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Isochronous transfer
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The isochronous data transfer used by IEEE 1394 is a transfer method that
guarantees the right to transmit or receive data at fixed intervals (125
microseconds). This makes it possible to transmit data in realtime. It is
particularly suitable for data of a realtime nature, such as video and audio.
This fixed interval (125 microseconds) is managed by the cycle master node, and
access rights are granted preferentially to the cycle master
node.
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Arbitration
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In order for a node to transmit data packets, it must obtain bus access
rights. The root node arbitrates bus access rights, and ultimately grants
access rights to a single node. It is not possible for multiple nodes to
simultaneously access a single bus.
Isochronous sub-actions
Arbitration is performed as follows:
- The node that wishes to transmit a data packet first transmits a "request" to
the root node.
- Upon receiving this request, the root node transmits "permission" to the node
that sent the request.
- The node that transmitted the request receives the "permission", thereby
obtaining access rights.
- The node that obtained access rights may then transmit data packets.
- The request and permission can be transmitted much faster if the node is closer
to the root node (that is, fewer hops between the node and the root). Consequently,
the star topology can handle the bus more efficiently than a daisy-chain
topology.
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