Data Communication: CBSE Class 12 Computer Science Notes and Questions

What Data Communication is about

Weightage: This chapter is part of Unit 2, Computer Networks, which carries 10 of the 70 theory marks together with Computer Networks.Source: CBSE Computer Science (083) syllabus 2026-27, distribution of marks, Unit 2 Computer Networks

Key concepts

Communication and its five components

NCERT section NCERT 11.1, 11.2

Data communication is the exchange of data between two or more devices (computers, phones, printers, servers) over a transmission medium. It needs five components: the sender (the device that sends the message), the receiver (the device that gets it), the message (the data itself: text, numbers, images, audio, video), the communication medium or channel (the physical or wireless path, such as a cable or radio waves), and the protocol (the set of rules both sides follow for format, timing and error handling, so that they understand each other).

Worked example

A student sends her project report from her laptop to the teacher's e-mail, which the teacher reads on a phone over Wi-Fi. Identify the five components.

Try it first, then show the solutionSolution

1. Sender: the student's laptop. Receiver: the teacher's phone.
2. Message: the project report (the e-mail and attachment).
3. Medium: Wi-Fi radio waves and the Internet's cables between them.
4. Protocols: SMTP to send the mail; POP3 (or a similar protocol) to fetch it; TCP/IP underneath.

Answer: Laptop, phone, report, Wi-Fi and Internet links, SMTP/POP3/TCP-IP.

Bandwidth and data transfer rate

NCERT section NCERT 11.3

The capacity of a channel is measured in two ways. Bandwidth is the range of frequencies a channel can carry: the highest frequency minus the lowest, measured in hertz (1 kHz = 1000 Hz, 1 MHz = 1000 kHz, 1 GHz = 1000 MHz). Higher bandwidth allows a higher data rate. Data transfer rate (bit rate) is the number of bits sent per second, measured in bps. NCERT uses binary multiples: 1 Kbps = 1024 bps, 1 Mbps = 1024 Kbps, 1 Gbps = 1024 Mbps, 1 Tbps = 1024 Gbps. (Network equipment often uses 1000 instead; in the board exam follow the convention given in the question, else NCERT's.) Note Mbps (megabits) versus MBps (megabytes): 1 byte = 8 bits.

Worked example

(a) A signal uses frequencies from 300 MHz to 1 GHz. Find its bandwidth. (b) Convert 2 Gbps to Kbps using NCERT's units. (c) How long does a 40 MB file take on a 8 Mbps line (take 1 MB = 8 Mb, ignore overheads)?

Try it first, then show the solutionSolution

1. (a) 1 GHz = 1000 MHz, so bandwidth = 1000 - 300 = 700 MHz.
2. (b) 2 Gbps = 2 × 1024 Mbps = 2048 Mbps = 2048 × 1024 Kbps = 2,097,152 Kbps.
3. (c) 40 MB = 320 Mb; 320 / 8 = 40 seconds.

Answer: (a) 700 MHz (b) 2,097,152 Kbps (c) 40 s

IP address

NCERT section NCERT 10.6.2 (IP address)

An IP address (Internet Protocol address) is a numeric label that identifies a device on a network so data can be delivered to it. IPv4 uses 32 bits written as four decimal numbers from 0 to 255 separated by dots, e.g. 192.168.1.10. Because 32 bits give only about 4.3 billion addresses, IPv6 uses 128 bits written as eight groups of hexadecimal digits separated by colons, e.g. 2001:db8:0:0:0:0:0:1. An IP address can change when a device joins a different network, unlike the MAC address fixed on its network card.

Worked example

Which of these are valid IPv4 addresses? (i) 192.168.10.5 (ii) 256.10.1.1 (iii) 10.0.0 (iv) 172.16.254.1

Try it first, then show the solutionSolution

1. Each of the four parts must be 0-255 and there must be exactly four parts.
2. (ii) has 256, which is above 255: invalid.
3. (iii) has only three parts: invalid.
4. (i) and (iv) satisfy both rules.

Answer: (i) and (iv)

Switching techniques: circuit and packet switching

NCERT section NCERT 11.5

Switching decides how data finds its way through a network of switches and routers. In circuit switching, a dedicated path is set up between sender and receiver before communication starts, kept for the whole session and released at the end, like the old landline telephone call: the data follows one fixed route and arrives in order, but the path is reserved even when nobody is speaking. In packet switching, the message is broken into small packets, each carrying the destination address and a sequence number; packets travel independently, possibly by different routes, and are reassembled in order at the receiver. No path is reserved, so links are shared efficiently; the Internet uses packet switching.

