USB Transfer Types

Article
06/30/2006

The Microsoft® Windows® CE .NET operating system (OS) supports all four types of data transfer defined in the Universal Serial Bus Specification, Revision 1.1. Data transfer takes place between the memory buffer on the host and an endpoint on the USB device. Data is organized into packets before it is transferred. The transfer type that is used depends on the pipe on which the transfer is being issued. This is determined by the USB device on your system. Device drivers for USB devices can use any of the following transfer types shown in the following table.

Transfer type	Description
Control transfers	Control transfers are bidirectional transfers that are used by the USB system software mainly to query, configure, and issue certain generic commands to USB devices. Control transfers typically take place between the host computer and the USB device's endpoint 0, but your control transfers may use other endpoints.
Isochronous transfers	Isochronous transfers provide guaranteed amounts of bandwidth and latency. They are used for streaming data that is time-critical and error-tolerant or for real-time applications that require a constant data transfer rate. For example, an Internet telephony application that carries a conversation in real time is a good candidate for isochronous transfer mode. Isochronous data requires guaranteed amounts of bandwidth and guaranteed maximum transmission times. For isochronous transfers, timely data delivery is much more important than perfectly accurate or complete data transfer.
Interrupt transfers	Interrupt transfers are used mainly to poll devices to check if they have any interrupt data to transmit. The device's endpoint descriptor structure determines the rate of polling, which can range from 1 through 255 milliseconds. This type of transfer is typically used for devices that provide small amounts of data at sporadic, unpredictable times. Keyboards, joysticks, and mouse devices fall into this category. It is also used to signal the presence of data on other endpoints in the device.
Bulk transfers	Bulk transfers are for devices that have large amounts of data to transmit or receive and that require guaranteed delivery, but do not have any specific bandwidth or latency requirements. Printers and scanners fall into this category. Very slow or greatly delayed transfers can be acceptable for these types of device, as long as all of the data is delivered eventually. However, in the absence of any other demands for bus bandwidth, bulk transfers are processed as quickly as possible.

USB Transfer Types

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