp. 221–222) becomes straightforward:

• Definition questions (Q1, Q2): open with the definition, then list the five components with a one-line role each — sender sends, receiver receives, message is the data, media is the path, protocols are the rules.
• Comparison questions (Q5): use a two-column table (e.g. wired versus wireless) and close with a one-line conclusion.
• Calculation questions (Q9, Q13): show every step with units. Q9 is a pure conversion — expand Gbps to bps using the powers-of-2 ladder. Q13 uses bandwidth = maximum frequency − minimum frequency, after putting both frequencies in the same unit.
• Short notes (Q11a–e): define the term, give one example or application, then state why it matters.
• Explain with example (Q12): give the definition of protocol, then illustrate it with one concrete protocol such as HTTP or SMTP.

DNS note: DNS appears as short note (d) in Q11, but it is not defined anywhere in the chapter text, so keep that answer brief — two or three lines connecting the idea of naming a computer to the IP-address system that TCP/IP assigns to every node. Do not pad it with detail the chapter never gives.

Chapter at a Glance: Revision Summary

Frequently Asked Questions About Data Communication

What are the five components of data communication?

The five components are sender, receiver, message, communication media and protocols (NCERT, p. 204). The sender and receiver are together called nodes; the message is the data exchanged; media is the path it travels; protocols are the rules both sides follow.

What is the difference between bandwidth and data transfer rate?

Bandwidth is the range of frequencies a channel can carry — the difference between maximum and minimum frequency — measured in Hz. Data transfer rate is the number of bits transmitted per second, measured in bps. A higher bandwidth is what makes a higher data transfer rate possible (NCERT, p. 205).

Which communication mode do walkie-talkies and mobile phones use?

A walkie-talkie uses half-duplex — two-way communication but one direction at a time (push-to-talk to transmit). Mobile phones use full-duplex — both parties transmit and receive simultaneously (NCERT, pp. 206–207).

Why is optical fibre unidirectional and how does that affect full duplex communication?

Light in an optical fibre travels in one direction, so a single cable carries data only one way. Because full duplex needs both directions at the same time, two optical fibre cables are required for full-duplex communication (NCERT, p. 212).

What is the difference between circuit switching and packet switching?

Circuit switching establishes a dedicated path before communication begins and all packets follow that same path — the old telephone system is the classic example. Packet switching splits the message into packets with a header and a message part; packets travel independently, possibly on different routes, and are reassembled at the destination. The Internet uses packet switching (NCERT, p. 208).

Which wireless waves can pass through walls and which need line of sight?

Radio waves of frequency 3–300 KHz can penetrate walls. Microwaves cannot penetrate walls, hills or mountains and need line-of-sight propagation — both antennas must face each other. Infrared also cannot penetrate walls and works only for short-distance point-to-point communication (NCERT, p. 213).

Reference: NCERT Class 12 Computer Science textbook, chapter Data Communication.