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Originally Posted by Dave Flowerday
You may want to recheck your numbers. USB 1.0 (aka "low speed USB") is 1.5Mbps, quite a bit more than 115,200bps used for serial. At any rate, nearly all devices support at least USB 1.1 (aka "full speed USB") which is 12Mbps. USB 2.0 ("high speed") is 480Mbps.
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Your right, my bad.
USB 1.0 "low speed" comes out to 1500000 bits/second. This would include any encoding, bit shuffling, USB overhead, etc.
Serial comes out to 92160 data bits/s (assuming 8N1 configuration, which is fairly standard; this is found by multiplying the baud rate by 80%=0.8).
Also, USB is half-duplex query-based. USB is succebtable to congestion (that is, a lot of devices on the same hub will slow down bandwidth). Every USB packet is at least 22 bytes long (for the header).
Serial is simple; the Tx and Rx lines just transmit data at the baud. So there is no overhead (handshaking and transmission control is often done with secondary lines).