USB Bulk Transfers

导读：
　　Bulk transfers are useful for transferring data when time isn't critical. A bulk transfer can send large amounts of data without clogging the bus, because the transfers defer to the other transfer types and wait until time is available. Uses for bulk transfers include sending data from the host to a printer, sending data from a scanner to the host, and reading and writing to a disk. On an otherwise idle bus, bulk transfers are the fastest transfer type.
　　Availability
　　Only full- and high-speed devices can do bulk transfers. Devices aren't required to support bulk transfers, though a specific device class may require it.
　　Structure
　　A bulk transfer consists of one or more IN or OUT transactions. A bulk transfer is one-way. A transfer's transactions must all be IN transactions, or all OUT transactions. Transferring data in both directions requires a separate pipe and transfer for each direction.
　　A bulk transfer ends in one of two ways: when the requested amount of data has transferred, or when a data packet contains less than the maximum data, including a zero-length packet.
　　To conserve bus time, the host uses the PING protocol in some high-speed control transfers. If a high-speed bulk OUT transfer has more than one data packet and if the device returns NYET after receiving one of these packets, the host uses PING to find out when it's OK to begin the next data transaction. In a bulk transfer on a high-speed bus with a low- or full-speed device, the host uses split transactions for all of the transfer's transactions.
　　Data Size
　　A full-speed bulk transfer can have a maximum packet size of 8, 16, 32, or64 bytes. For high speed, the maximum must be 512 bytes. During enumeration, the host reads the maximum packet size for each bulk pipe from the device's descriptors. The amount of data in a transfer may be less than, equal to, or greater than the maximum size. If the amount of data won't fit in a single packet, the host completes the transfer using multiple transactions.
　　Speed
　　The host controller guarantees that bulk transfers will complete eventually, but doesn't reserve any bandwidth for the transfers. Control transfers are guaranteed to have 10 percent of the bandwidth at low and full speeds, and20 percent at high speed. Interrupt and isochronous transfers may use the rest. So if a bus is very busy, a bulk transfer may take very long. However, when the bus is otherwise idle, bulk transfers can use the most bandwidth of any type, and they have a low overhead, so they're the fastest of all. When an endpoint's maximum packet size is less than the maximum, some hosts schedule no more than one packet per frame, even if more bandwidth is available.
　　At full speed on an otherwise idle bus, up to nineteen 64-byte bulk transfers can transfer up to 1216 data bytes per frame, for a data rate of 1.216 Megabytes per second. This leaves 18% of the bus bandwidth free for other uses. The protocol overhead for a bulk transfer with one data packet is 13 bytes at full speed and 55 bytes at high speed.
　　At high speed on an otherwise idle bus, up to thirteen 512-byte bulk transfers can transfer up to 6656 data bytes per microframe, for an impressive data rate of 53.248 Megabytes per second, using all but 2% of the bus bandwidth. The protocol overhead for a bulk transfer with one data packet is 55 bytes.
　　Detecting and Handling Errors
　　Bulk transfers use error detecting. If a device doesn't return an expected handshake packet, the host tries up to twice more. The host will also retry without limit on receiving NAK handshakes. Bulk transfers use data-toggle bits to ensure that no data is lost.

本文转自
http://www.usb-programming.com/bulk-transfers.html