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>2.5. Isochronous Transmissions</A
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> Nodes can allocate a channel and bandwidth for isochronous transmissions
at the iso manager to broadcast timing critical data (e.g. multimedia
streams) on the bus. However these transmissions are unreliable, there
is no guarantee that every packet reaches the intended recipients (the
software and hardware involved also take iso packets a bit more
lightly). After a cycle start packet, the isochronous cycle begins and
every node can transmit iso packets, however only one packet per channel
is allowed. As soon as a gap of a certain length appears (i.e. no node
sends anymore), the iso cycle ends and the rest of the time until the
next cycle start is reserved for asynchronous packets.
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> The channel register on the iso manager consists of 64 bits, each of
which signifies one channel. A channel can be allocated by any node by
doing a compare-swap lock request with the new bitmask. Likewise the
bandwidth can be allocated by doing a lock request with the new value.
The bandwidth register contains the remaining time available for every
iso cycle. Since you allocate time, the maximum data you are allowed to
put into an iso packet depends on the speed you will send at.
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> On every bus reset, the resource registers are resetted to their initial
values (all channels free, all bandwidth minus some amount set aside for
asynchronous communication available), this has to happen since the
isochronous manager may have moved to another node. Isochronous
transmissions may continue with the old allocations for 1000ms. During
that time, the nodes have to reallocate their resources and no new
allocations are allowed to occur. Only after this period new
allocations may be done, this avoids nodes losing their allocations over
a bus reset.
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> libraw1394 does not provide special functions for allocating iso
resources nor does it clean up after programs when they exit. Protocols
exist that require the first node to use some resources to allocate it
and then leave it for the last node using it to deallocate it. This may
be different nodes, so automatic behaviour would be very undesirable in
these cases.
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