A P P E N D I X
A
USB 2.0 specification primer
USB topology
The USB physical network is a tiered star topology. A hub is at the center of each star. Each wire
segment is a point-to-point connection between the host and a hub or function, or a hub connected to
another hub or function. This figure illustrates the topology of a USB network.
Host (Tier 1)
Root
hub
Hub 1
Tier 2
Function
Hub 2
Tier 3
Function
Function
Tier 4
Hub 3
Hub 4
Function
Tier 5
Hub 5
Function
Function
Tier 6
Hub 6
Hub 7
Tier 7
Function
Timing constraints for hub and cable propagation times allow a maximum of seven tiers, including the
root tier. In seven tiers, a maximum of five non-root hubs can be supported in a communication path
between the host and any device. Note that a compound device occupies two tiers; therefore, it
cannot be attached at tier seven where only functions can be used.
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