NCERT Solutions Class 12 Computer Science Chapter 11 Data Communication

Write this as five labelled headings, not a paragraph. Examiners look for the five named components, each with a one-line role, so give each one its own line to secure full marks. A good order is message, sender, receiver, medium, protocol, because that matches the natural left-to-right flow of a single transmission.

• The sender and the receiver are roles, not fixed machines. In a chat between two phones, each phone is a sender at one instant and a receiver the next.
• Do not treat the protocol as an afterthought. It is the component that turns five separate pieces of hardware into a working communication system.
• If the question carries three or more marks, add that the message can be text, numbers, images, audio or video, because that extra detail often carries a separate mark.

Answer: Five components: message, sender, receiver, transmission medium, protocol. Sender and receiver are roles the same device can swap between.

The single mistake that costs this mark is reading "both directions" and stopping there. Half-duplex also allows both directions, so the deciding word is "simultaneously", and only full-duplex meets it. Underline that one word before you answer.

• Attach a real device to each mode: simplex is a radio, half-duplex is a walkie-talkie, full-duplex is an ordinary phone call.
• For a longer answer, add that full-duplex needs more capacity because both directions are live together, which is why such links generally cost more.

Answer: Full-duplex, sends and receives at the same time. Watch the word "simultaneously", it rules out half-duplex.

The whole question hangs on one idea: speed and area pull in opposite directions. Once you fix "smaller area means faster, larger area means slower", both answers come free. This rule also protects you against the distractor where a question hopes you pick WAN as fastest because the Internet feels powerful.

• A LAN uses short cables one organisation owns, so the links are clean and free of long-distance delay. A WAN stitches together public carriers and leased lines, adding delay at every hop.
• Draw a quick three-row table even for a one-mark question, because it forces one value per network and makes the inverse pattern visible at a glance.

Answer: Highest speed: LAN. Largest area: WAN. Remember, area up means speed down.

The marker is checking the three names and a one-line distinguishing feature for each. Pair each cable with its most memorable property: twisted-pair is the cheap everyday cable, coaxial is the shielded cable-TV cable, and optical fibre is the light-carrying glass cable with the highest bandwidth.

• The two wires of a pair carry equal and opposite signals, so outside noise that hits both equally gets cancelled at the receiver. Twisting keeps them close so they pick up identical noise.
• Mentioning interference handling, bandwidth and distance for each cable turns a plain list into a comparison, and CBSE markers reward the comparison.

Answer: Twisted-pair (cheap, short), coaxial (shielded, medium), optical fibre (light-based, highest bandwidth and longest reach).

For a "compare" question the examiner expects a two-column table with a clear basis per row. Pick four or five strong points (path, mobility, speed, interference, security) and you cover almost every variant. A table also stops you from describing only one side and forgetting the matching point.

• There is no winner, only a trade-off, and saying so explicitly often earns the final mark. Wired wins on speed and security; wireless wins on mobility and easy coverage.
• Real networks mix both: an optical-fibre backbone between buildings feeds Wi-Fi access points inside each room.

Answer: Wired: guided, fast, steady, secure, fixed. Wireless: unguided, mobile, easy to deploy, but slower and more open to noise. It is a trade-off, not a winner.

This is a one-mark recall question, and "optical fibre" earns the mark on its own. But a single extra line about how the light travels, by total internal reflection, guards you against a careless slip and prepares you for the follow-up "explain how it works".

• Light carries no electric charge along the cable, so electromagnetic noise cannot touch it. That is the deep reason optical fibre is both the fastest and the most interference-free wired medium.
• If marks allow, name the consequence: highest bandwidth and immunity to electrical noise, which is why backbone and undersea links are almost all fibre.

Answer: Optical fibre cable. Light pulses travel through a glass strand by total internal reflection, giving very high bandwidth with no electrical interference.

The cleanest way to recall this list is to trace each point to a single cause. The advantages all flow from the signal being light: huge bandwidth, no electrical-noise disturbance, little loss over distance. The disadvantages all flow from the medium being thin glass: costly to make, brittle, and needing precise skilled splicing.

• The bandwidth advantage is enormous because light oscillates at frequencies far above any electrical signal, so one fibre can carry many data streams using different colours of light.
• If the question carries four or more marks, give at least three advantages and two disadvantages with one reason each, since marking schemes award per valid point.

