USB Transfer Types
Universal Serial Bus (USB) transfer types refer to the mode of communication used between the Host and a device's endpoints. The transfer type determines the frequency and length of the transactions used to communicate with the endpoint. Transfer types can also assign a Cyclical Redundancy Checksum (CRC) to be verified with each packet transmitted. Transfer types are set by the device and are communicated to the Host during the enumeration process.
To facilitate Host software development and allow for interchangeability between different manufacturers' products, the USB Developers Forum has approved specifications for device classes. Device classes provide endpoint numbers, transfer types, and endpoint grouping for devices.
Consideration for Selecting Transfer Types
Latency
How quickly does the Host need to service the endpoint? To establish a deterministic latency, transactions must be scheduled at fixed intervals. For example, if an endpoint is scheduled to be serviced every 10 frames, the system would have a response time of 10 ms, using a full-speed USB. To guarantee latency, the endpoint is serviced at the prescribed interval regardless of pending activity. Allocating time for each frame to service, inactive devices limits the number of devices that can enumerate due to the finite free time in the frame.
Error Checking
Does the payload in the transaction need an accompanied CRC for reliable delivery? Most USB implementations use a hardware CRC generator to append the CRC to the message. To allow economical silicon solutions to be developed, USB designers have limited the packet size for transactions requiring CRC. Reduced packet size may limit the data rate of the device.
Bandwidth
The number of packets and the length of each packet affect the bandwidth. Increasing the size of a transaction will increase the bandwidth, but long packets do not provide for CRC checking. In addition, reserving portions of the frame for long high bandwidth transfers will limit the number of potential devices which can be enumerated to the Host. Increasing the number of times CRC-enabled transactions can be sent in a frame will increase bandwidth, but this approach poses the same enumeration dilemma presented by long frames.
Control and Configuration
A portion of the frame must be made available for the Host to enumerate new devices and send a control command to currently enumerated devices.
Transfer Types Used by USB
Short packets with CRC, scheduled at fixed periodic intervals. While the following diagram shows the transfer running every frame, interrupt transfers could be scheduled to run on intervals of frames, such as every 10 frames. Interrupt transfers will always occur regardless of whether or not there is any data to be transferred.
| Interrupt Transfers | |
|---|---|
| Benefits | High-reliability data transfers with a fixed response time |
| Drawback | Bandwidth may be limited (64 KBytes for Full-Speed USB) |
| Typical Use | Mice, Keyboards, and Medical Devices |
| Notes | Up to 90 percent of the frame can be allocated for Interrupt endpoints. The maximum length of the transfer depends upon the frame size used. |
Isochronous Transfers
Longer packets without CRC. Isochronous transfers are scheduled at fixed periods. The frame bandwidth is released by the Host if communication with the endpoint is not needed.
Isochronous Transfers | |
|---|---|
| Benefits | High bandwidth |
| Drawback | No CRC hardware. If a CRC is needed it must be done in software. Long packets can limit the number of devices being enumerated. |
| Typical Use | Audio/Video streaming, serial port emulation |
| Notes | Up to 90 percent of the frame can be allocated for interrupt endpoints. When not in use the bandwidth used will be released. The maximum length of the transfer depends upon the frame size used. |
Bulk Transfers
Short packets with CRC. Bulk transfers are not scheduled, they run when there is available bandwidth in the frame. Multiple bulk transfers can run in a single frame if there is bandwidth available
| Bulk Transfers | ||
|---|---|---|
| Benefits | High reliability with the potential for high bandwidth | |
| Drawback | Bandwidth may vary depending upon the number of interrupt endpoints enumerated and the activity of enumerated Isochronous endpoints. | |
| Typical Use | Mass Storage and Printers | |
| Notes | Will take advantage of unused Isochronous bandwidth. The maximum length of the transfer depends upon the frame size used. |