USB communication explained in a simplified way, consists of 3 basic elements that forms the chain of communication.
- USB Host Controllers
- USB Hubs
- USB Devices (Functions)
USB operates in 7 Tiers (layers) of communication.
Tiers describe in which order ports, hubs and devices (also called "Functions") are connected
The USB host controller always occupies Tier 1 and sometimes Tier 2
Both hubs and USB extenders occupy Tiers 2 through 6. Hubs can depending on their size occupy 2 Tiers.
Devices (Functions) are always Tier 7, since it is the end of the daisy-chain.
Issues can occur when a USB tree goes deeper than Tier 7 with hubs. Then devices are simply not discovered. Take this example:
Laptop [USB Host Controller] (Tier 1 - 2)
Solvo (Tier 3)
USB Extender (Tier 4)
7 port USB Hub (Tier 5 - 6)
Interactive Whiteboard with built-in USB Hub (Tier 7)
Then devices plugged into the whiteboard will not be discovered and working. In order to solve it, change the chain of USB connections, so that the devices will be connected on a lower Tier level.