Figure 4-39 Receive window after false acknowledgement of S3.

Given the periodic nature of the traffic under consideration, the device may know the presentation time that will be associated with each burst sequence number. This information may be used by the function endpoint to transmit “false” acknowledgements for stuck packets before they expire to keep the flow going. With this type of implementation a host will not receive accurate packet error rate information for the link. However, the host may still have knowledge of the link condition by tracking how close the packet presentation times are to the Wireless USB channel time before the packets are acknowledged. This type of implementation may not be possible if data does not follow predictable patterns. It could also lead to some amount of unnecessary data discard in error scenarios.

Although it is not guaranteed that the host will not begin a discard sequence (the host could fail to receive the false acknowledge handshake), with this type of implementation a device will still be able to re-synchronize its receive window with host’s transmissions without processing the WISOCH_DISCARD IE.

4.11.7.3Presentation time unaware implementations

A device’s ISO endpoint receiver implementation may not be capable of processing the timestamp associated with each Isochronous data packet. In this case, the device must be able to resynchronize with the host based on the information contained in the WISOCH_DISCARD IE and according to the mechanism described in Section 4.7.5.

The data sink may delay the consumption of received packets by as many periods as the number of packets discarded (as indicated in the WISOCH_DISCARD IE).

During a prolonged discard sequence the device may receive data packets subsequently discarded by the host and accounted for in the WISOCH_DISCARD IE (the host did not receive the handshake packets from the device).This can be dealt with in several ways.

In a basic implementation, like the one illustrated in Figure 4-40, when receiving a WISOCH_DISCARD IE the device would flush all data which has not been consumed by the data sink, and reset its receive window as indicated by the discard IE.

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Figure 4-40 Receive window after receiving WISOCH_DISCARD IE and burst for sequences S6, S7, and S0 correctly.

A more sophisticated device could implement some mechanism (e.g. a counter) to correctly determine how many of the packets indicated by the host as discarded were received in past burst transmissions. It will then update accordingly its current receive window. This type of device must know the expected presentation time for each packet based on its sequence number.

An example of how this type of device would update its receive window is depicted in Figure 4-41. In this type of implementation data is never discarded after being received successfully.

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