I thought this forum would help me to find an [answer] for my questions. But this is a complete [waste] of time.
I really wanted to learn more about arduino with all of you, but you choose to ignore me.
Well that all sounds a bit like a dramatic outburst of frustration, but anyway:
If you cant use 'ack' to measure the round trip delivery time, then you have to devise your own method of achieving the same thing.
You have to send a message from A to B. A sets a timer when it sends the message. B accepts the message and sends a response back to A. A records the time it receives the response back from B. The difference between these two times, that is the time of the sending of the original message and the time of receiving the response back, is the round trip delivery time.
This is not a trivial exercise, but it is also not extremely difficult. Do you have to show a complete working solution or only show the outline of a solution ?