Two nodes P and Q are connected by a shared broadcast medium. P starts transmitting at t = 0 after c

Q: What is GATE-CS?

GATE-CS is a competitive examination conducted in India. This question is from the 2018 GATE-CS Computer Networks paper.

Q: How can I prepare for GATE-CS Computer Networks?

To prepare for GATE-CS Computer Networks, practice previous year questions (PYQs), understand concepts thoroughly, and solve mock tests regularly. This 2018 question covers Data Link Layer (Framing, Error Detection, MAC, Ethernet Bridging) topic.

Q: What is the difficulty level of this question?

This GATE-CS 2018 Computer Networks question has a medium difficulty level and is marked as an important question.

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Question

Two nodes P and Q are connected by a shared broadcast medium. P starts transmitting at t = 0 after completing its carrier-sense phase. Q has a packet to transmit at t = 0 and begins carrier-sensing for 5 time units. If Q detects P's transmission during its carrier-sense phase, Q will wait; otherwise Q begins transmitting. The signal propagates at 10 meters per time unit. The transmission lasts 20 time units. The maximum distance d (in meters, rounded to the closest integer) between P and Q that allows Q to detect P's transmission and avoid a collision is _____.

Accepted Answer

Correct Answer: 50 metersStep-by-step Solution:P begins transmitting at t = 0. P's signal propagates outward at 10 meters per time unit.Q performs carrier sensing from t = 0 to t = 5 time units. For Q to detect P's transmission and refrain from transmitting, P's signal must arrive at Q's location before Q's carrier-sense window ends — that is, before t = 5.The time for P's signal to reach Q = d / 10 (where d is the distance in meters).Condition for Q to detect P: d / 10 ≤ 5, which gives d ≤ 50 meters.The maximum distance that guarantees Q detects P's signal and avoids a collision is d = 50 meters.Key Concept: In CSMA (Carrier Sense Multiple Access), a node listens for channel activity before transmitting. A collision occurs when the propagation delay exceeds the carrier-sense duration: the transmitting node's signal has not yet reached the sensing node by the time it finishes sensing and begins its own transmission. The maximum safe separation is: d_max = propagation speed × carrier-sense duration = 10 × 5 = 50 meters. Beyond this distance, the channel appears idle to Q during its entire sensing window, causing Q to transmit and collide with P's ongoing transmission.

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More Questions from Data Link Layer (Framing, Error Detection, MAC, Ethernet Bridging)

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