Chapter 6: LS/FS Transfer Types & Scheduling

Isochronous Transfers

Isochronous transfers are typically used by real-time applications that must establish a synchronous connection with another device. For example, audio applications must transfer information synchronously to avoid distortion in the audible output (e.g., CD audio, speakers, and digital telephones). Isochronous data delivery is characterized by the need to provide data on a timely basis and is used where timeliness is more important than verifying accurate delivery of data. For this reason valid data delivery is not guaranteed during isochronous transfers.

The following headings characterize isochronous transfers from the USB perspective regarding direction of transfer, service period, packet size and bandwidth capability, and error detection and handling. For a more complete discussion of synchronous devices and their use of isochronous transfers, refer to “Establishing Synchronous Connections” on page 125.

Direction of Transfers

Isochronous transfers are unidirectional. No single endpoint can transfer information in both directions. Isochronous devices that need to both send and receive data must implement two endpoints, one for sending and one for receiving data.

Service Period

An isochronous transfer is scheduled to be performed regularly over the USB during consecutive 1ms frames. This ensures that a constant data rate can be maintained.

Bandwidth Allocation

The data payload during an isochronous transfer is limited to 1023 bytes during each frame and can only be performed at full speed (i.e., low-speed devices cannot support isochronous transfers). The endpoint descriptor defines the maximum payload supported. If an isochronous endpoint requires more bus bandwidth than is available, the device will not be configured. This is because isochronous pipes require guaranteed bandwidth to function correctly.

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USB System Architecture

Table 6-2: Full-Speed Isochronous Bandwidth

Error Recovery

Since a constant data transfer rate is essential for isochronous applications, error detection and recovery are not supported. Recovery involves transferring data again in the event that errors are detected. Attempting retransmission of the data may result in a loss of synchronization between the USB device and the application or target device. Therefore, no transfer retries are permitted.

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