Looking at the pins of USB 3.0 receptacles, I can see that there are separate transmission and reception pairs, however for USB 2.0 there is only one "data" pin pair. How does USB 2.0 ensure that the devices aren't communicating simultaneously?
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USB is strictly master-slave. The device does not transmit unless the host tells it to transmit.
Even so called "interrupt" mode is really polling: for example, every 8 milliseconds (or less if you got a gamer mouse), the PC asks the mouse "what is your position" and the mouse replies.
Same if you have a USB-serial interface for example. When the interface receives data on the serial line, it will not transmit it to the PC. Instead it will wait for the PC to initiate the transaction and ask for the data.
This webpage has a good explanation about the packets that are exchanged. Basically, keep in mind that USB was implemented to allow the dumbest and cheapest possible peripheral to function, which means most of the intelligence is in the host, host usb controller, OS, and drivers. This is very apparent when reading the spec.
Firewire (for example) has a completely different philosophy, it is much more powerful, it's multi-master so devices can talk to each other without help from a host/master. It is actually much closer in its philosophy to something like token ring with isochronous transfers slapped on top, than to USB. However "multi master" means it requires a powerful microcontroller in the devices, running a complex software stack. It is therefore more expensive, and thus limited to expensive products like camcorders and fast hard drive enclosures. A firewire mouse makes no sense, it would be too expensive. That's one of the reasons why FireWire failed.