4. Open-ended quiz

Checking criteria: (1) application of the course materials discussed after midterm, (2) concreteness, (3) originality, (4) [no] jumps in the protocol procedure, (5) possibility of performance calculation of the designed scheme, (6) meeting the design requirements

 

Q. We have discussed the hidden terminal problem, and how it can be solved through the use of RTS/CTS. But then, there is also a problem called the exposed node problem. As shown in the following Wikipedia illustration, S1 and S2 cannot transmit simultaneously because they worry that their transmissions collide. The problem is that even if they transmit simultaneously, the colliding transmission will have no problem reaching their respective destinations R1 and R2. It is because R1 is beyond the communication range of S2, and R2 is beyond the communication range of S1. Now, design a heuristic that solves the exposed node problem. Make sure that your solution does not worsen the hidden terminal problem. [Hint: RTS/CTS may come to the rescue again. In particular, CTS can be sent to the transmitter itself.]

 

Determining that the channel is busy and causing a time delay to avoid collision. This unnecessary prevention is called Exposed node ploblem. A method to solve the exposed node problem is to use CTS even if there is a terminal in an environment that cannot detect a carrier for a signal of a transmitting terminal using RTS/CTS. If the base station can receive the transmitted CTS, it is possible to determine the existence of the transmitting terminal, so the Exposed node problem can be solved.

In other words, when the terminal hears an RTS from an adjacent terminal other than the corresponding CTS, the node is an exposed terminal and transmission to other adjacent terminals is allowed.