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once all of the ases associated with a cis have been disabled the initiator can disable any
cises which are still enabled and tear down the associated data paths when all of the cises
in a cig across all acceptors have been disabled the cig should be moved to the inactive
state
section configuring an ase and a cig
the source ase state machine
a source ase behaves slightly differently as we need a confirmation from the initiator t o
ensure a clean transition this introduces a disabling state it is only used for source ases
and is shown in the source ase state machine of figure
figure state machine for a source ase
for a source ase the acceptor needs confirmation from the initiator that the initiator is
ready to stop receiving audio data the acceptor can autonomously stop streaming but the
receiver stop ready command results in a clean termination of streaming this is important
if there is a potential requirement to reuse the ase having notified the fact that i t is in the
disabling state the acceptor should wait for a command from the initiator to tell it to stop
streaming and move to the qos configured state from there the source ase can be moved
to the releasing state but if it does it will not be possib le to reuse the cis without taking the
ase through the state machine again
once in the releasing state whether by a direct transition from the streaming state for a sink
ase a direct transition from the disabling state for a source ase or being released from the
qos configured state for either both initiator and acceptor should tear down their data paths
and terminate the cis the acceptor can choose to transition to either the idle or codec
config ured states both of which operations are performed autonomously if the cis is
bidirectional it should not be terminated until both sink and source ases are in the releasing
state when the final cis is terminated the cig moves from the inactive cig state to the
no cig state
chapter setting up unicast audio streams
autonomous operations on an ase
weve already seen that an acceptor can autonomously transition a sink ase from the
enabling state to the streaming state and from the releasing state to the idle st ate or the
codec configured state these are not the only occasions where an acceptor can act
autonomously with the exception of the transitions shown in table all of the state
machine transitions can be performed by either the acceptor or initiator however in most
cases moving around the state machine is performed as described above
ase type current state next state initiating device
sink and source codec configured qos configured initiator
sink and source qos configured qos configured initiator
sink and source qos configured enabling initiator
source disabling qos configured initiator
sink and source releasing codec configured acceptor
sink and source releasing idle acceptor
table ase transitions which are confined to initiator or acceptor actions
acl link loss
if the acl link is lost between an acceptor and an initiator all cises to that acceptor are
disconnected where there are other acceptors involved in the cig the acceptor should
move all of the ases associated with the lost acl link to the qos configure d state so that
the cises can be reenabled if the link returns the acceptor will notify this change of state
although it is questionable whether the initiator will receive the notification
as acceptors are not necessarily aware of the existence of a ny other acceptors they may
not know whether the cig is still active or streaming to other acceptors if they dont receive
a reconnection request after a link loss they may employ an implementation specific timeout
to return their ases to the idle st ate on a later reconnection of the acl link the initiator
should read the ase characteristics of t hat acceptor to check its state if it was released from
the qos configured state because of a timeout the initiator will normally need to tear down
and reestablish the entire cig remember that there is an asymmetry between initiator and
acceptor the ase state machine resides on the acceptor whereas the cig state machine is
on the initiator an acceptor is never aware of the cig state it is what th e initiator which
has a global view of the acl connection status uses to drive the ase states
section handling missing acceptor s
handling missing acceptors
before starting to configure ases cap requires that an initiator connects to all of the
acceptors in th e target coordinated set in the real world there will be occasions when not
all of them are present either because one of them is turned off missing or out of range in
this case the initiator should go ahead with setting up the members of the coor dinated set
which it can find how long it waits before this happens and whether it involves user feedback
as well as the choice of audio channels to send to the available acceptor are all
implementation specific the initiator should continue to search for the missing acceptors
adding them in when it discovers them
the nature of a cig is that once it is active additional cises cannot be configured hence
if the initiator only configures cises for the acceptors it can find then if missing ones tur n
up and no cis had been scheduled for them it would have to tear down the cig reconfigure
it and re establish the cises which will disrupt the audio streams
to prevent that break in the a udio an initiator can schedule the cig with cached values for
any missing ases if it detects the presence of the missing acceptor at a later point it can
configure their ases then enable the associated cises in the active cig as they have already
been scheduled and start the transfer of audio data cap doesn t describe this process its
an extension of functionality by using bap procedures in addition to cap ones but it can
result in a better user experience
preconfiguring cises
a similar case exists where an initiator knows that an acceptor is likely to participate in a
number of different use cases which have different ase configurations a common example
of this is streaming music the analogue of adp which just needs sink ases to enable the
cises carrying audio from initiator to acceptor but which may be interrupted by incoming
phone calls where a return audio stream from the microphones is required to make this
transition faster the initiator can configure a set of ases which fulfil both use cases ie two
sink ases and two sourc e ase so that when it wants to transition between the two
applications all it needs to do is enable or disable the source ases as opposed to tearing
everything down and restarting
the downside to this is that the airtime for the return isochronous st ream is always allocated
although it may never be used a stereo stream for high reliability takes up around
of the airtime if it is scheduled to include a return stream that increases to
by comparison a bidirectional s tream takes only of airtime there is also a
discrepancy in the latency requirements of the two applications for music streaming which
generally uses the high reliability qos settings the overall latency for a stream with
a return i s just over ms thats a lot longer than the ms you would get with a
bidirectional low latency stream table illustrates the effect of different qo s