15
English
Technical Explanations
■
Isochronous transfer
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
In order for a node to transmit data packets, it must obtain bus access rights. The root node
arbi-
trates 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.
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.
Figure 7: Isochronous sub-actions
5. Bandwidth Issues
125
µ
s
Isochronous portion
Isochronous portion
Arbitration
Data packet
ch1 ch2 ch3 ch4 ch5 .....
Time
mLAN-body(E).book Page 15 Monday, October 2, 2000 1:51 PM
