GATE CS 2018 Q.65 (CS Q.55): OSI/TCP-IP Layering, Physical Layer and Switching

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 based 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 ongoing transmission is _____.

Your answer

Official answer and solution

Official answer: 50
From: Official GATE 2018 answer key

Solution

Determine the maximum propagation delay allowing carrier-sense detection before transmission commences.

Node P begins transmission at $t = 0$ . The signal travels at speed $v = 10\text{ meters per unit time}$ . The propagation delay to reach node Q is $\tau = \frac{d}{v} = \frac{d}{10}$ .

Node Q begins carrier-sensing at $t = 0$ and continues sensing until $t = 5$ units of time.

If P's transmission reaches Q at or before $t = 5$ (i.e. $\tau \le 5$ ), Q will detect the transmission during its carrier-sense period, wait for P to finish, and refrain from transmitting, successfully avoiding collision.

If $\tau > 5$ , P's transmission arrives after Q has already finished carrier-sensing; Q will detect nothing, begin transmitting at $t = 5$ , and cause a collision.

Thus, collision is avoided if and only if:

\frac{d}{10} \le 5 \implies d \le 50\text{ meters}

Final answer

The answer is the one in the official GATE answer key. The solution is this site's own explanation.

Source

Paper: GATE 2018 Computer Science and Information Technology (CS)
Question: 65 of 65, printed as CS Q.55
Section: Computer Science and Information Technology

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