Computer Networks · TEE · papers

Q3(a) · CSMA/CA

2024-02-06 · 2022-23

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

Q3(a). Describe the working of CSMA/CA with the help of flow diagram. [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.

CSMA/CA: optional reservation, data, acknowledgement — CSMA/CA: reduce collision risk before sending

Understand it step by step

A wireless station senses the medium and follows the required interframe waiting and contention rules.
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.
When its counter reaches zero, it transmits. A unicast receiver normally sends an ACK after successful reception. Missing ACK triggers retry logic.
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.
Wireless stations cannot generally rely on Ethernet-style collision detection while transmitting. Collision avoidance does not eliminate all losses or collisions.

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.

Hidden terminals: A and C cannot sense each other — Hidden terminals and RTS/CTS

Understand it step by step

Suppose A and C are outside each other’s sensing range, but B can receive both.
A senses no signal from C and may transmit; C may make the same decision. Their frames can collide at B.
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.
RTS frames can themselves collide and wireless conditions still matter. Describe RTS/CTS as mitigation, not an absolute guarantee.

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.