Worked example

Give two differences between circuit switching and packet switching, and say which suits sending an e-mail with an attachment.

Try it first, then show the solutionSolution

1. Path: circuit switching reserves one dedicated path for the session; packet switching reserves none and packets may take different routes.
2. Use of links: in circuit switching the path is wasted when idle; packet switching shares links among many users.
3. E-mail data can be split, sent independently and reassembled: packet switching.

Answer: Dedicated path vs no reserved path; idle reservation vs shared links. E-mail: packet switching.

Wired (guided) media

NCERT section NCERT 11.6.1

Medium	Construction	Pros	Cons	Typical use
Twisted pair	two insulated copper wires twisted together; UTP (unshielded) and STP (shielded)	cheap, flexible, easy to install	low bandwidth; suffers interference; short distances (about 100 m per run)	telephone lines, LANs (with RJ45)
Co-axial cable	copper core, insulation, braided metal shield, plastic cover	better shielding and higher bandwidth than twisted pair; longer runs	thicker, costlier, harder to install	cable TV, older LANs
Fibre-optic cable	thin glass core with cladding; carries light	highest bandwidth, long distances, immune to electrical interference, light	expensive; difficult to install and join; needs care	backbones, links between buildings, ISP lines

Worked example

Choose the most suitable wired medium: (a) connecting two buildings 400 m apart with very high-speed data (b) connecting 20 computers in one lab cheaply (c) a cable TV connection at home.

Try it first, then show the solutionSolution

1. (a) Long distance, high speed, no interference: fibre-optic.
2. (b) Short runs, low cost: twisted pair (UTP with RJ45).
3. (c) Shielded, higher frequency over a moderate distance: co-axial.

Answer: (a) Fibre-optic (b) Twisted pair (c) Co-axial

Wireless (unguided) media

NCERT section NCERT 11.6.2

Wireless media carry data as electromagnetic waves through the air.

Medium	Frequency (NCERT)	Properties	Uses
Radio waves	about 3 kHz - 1 GHz	travel in all directions (omnidirectional); can pass through walls; long range at lower frequencies	AM/FM radio, cordless phones, walkie-talkies, Wi-Fi and Bluetooth use radio frequencies
Microwaves	about 1 GHz - 300 GHz	travel in straight lines (line of sight), need aligned antennas on towers; cannot pass through obstacles well	mobile phone networks, satellite links, long-distance point-to-point links
Infrared	about 300 GHz - 400 THz	very short range, line of sight, cannot pass through walls (so secure within a room)	TV remote controls, short-range device-to-device links

Worked example

Which wireless medium suits: (a) a TV remote (b) a link between two towers 30 km apart (c) a walkie-talkie used inside a building?

Try it first, then show the solutionSolution

1. (a) Short range, line of sight, confined to a room: infrared.
2. (b) Point-to-point line-of-sight link over long distance: microwaves.
3. (c) Must go through walls in all directions: radio waves.

Answer: (a) Infrared (b) Microwaves (c) Radio waves

Network protocols

NCERT section NCERT 11.8

A protocol is a set of rules for communication. The nine in the syllabus:

Protocol	Full form	Job
HTTP	HyperText Transfer Protocol	fetches web pages between browser and web server
HTTPS	HyperText Transfer Protocol Secure	HTTP with encryption, for logins and payments
FTP	File Transfer Protocol	uploads and downloads files between computers
PPP	Point-to-Point Protocol	direct link between two nodes, e.g. a dial-up or DSL link to an ISP
SMTP	Simple Mail Transfer Protocol	sends e-mail from a client to a server and between mail servers
POP3	Post Office Protocol version 3	downloads e-mail from the server to the user's device
TCP/IP	Transmission Control Protocol / Internet Protocol	IP addresses and routes packets; TCP splits data into packets, ensures delivery and reassembles them in order
TELNET	Teletype Network	logs in to and controls a remote computer through a text terminal (not encrypted)
VoIP	Voice over Internet Protocol	carries voice (and video) calls as packets over the Internet

Worked example

Name the protocol used: (a) to send an e-mail (b) to download e-mails to your phone (c) to make a WhatsApp voice call (d) to open a bank's login page securely (e) to upload a website's files to the hosting server.

Try it first, then show the solutionSolution

1. (a) Sending mail: SMTP. (b) Fetching mail to the device: POP3.
2. (c) Voice over the Internet: VoIP.
3. (d) Secure web page: HTTPS. (e) Transferring files: FTP.

