Project ForgeConsider a simple communication system where multiple nodes share a broadcast medium using carrier-sense based medium access. A node carrier-senses for 5 units of time; if it detects no transmission, it starts transmitting in the next time unit; if it detects a transmission, it waits for that transmission to finish, then carrier-senses for 5 units again. Once transmitting, nodes do not perform collision detection. All transmissions last 20 units of time. The signal travels at 10 meters per unit time. Two nodes P and Q are at distance d meters apart. P starts transmitting at t=0. Node Q has a packet to transmit at t=0 and begins carrier-sensing at t=0. The maximum distance d (rounded to the closest integer) that allows Q to successfully avoid a collision is ____.
Explanation
P's transmission (starting at t=0) reaches Q after a propagation delay of d/10 time units. For Q to detect it and correctly hold off, this signal must arrive within Q's 5-unit carrier-sense window, i.e. before Q decides to transmit at t=5: d/10 ≤ 5, so d ≤ 50 meters.
Source: GATE CS 2018 (2018)
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