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this allows an acceptor to make a decision on what different initiators can do such as
controlling which devices can stream m edia to it or establish an audio stream for a phone call
on a per client basis how this is managed is up to the implementation and it is down to the
acceptor to manage the different namespaces required to do this when implemented it
provides a way fo r acceptors to set connection policies based on use cases which was not
possible with bluetooth classic audio profiles
ascs the audio stream control service
pacs specifies the way that an acceptor exposes its properties and what it is available to do
at any point in time for most devices it will include a wide range of options of codec settings
and contexts whether it can act as a source andor sink and which audio locations it
support s that range needs to be narrowed down to individual sets of parameters to establish
audio streams typically because of current resource constraints such as when an acceptor is
supporting multi ple audio streams which may need different parameters th is is where the
audio stream control service comes into play by defining endpoints for each audio stream
the audio stream control service is implemented on an acceptor and defines how an
initiator can configure and establish an audio stream between the two of them using the
procedures defined in bap
chapter setting up unicast audio streams
audio stream endpoints
at the heart of the audio stream control service is the concept of audio stream endpoints
an audio stream endpoint or ase is defined as the origin or d estination of a unicast audio
stream in an acceptor an initiator does not contain any ases it configures the ases on
the acceptors if audio data flows into an ase its a sink ase if it flows out of it its a
source ase both of which are described by read only characteristics an acceptor must
expose at least one ase for every audio stream it is capable of supporting
for each ase an acceptor maintains an instance of a state machine for each connected
initiat or the state of an ase is controlled by each initiator although in some cases an
acceptor can autonomously change the state of an ase whereas a sink and source pac
provide an initiator with device wide properties a sink and source ase represent the c urrent
state and properties of each connection these can be read using the sink or source ase
characteristics which will return the information shown in table
field size
octets description
aseid a unique id for each client namespace
asestate x idle
x codec configured
x qos configured
x enabling
x streaming
x disabling
x releasing
state specific ase parameters varies depends on the state see ascs
tables to
table ase characteristic format
figure shows the state machine for a sink ase the state machine for a source is slightly
more complex as it contains an additional disabling state well start with the sink ase state
machine as the journey through to enabling an ase is essentially the same up until the
streaming state then look at the differences when we get to the disabling state
section ascs the audio stream control service
figure the state machine for a sink ase
the ascs defines how an initiator moves each ase on an acceptor through these states by
writing a succession of commands to the ase control point characteristic using the opcodes
listed in table they are defined in section of ascs
opcode operation description
x config codec configures an ases codec parameters
x config qos configures the ases preferred qos parameters
x enable applies the cis parameters and starts coupling an ase
to a corresponding cis
x receiver start
ready completes the cis establishment and signals that an
ase is rea dy to receive or transmit audio data
x disable starts to decouple an ase from its cis
x receiver stop
ready
source ase only signals that the initiator is ready to stop receiving data
and completes decoupling a source ase from its cis
x update metadata updates metadata for an ase
x release returns an ase to the idle or codec configured state
table ase control point operation opcodes
opcodes x config codec and x config qos can be used to transition the state of
an ase or update the configuration of an ase which is already in that state opcode x
updates the metadata but doesnt change the state all of the other opcodes result in a
transition to another state in the ase state machine
chapter setting up unicast audio streams
an initiator can perform an ase control point operation on a single ase or multiple ases
on an acceptor as long as the ases are in the same state if the cig includes multiple cises
either on one or multiple acceptors the initiator should ensure that it has collected the
relevant information from every ase on every acceptor before moving the ases on each
acceptor to the enabling state thats repeating what cap says which is to perform the bap
procedures on all members of a coordinated set in the correct order in most cases all of
the acceptors within a cig will be members of a coor dinated set however ad hoc sets are
allowed in cap where the acceptors can be allocated to work together without being
members of a coordinated set in which case the order of configuration is left to the
implementation
a feature of ases is that an acceptor supports a separate instance of each ase for every
current connection to an initiator if there are multiple initiators with active acl connections
to an acceptor the acceptor will maintain an independent set of ase values for each initiator
it maintains a different aseid namespace for each initiator whilst the aseid must be
unique within each namespace the same aseid can exist in different namespaces that
means that each aseid and its corresponding handle remains unique managing tha t is
down to implementation a change to an ase in one namespace does not affect the state of
the same ase in a different namespace although the change may result in an application
moving a cis from one ase to another one as a result of that change but that is
implementation specific
where acceptors have built up a connection history to multiple initiators they may decide to
maintain sets of ases with cached configurations this simplifies the process of transitioning
to audio streams from a differ ent initiator although the specification allows an acceptor to
have concurrent audio streams enabled with multiple initiators in practice that requires a lot