I have the following scenario: PC-A would like to send data to PC-B. PC-A is contending for the wireless resource and so is PC-C. At present, the medium is idle, hence, PC-A gets to use the air first, since it has a shorter back-off timer.

Question: Once PC-A finishes sending its data to PC-B, who will be able to use the wireless medium next? Is it PC-C or will it be PC-B with the ack frame.

If PC-C is to use the medium next, then, will PC-A re-transmit its data, since its initial frame was not acknowledged by PC-B


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I hope this link can help you: https://www.tlm.unavarra.es/~daniel/docencia/arss/arss09_10/slides/31y32-CSMA-CA.pdf


Short answer: No device gets automatic priority.

Is it Ad Hoc (device-to-device), or is a Wifi AP (switch/router) involved?

In Wifi AP: Wifi Access Points use CSMA/CA with RTS/CTS
The first image in the CSMA/CA Wiki link should be easy to understand.
Since WifiAP wants to forward the received packet to PCb, PCc wants to talk, and the others have nothing to say; either WifiAP or PCc will go first depending on their random backoff timers.

In Ad Hoc: I believe the same protocols are used (someone please correct me if I'm wrong). But the middle man WifiAP is removed, which would match your diagram. In this case either PCb or PCc will go next, but again it depends on the random backoff timers.

This link might help you understand RTC/CTS more completely.

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