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The only difference that I can see is that there's no physical object to signify that the space is taken, and so Tony might end up parking on top of someone else. That's not good, since the other person doesn't get the service they payed for

And that's the only issue I have. It's as if Tony were a blind driver and decided that the best way to see if there was a parking space was to drive his car into the space. If he got in, great. If he hit another car, oh well. And the people coming back are also blind, ramming into his car that's in their assigned space.

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if there were a reliable way of detecting that a specific MAC address has gone away for good, then what's the issue?

But there is no reliable way. Someone idle is completely invisible.
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Bitt Faulk