Answer: (a) SMTP (b) POP3 (c) VoIP (d) HTTPS (e) FTP

Data Communication in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Topics in this chapter

• Network ProtocolsComing soon
• Transmission MediaComing soon
• Switching TechniquesComing soon
• Communication Components, Bandwidth, Data Rate and IP Address
• Network Protocols: HTTP, HTTPS, FTP, SMTP, POP3, TCP/IP, TELNET, VoIP, PPP
• Switching Techniques and Transmission Media

Data Communication: syntax and key facts

Bandwidth

Bandwidth = highest frequency - lowest frequency, in Hz (1 kHz = 1000 Hz; 1 MHz = 1000 kHz; 1 GHz = 1000 MHz)

Higher bandwidth allows a higher data transfer rate.

Data transfer rate units (NCERT)

1 Kbps = 1024 bps; 1 Mbps = 1024 Kbps; 1 Gbps = 1024 Mbps; 1 Tbps = 1024 Gbps

Mbps = megabits per second; MBps = megabytes per second (× 8 for bits).

Transfer time

time = size in bits / data transfer rate in bps

Convert bytes to bits first (× 8).

IPv4 vs IPv6

IPv4: 32 bits, 4 decimal parts 0-255 separated by dots. IPv6: 128 bits, 8 hexadecimal groups separated by colons.

A part above 255 makes an IPv4 address invalid.

Media ranking

Speed and distance: twisted pair < co-axial < fibre-optic. Cost: the same order.

Wireless: radio (all directions, through walls), microwave (line of sight), infrared (short, in-room).

Mail pair

SMTP sends mail; POP3 receives (downloads) mail

HTTP vs HTTPS: the S means encrypted.

How the board asks Data Communication

The question patterns to expect, and the method that works for each.

Tricks to remember Data Communication

Short memory aids. Say each one aloud once, then use it on a question the same day.

• "Smart Rabbits Make Many Plans"

  Sender, Receiver, Message, Medium, Protocol.

  Use it when: Listing the five components.

• SMTP Sends, POP3 Pulls

  S for Send; P for Pull (download).

  Use it when: E-mail protocol questions.

• Twist, Coax, Glass

  Cost, speed and distance rise in the order twisted pair → co-axial → fibre (glass).

  Use it when: Comparing wired media.

• Radio roams, Micro aims, Infra stays in

  Radio goes in all directions and through walls; microwaves aim in a straight line; infrared stays inside the room.

  Use it when: Choosing a wireless medium.

• 1024 ladder

  bps → Kbps → Mbps → Gbps → Tbps, each step × 1024 (NCERT).

  Use it when: Unit conversions.

Data Communication: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In the board paper this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Data Communication

The slips examiners see most often, and what to write instead.

• ✗Mistake: Mixing up bandwidth and data transfer rate

  Both describe capacity, but bandwidth is a frequency range (Hz) and data rate is bits per second (bps).

  ✓Do this: Write the unit with every answer: Hz for bandwidth, bps for data transfer rate.

  Usually costs: 1 mark

• ✗Mistake: Using POP3 for sending e-mail

  POP3 downloads mail from the server; SMTP sends it.

  Wrong working

  Protocol to send e-mail: POP3

  Right working

  Protocol to send e-mail: SMTP

  ✓Do this: SMTP Sends, POP3 Pulls.

  Usually costs: 1 mark

• ✗Mistake: Forgetting to convert bytes to bits

  File sizes are in bytes (MB) but line speeds are in bits (Mbps).

  Wrong working

  40 MB on 8 Mbps takes 40 / 8 = 5 s

  Right working

  40 MB = 320 Mb; 320 / 8 = 40 s

  ✓Do this: Multiply bytes by 8 before dividing by the speed.

  Usually costs: 1 mark

• ✗Mistake: Suggesting twisted pair or radio for a long, high-speed inter-building link

  The board expects fibre-optic for long, high-bandwidth, interference-free wired links.

  ✓Do this: Answer 'fibre-optic cable' and give the reason: high bandwidth, long distance, immune to electromagnetic interference.

  Usually costs: 1 mark in the network case

• ✗Mistake: Writing that circuit switching sends packets by different routes

  That is packet switching; circuit switching uses one dedicated path for the whole session.

  ✓Do this: Circuit = one reserved path; Packet = independent packets, no reservation, reassembled at the end.

  Usually costs: 1 mark

NCERT exercise guide: Data Communication

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Data Communication

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

1. Q1 · 1 mark · Very short answerBoard level

   Expand: (a) PPP (b) TELNET (c) HTTPS (d) TCP/IP

   Hint

   Full forms.

   Answer

   (a) Point-to-Point Protocol (b) Teletype Network (c) HyperText Transfer Protocol Secure (d) Transmission Control Protocol / Internet Protocol

2. Q2 · 1 mark · MCQBoard level

   Which medium is least affected by electromagnetic interference? (a) UTP (b) STP (c) Co-axial (d) Fibre-optic

   Hint

   It carries light.

