Connecting other devices to the same host means sharing the available maximum bandwidth. USB is a tree connection, one host to many devices.If data is cached, this will distort numbers (seem faster), always be aware of the tricks the system or OS uses to speed things up.This means for 1 GB it takes 18,78 seconds (18,78 x 53,248 MBps = 1 GB). The theoretical maximal payload data limit for USB 2.0 is 53.248.000 Bps and not 60.000.000. In decimal a megabyte is 1.000.000 bytes = 1 MB (*). = 53.248.000 bytes per second netto BULK data payload maximum.A BULK transfer guarantees the data delivery, only when the device acknowledges the reception, the data is considered sent and the host frees the local BULK buffer with the 512 bytes of data. A BULK transfer has a maximum of 512 payload bytes. USB 2.0 microframes (one per 125us) can have a maximum of 13 BULK transfers per microframe. And protocol overhead eats into the maximum: data is bidirectional (BULK data needs acknowledge), IN/OUT tokens start a transfer, sync patterns. The 60 MBps (480 MHz is the actual clock frequency for the serial data) is the maximum bitrate. USB2.0 is specified as 480 Mbps = 60 MBps but due to the specification and protocol overhead, the maximum is 53,248 MBps. Actual is more important than theoretical throughput. Doing USB validation, this is my technical conclusion: it is theoretical and there is actual.
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