The timely delivery of isochronous data is ensured at the expense of potential transient losses in the data stream. In other words, any error in electrical transmission is not corrected by hardware mechanisms such as retries. In practice, the core bit error rate of the USB is expected to be small enough not to be an issue.
USB isochronous data streams are allocated a dedicated portion of USB bandwidth to ensure that data can be delivered at the desired rate. The USB is also designed for minimal delay of isochronous data transfers.