1. 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. In order to help detect close contacts that enable Covid-19 infections, you have decided to implement a communication-based smartphone app that records such contacts. The distance between two smartphones that defines the close contact is 1 meter, and the infection between the smartphone holders is likely in the close contacts that last over 5 minutes. You do not want to miss any true close contact events, or record any “false” contacts that do not satisfy the distance requirement. Design a wireless protocol that will be used by the app. Among others, elaborate on the three DLC functionalities in your protocol.

 

 

Basically, the well-known location-based technologies are GPS, WPS (using WIFI router), and Bluetooth-based beacons. In this problem, since there is a condition for more than 5 minutes within a distance of 1m, it can be seen that the app must use Bluetooth, except for GPS, which is difficult to implement indoor location recognition, and WPS, which has a large error.

 

A protocol that satisfies the 3 requirements of 1m, 5 minute condition check, and record by giving individual IDs should be designed. DLC functions are framing, error control, and flow control. Each individual ID is assigned and the value and ACK are put into the frame. If the distance increases, the probability of a damaged ACK arriving increases, so if a damaged ACK arrives, simply discard it (because wireless communication is a very rough environment). do. This can be seen as an initial version of the stop and wait protocol, and flow & error control can be performed through it. It can also be used to correct the distance error by statistically calculating the damage probability according to the distance. By reducing the number of packet transmissions, the number of collisions can be reduced. The number of collisions can be reduced by transmitting packets at prime number time instead of continuously sending and receiving for 5 minutes. Conflicts cannot be avoided because of clustering. This can be supplemented to some extent by the grouping method described later.

 

The next thing to consider is the procedure for deciding whether to transmit every moment in the random access and contention method. With ALOHA, the basic random media access method, data can be corrupted when there is a frame collision. Therefore, if the number of broken data increases, eventually there are many collisions = there are many people around. Conflicts can be reduced by choosing a method that checks for close contact. Such protocols can better detect cluster infections.