GATE | GATE CS 2018 | Question 38

Consider a simple communication system where multiple nodes are connected by a shared broadcast medium (like Ethernet or wireless). The nodes in the system use the following carrier-sense the medium access protocol. A node that receives a packet to transmit will carrier-sense the medium for 5 units of time. If the node does not detect any other transmission in this duration, it starts transmitting its packet in the next time unit. If the node detects another transmission, it waits until this other transmission finishes, and then begins to carrier-sense for 5 time units again. Once they start to transmit, nodes do not perform any collision detection and continue transmission even if a collision occurs. All transmissions last for 20 units of time. Assume that the transmission signal travels at the speed of 10 meters per unit time in the medium.

Assume that the system has two nodes P and Q, located at a distance d meters from each other. P starts transmitting a packet at time t = 0 after successfully completing its carrier-sense phase. Node Q has a packet to transmit at time t = 0 and begins to carrier-sense the medium.

The maximum distance d (in meters, rounded to the closest integer) that allows Q to successfully avoid a collision between its proposed transmission and P’s on going transmission is _______ .



Note –This was Numerical Type question.
(A) 50
(B) 150
(C) 20
(D) 60


Answer: (A)

Explanation: Node that receives the packet to transmit will carrier-sense the medium for 5 units. Any packet which arrives within 5 unit will be sensed and keep the channel busy. Given that Signal speed is 10 meter/time.

Which means in 5 unit of time packet can travel 50 meters in max, that allow Q to successfully avoid the collision.

So, option (A) is correct.

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