   Answer

   (d) Fibre-optic

3. Q3 · 1 mark · Very short answerBoard level

   Convert 3 Mbps to bps using NCERT units.

   Hint

   3 × 1024 × 1024.

   Answer

   3,145,728 bps

4. Q4 · 1 mark · Assertion-reasonHOTS / competency

   Assertion (A): Infrared is suitable for communication between two buildings.
   Reason (R): Infrared waves cannot pass through walls.

   Hint

   Is A true?

   Answer

   (d) A is false but R is true: infrared is short-range and blocked by walls, so it does not suit inter-building links.

5. Q5 · 2 marks · Short answerBoard level

   Name the component of data communication in each: (a) the rules both phones follow during a call (b) the photo being shared (c) the Wi-Fi signal (d) the phone that receives the photo.

   Hint

   Five components.

   Answer

   (a) Protocol (b) Message (c) Communication medium (d) Receiver

6. Q6 · 2 marks · Short answerHarder board level

   A 750 MB video is sent over a 25 Mbps line. How long does it take? (1 MB = 8 Mb)

   Hint

   Convert to bits first.

   Answer

   750 × 8 = 6000 Mb; 6000 / 25 = 240 s (4 minutes).

7. Q7 · 2 marks · Short answerBoard level

   Differentiate between bandwidth and data transfer rate with units.

   Hint

   Hz vs bps.

   Answer

   Bandwidth: range of frequencies a channel can carry (highest minus lowest), in Hz. Data transfer rate: number of bits sent per second, in bps (Kbps, Mbps, Gbps). Higher bandwidth allows a higher data rate.

8. Q8 · 2 marks · Short answerHOTS / competency

   Why can two packets of the same message arrive in a different order, and how is the message still read correctly?

   Hint

   Different routes; sequence numbers.

   Answer

   In packet switching each packet is routed independently, so packets may take paths of different lengths or congestion. TCP numbers the packets and reassembles them in order at the receiver, asking again for any that are missing.

9. Q9 · 2 marks · Short answerHOTS / competency

   A village 40 km from the nearest town, with hills between them, needs Internet. Suggest a medium and justify. Which protocol would a farmer use to talk to an expert by video?

   Hint

   No cable; no line of sight across hills.

   Answer

   Satellite (microwave) link: works over long distances and difficult terrain where cable cannot be laid and towers cannot see each other. Video talk: VoIP.

10. Q10 · 1 mark · Very short answerHarder board level

    Why is TELNET considered unsafe for logging in to a remote computer?

    Hint

    Compare with HTTPS.

    Answer

    TELNET sends data, including the password, as plain text without encryption, so anyone intercepting the traffic can read it.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1. 1.Name the five components of data communication.Show answer

   Sender, receiver, message, communication medium, protocol.

2. 2.In which unit is bandwidth measured?Show answer

   Hertz (Hz).

3. 3.How many bits are there in an IPv4 address and in an IPv6 address?Show answer

   32 bits and 128 bits.

4. 4.Which switching technique does the Internet use?Show answer

   Packet switching.

5. 5.Which wired medium carries light signals?Show answer

   Fibre-optic cable.

6. 6.Which wireless medium is used by a TV remote?Show answer

   Infrared.

7. 7.Expand VoIP and state its use.Show answer

   Voice over Internet Protocol: making voice or video calls over the Internet.

8. 8.Which protocol lets you log in to a remote computer through a text terminal?Show answer

   TELNET.

9. 9.1 Gbps equals how many Mbps (NCERT)?Show answer

   1024 Mbps.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Components: sender, receiver, message, medium, protocol.
• ☐Bandwidth: range of frequencies, in Hz. Data transfer rate: bits per second.
• ☐NCERT units: each step from bps to Tbps is × 1024; 1 byte = 8 bits.
• ☐IPv4: 32 bits, four parts 0-255. IPv6: 128 bits, hexadecimal groups.
• ☐Circuit switching: dedicated path for the session. Packet switching: independent packets, reassembled.
• ☐Twisted pair: cheap, short, LANs. Co-axial: shielded, cable TV. Fibre: light, fastest, longest, no interference, costly.
• ☐Radio: all directions, through walls. Microwave: line of sight, towers and satellites. Infrared: short range, in-room.
• ☐HTTP/HTTPS web; FTP files; SMTP send mail; POP3 receive mail; PPP point link; TCP/IP packets and addresses; TELNET remote login; VoIP calls.

Data Communication: frequently asked questions