Physical Layout
A USB network is built from USB hubs connected downstream to USB ports, which themselves may stem from USB hubs. USB hubs can extend a USB network to a maximum of 127 ports. The USB specification requires that bus-powered hubs may not be connected in series to other bus-powered hubs.
USB ports are often closely spaced. Consequently, plugging a device into one port may physically block an adjacent port, particularly when the plug is not part of a cable but is integral to a device such as a USB flash drive. A horizontal array of horizontal sockets may be easy to fabricate, but may cause only two out of four ports to be usable (depending on plug width).
Port arrays in which the port orientation is perpendicular to the array orientation generally have fewer blockage problems. External "Octopus" or "Squid" hubs (with each socket at the end of a very short cable maybe 2 inches long), or "star" hubs (with each port facing in a different direction, as pictured) avoid this problem completely.
Read more about this topic: USB Hub
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