Structure of Bus

In spite of the fact that most USB connections seem to run from computer to device (point to point) this hardly needs to be the case. The internal structure of the bus is really a tree rather than a link. Every node in the tree represents a hub. The root hub is present inside the computer with the two USB sockets present on the computer being nodes connected to the root hub. Thus a compound USB device which consists of a keyboard and the mouse with one USB link going to the host may be represented as follows:

 

Figure 1: Layout of Bus

where the boxed region represents the device. The internal hub is invisible to the user.
This raises a couple of points. First, obviously the computer needs to be aware and able to address either of the devices. This is done by way of assigning device addresses to each of the devices on the bus. Also, in spite of the fact that the function and the location of the internal hub is different from the devices, as far as the computer is concerned, it treats the hub in the very same way as it treats the other devices.