A Steady Data Stream

Unlike asynchronous and synchronous communication, which both involve elaborate error checking mechanisms, the driving force behind isochronous communication is a fast, steady, uninterrupted data stream. Isochronous clocking information is derived from or included in the data stream, and the delay factor is dependent on a channel's characteristics and can be logically determined. Communication can be disrupted if the transmitter does not maintain a constant transfer rate, or if the receiver has an insufficient buffer to store data at the rate it is arriving and then hold it until it can be processed by software. To maintain data transfer speed, error checking is often omitted. Though software can be written to track errors, there is no hardware mechanism by which to request retransmission of corrupted data.

Isochronous communication is best suited for applications where a steady data stream is more important than accuracy. A good example is video conferencing where infrequent small “blips" in the data stream are tolerable, however, long pauses between a transmission and a response are not.

To ensure that isochronous transfers are not bogged down by other devices, the USB specification sets aside bandwidth for them.