last Set Interface DS request it must treat the Set Interface request as it normally would in a situation that did not involve dynamic switching.

•If a device receives a Set Interface DS request and does not receive a Set Interface request for an extended period of time it must follow the rules outlined previously in this section. If the buffer for an isochronous OUT endpoint empties or the buffer for an isochronous IN endpoint starts to overflow the device may undertake vendor specific error reporting/handling steps at any time. The host must attempt to prevent this situation from occurring. However, a dynamic switch will typically be an attempt to avoid problems due to link degradation and these errors may occur.

•A host must not attempt to make a dynamic switch to an interface with larger bandwidth requirements if it does not already have allocated bandwidth to support the change.

•A host must not attempt to send a Set Interface DS request with a switch time earlier than the current Wireless USB Channel time.

•A host must not send data in the pre-switch format to an isochronous OUT endpoint with presentation times after the switch time.

•A host must not send data in the post-switch format to an isochronous OUT endpoint with presentation times before the switch time.

•A host must not make any requests for data from an isochronous IN endpoint that only has data in the post-switch format buffered before performing the Set Interface step in the switch process.

4.11Special Considerations for Isochronous Transfers

This section begins with an overview of the key features of wired USB isochrony. It then describes the challenges with wireless media that prevent this model from being used with Wireless USB. The section concludes with a high level overview of the Wireless USB isochronous model.

4.11.1Summary Of Key Features Of USB Wired Isochrony

This section presents a summary of the key features of the wired USB isochronous transfer model. The Wireless USB approach to isochronous transfers has significant differences from the wired model.

Consider a full speed wired USB device with an isochronous endpoint. When an isochronous stream is started the device has a contract guaranteeing the opportunity attempt to send or receive the requested amount of data each service interval. The amount of data to be moved is part of the endpoint descriptor for the isochronous endpoint. The delivery itself is not guaranteed (wired isochronous traffic does not use handshakes or retries), however wired USB bit error rates are required to be 10-9 or better. Therefore, the loss of data (when the requested amount of data is not sent or received in a service interval) is rare. The USB specification does not make any guarantees on the location of the service it provides to a wired USB isochronous endpoint in the service interval. The USB host sends Start of Frame (SOF) packets at the beginning of each frame. The SOF contains a frame index that rolls over every 2048 frames. The host is responsible for sending the SOF packets at regular 1 millisecond intervals. Figure 4-19 shows the worst case variation of service location for a FS wired USB device with a service interval of 1 millisecond (1 USB frame).

57

Figure 4-19. Worst Case Service Interval Jitter For FS Isochronous Endpoint

In this case device A has an isochronous endpoint that has been admitted to the bus. The endpoint is guaranteed an amount of time on the bus each frame as shown in Figure 4-19. There is no guarantee on the location of the service opportunity. The service interval could approach 2 frames or be as small as the duration of the SOF, reservation and associated guard band. These extremes could happen in successive frames as shown in Figure 4-19. In typical operation the service attempts are evenly spaced as shown in Figure 4-20.

Figure 4-20. Typical Service Interval For FS Isochronous Endpoint

The basic operation of a HS wired USB isochronous endpoint is very similar to the FS case. The frame size changes to 125 microseconds. These divisions are known as microframes. There is still an indexed SOF packet sent by the host at the beginning of each microframe. Figure 4-21 shows the typical and worst case service gaps for a HS isochronous device with a service interval of 1 microframe.

58