Answer: Advantages (from using light): high bandwidth, long reach, noise immunity, security, light weight. Disadvantages (from using glass): costly, fragile, hard to install and repair.

Memorise the lower and upper bounds as a pair, here 3 KHz and 1 GHz, and keep the units straight, because a single wrong prefix turns a correct answer into a wrong one. A quick check: 1 GHz is a thousand MHz and a million KHz, so the band spans six orders of magnitude.

• Above about 1 GHz the waves become microwaves, which travel in straight lines and need antennas to face each other, unlike radio waves that spread in all directions.
• Give the range first because it carries the mark, then add the omni-directional property and one use such as FM radio, since that detail often carries a separate mark.

Answer: 3 KHz to 1 GHz. Radio waves are omni-directional and pass through obstacles, which is why they carry AM, FM and TV broadcasts.

The single error that loses this mark is the wrong power of ten. Recite the prefix ladder every time: kilo is 10³, mega is 10⁶, giga is 10⁹, so 1 Gbps is exactly nine zeros. Then 18 Gbps is 18 followed by nine zeros, eighteen billion.

• For data rates the prefixes are decimal (10³, 10⁶, 10⁹), never 1024, because this is a rate in bits, not a storage size in bytes.
• The unit is Gbps with a small b for bits, so there is no factor of eight here; simply multiply by 10⁹.

Answer: 18,000,000,000 bps, that is 18 × 10⁹ bits per second (eighteen billion bits per second).

The mark is the expansion, Hypertext Transfer Protocol, written with that exact spelling. Since abbreviation questions often ask you to "name and explain", attach one line on what the protocol does: it carries requests and responses for web pages between a browser and a server.

• HTTP is a client-server protocol: the browser asks, the server answers. Each request is independent, which is why websites use cookies to remember you.
• Do not confuse HTTP with HTML. HTML is the language the page is written in; HTTP is the protocol that transfers that page across the network.

Answer: Hypertext Transfer Protocol, the client-server protocol that transfers web pages between a browser and a web server.

A "short note" question rewards structure over length. For each term give three lines in a fixed order: the full form where one exists, a one-sentence definition, and the unit or a real use. The same skeleton for all five keeps your answer tidy.

• Bandwidth versus data transfer rate: bandwidth is a channel property in hertz and sets the ceiling; data transfer rate is the actual bits delivered per second in bps. Bandwidth is the pipe width, rate is the flow.
• HTTP versus DNS: DNS runs first, turning the name into an IP address; HTTP runs next, fetching the page. DNS finds the house, HTTP delivers the letter.

Answer: HTTP transfers web pages; bandwidth (Hz) is the channel's frequency range; Bluetooth is short-range 2.4 GHz wireless; DNS maps names to IP addresses; data transfer rate (bps) is bits sent per second.

CBSE marks this question in two parts, the definition and the example, so structure your answer to hit both. Define a protocol as a set of agreed rules for communication, then name one real protocol and show the rule in action. HTTP is the strongest example because you can describe the request-response exchange in one sentence.

• Explain what would go wrong without a protocol: two computers on a perfect cable still fail if one sends in one format and the other expects another, like two people who speak different languages.
• Protocols are layered: HTTP for web meaning sits on TCP for reliable delivery, which sits on IP for addressing, so several protocols cooperate on one request.

Answer: A protocol is a set of agreed rules (syntax, semantics, timing) that let two devices communicate. Example: HTTP, where a browser's request and a server's response follow the same rules so the page loads.

The whole question is a one-line subtraction, yet students lose the mark by subtracting across mixed units, taking "1 GHz minus 500 MHz" as "1 minus 500" or "1 minus 0.5". Convert both numbers to the smaller unit first: 1 GHz is 1000 MHz, so the sum is 1000 MHz minus 500 MHz, giving 500 MHz.

• The signal is "composite" because it is made of many single frequencies mixed together; bandwidth measures how wide that mix is, and a wider band allows a higher data transfer rate.
• A sanity check: bandwidth can never exceed the highest frequency and is always positive, so 500 MHz (half the top frequency) is reasonable. You can also state it as 0.5 GHz or 500 × 10⁶ Hz.

Answer: 1000 MHz − 500 MHz = 500 MHz (equal to 0.5 GHz or 500 × 10⁶ Hz). Always convert to a common unit before subtracting.