Because all devices share the bus, a device has no guarantee that a particular rate or maximum latency will be available on attachment. If the bus is too busy to allow a requested transfer rate or maximum latency, the host refuses to complete the configuration process that enables the host to schedule transfers. The device’s driver can then request a configuration or interface that requires less bandwidth. To take full advantage of reserved bandwidth, the device driver and application software and device firmware must eliminate retries as much as possible.
The device should have data ready to send when the host requests it and should be ready to accept data when the host sends it.Of USB’s four transfer types, the fastest on an otherwise idle bus are bulk transfers,with theoretical maximums of around 1.2 MB/s at full speed, 53 MB/s at high speed, and 400 MB/s at SuperSpeed. Isochronous transfers can request the most bandwidth (1.023 MB/s at full speed, 24.576 MB/s at high speed, and 393 MB/s at SuperSpeed). Low speed doesn’t support bulk or isochronous transfers, and the maximum guaranteed bandwidth for a single low-speed transfer is 800 bytes per second.
Tags: bandwidth, configuration, interface, transfers, USB
