# Q2(b) · CSMA/CA and RTS/CTS

Subject: Computer Networks

Exam: TEE

Reader: https://deep-learning-study-room.vercel.app/cn/#tee/papers/paper-2025-feb-reexam/q2b

Canonical: https://deep-learning-study-room.vercel.app/read/cn/tee/papers/paper-2025-feb-reexam--q2b/

Actual paper; 100 marks attempted. Current-syllabus alignment is stated per question.

### Q2(b). Explain in detail CSMA/CA with the help of a diagram. Describe the role of RTS and CTS in avoiding collision in CSMA/CA. [10 marks]

### Answer

**Definition.** CSMA with Collision Avoidance uses carrier sensing and randomized access delays, with acknowledgements and optional reservation exchanges, to reduce collisions on a wireless shared medium.

**Simple meaning.** CA = pehle collision avoid karne ki koshish; guarantee nahi.

Swipe across the diagram to read its labels ↔

![CSMA/CA: optional reservation, data, acknowledgement](https://deep-learning-study-room.vercel.app/cn/fig/ca-sequence.566280e50f.svg)

Diagram: CSMA/CA: optional reservation, data, acknowledgement. Labels: CSMA/CA: optional reservation, data, acknowledgement | CSMA/CA: optional reservation, data, acknowledgement | Sender | Receiver | RTS (optional reservation) | CTS | DATA | ACK | Before data access: carrier sensing and a random backoff. | A busy medium freezes the eligible backoff countdown.

CSMA/CA: reduce collision risk before sending

## Understand it step by step

1. A wireless station senses the medium and follows the required interframe waiting and contention rules.
2. A random backoff counter counts down in eligible idle slots and freezes while the medium is busy. It resumes when permitted; it is not continuously counted through someone else’s transmission.
3. When its counter reaches zero, it transmits. A unicast receiver normally sends an ACK after successful reception. Missing ACK triggers retry logic.
4. RTS/CTS is an optional reservation exchange. Stations hearing reservation information defer for the indicated duration. It can reduce hidden-terminal data collisions but adds overhead.
5. Wireless stations cannot generally rely on Ethernet-style collision detection while transmitting. Collision avoidance does not eliminate all losses or collisions.

#### Practical example

Two laptops using one Wi-Fi channel contend for access. Random backoff reduces simultaneous starts; an ACK lets the sender know a unicast frame was received.

| Aspect | CSMA/CD | CSMA/CA |
| --- | --- | --- |
| Main action | Detect and abort a collision during sending | Reduce collision risk before sending |
| Typical teaching context | Shared half-duplex wired Ethernet | Wi-Fi contention access |
| Mechanisms | Carrier sensing, detection/jam, backoff | Carrier sensing, random backoff, ACK; optional RTS/CTS |

**Yaad rakho:** CD = detect during; CA = avoid before.

**Definition.** Hidden terminals are stations that cannot sense each other but whose transmissions can interfere at a common receiver.

**Simple meaning.** A aur C ek dusre ko nahi sunte, par dono B ko bhej rahe hain.

Swipe across the diagram to read its labels ↔

![Hidden terminals: A and C cannot sense each other](https://deep-learning-study-room.vercel.app/cn/fig/hidden-terminal.1ffcf794d5.svg)

Diagram: Hidden terminals: A and C cannot sense each other. Labels: Hidden terminals: A and C cannot sense each other | Hidden terminals: A and C cannot sense each other | A | B | C | Common receiver B hears both | B’s CTS can make a hidden sender defer. Collisions remain possible.

Hidden terminals and RTS/CTS

## Understand it step by step

1. Suppose A and C are outside each other’s sensing range, but B can receive both.
2. A senses no signal from C and may transmit; C may make the same decision. Their frames can collide at B.
3. With optional RTS/CTS, A first sends a short RTS to B. B replies CTS, which C can hear. C defers for the reservation duration.
4. RTS frames can themselves collide and wireless conditions still matter. Describe RTS/CTS as mitigation, not an absolute guarantee.

#### Practical example

Access points can hear devices at opposite ends of a room even when those devices cannot hear one another clearly. The access point’s CTS can inform both sides about a reservation.

A and C may both reach B but be unable to sense one another. Simultaneous transmissions can then collide at B. A short RTS followed by B’s CTS announces a reservation; a station hearing the CTS defers, reducing the risk of a long data-frame collision.

**Yaad rakho:** Hidden: senders invisible to each other, receiver common.

### CSMA/CA flow diagram

```
Frame ready → sense medium
Busy? yes → defer until eligible idle interval
          ↓
Choose a random backoff → count down eligible idle slots
Medium becomes busy? → freeze counter; resume after required idle wait
          ↓ counter reaches zero
Optional RTS → CTS received? no → retry/back off
          ↓ yes (or RTS/CTS not used)
Send DATA → ACK received? yes → success
                        no → retry/back off or fail at retry limit
```

A station hearing a duration reservation updates its network allocation vector and defers. RTS/CTS is optional; loss of CTS or ACK triggers recovery but does not by itself prove a collision, because channel errors can also cause loss.