Isochronous operations involve sending or receiving a constant stream of packets at a fixed rate of 8KHz. Unlike raw1394's asynchronous API, where you "push" packets to raw1394 functions at your leisure, the isochronous API is based around a "pull" model. During isochronous transmission or reception, raw1394 informs your application when a packet must be sent or received. You must fulfill these requests in a timely manner to avoid breaking the constant stream of isochronous packets.
A raw1394 handle may be associated with one isochronous stream, either transmitting or receiving (but not both at the same time). To transmit or receive more than one stream simultaneously, you must create more than one raw1394 handle.