A USB host may implement multiple host controllers and each host controller may provide one or more USB ports. The maximum number of tier levels are five. Inclusion of hubs allows for branching into a tree structure. Additional USB hubs may be included in the tiers. It consists of a host, one or many downstream USB ports, and peripheral devices connected to ports in a tiered-star topology. The architecture of USB topology is asymmetrical. This makes differential signaling immune to electrical interference. If interference acts on the differential pair, it modifies both the lines similarly, but does not affect the difference between the lines. The signal at the receiving end is interpreted as the difference between the two lines that make up the differential pair. In differential signaling, each signal is transmitted using a differential pair-the signal carried by one wire is the same level as the one carried by the other wire, but in opposite polarity. When a signal is transmitted through a single wire, surrounding electrical interference acts on it and can potentially modify the signal levels, causing the signal to be detected incorrectly (a 1 getting detected as 0 and vice versa).
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