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Rev

I'm pretty sure this isn't true. The host controller manages 1:1 connections to each of the child devices back to the root USB controller. These connections may not be full-duplex, but as far as I'm aware they aren't shared buses. They're physically separated and don't do broadcast.
– Polynomial
Commented Jun 24, 2013 at 10:59
"Each of these hubs have multiple downstreams and exactly one upstream [...] it means that whatever data sent by the hub is sent to all child hubs and devices, while data sent by the hubs and devices are only sent upwards to Root Hub." - this is definitely incorrect. USB uses unique addressing for downstream and horizontal traffic, and round-robin polling to determine who gets to send upstream traffic back to the root host. There's no downstream broadcast.
– Polynomial
Commented Jun 24, 2013 at 11:07
@Polynomial Hmmm.. My understanding of this comes from my colleague's explanation of the whole process. But I've seen it with my own eyes how data copied to one USB stick are mirrored on the other. Although, now that you say it, I could be wrong because the demonstration was on two USB sticks connected to an external USB hub. Do external USB hubs behave differently?
– Adi
Commented Jun 24, 2013 at 11:08
3

I think it's to do with the differentiation between control and data packets, which can both be broadcast and unicast data. The spec doesn't explicitly prevent either from being broadcast, but by default the data packets should be unicast. I'd imagine one of the USB sticks designated itself master, and the other slave, and relayed the unicast traffic horizontally.
– Polynomial
Commented Jun 24, 2013 at 11:22

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