In its simplest form, data is transmitted and the sender waits for an acknowledgment from the receiver whether the data was received properly or not. If the data was received properly, the next data is sent and the process continues. If the data was received improperly or this acknowledgment was never returned, an error has occurred and the sender must either re-send the original data or initiate some error recovery process. In actual practice, the sender transmits multiple data and uses a queuing process to keep track of corresponding acknowledgments. Asynchronous mode therefore transports data whenever the acknowledgment process is satisfied.
Isochronous mode implies uniform in time.
The sending device first negotiates with the other devices on the bus for this bandwidth. Once granted, that bandwidth belongs to the sending device and no other device can contend for it. The sending device now transmits a packet of data at uniform time intervals filling up its bandwidth allotment. This process is extremely efficient as no bus time is wasted waiting for acknowledgments or contesting requests from other devices for bus time. Isochronous mode is really what makes 1394 technology so great and perfect streaming digital video, audio, and mission critical data.