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each ase to the codec configured state the initiator has already determined which codec
configurations are supported so it now selects the config uration it wants for its current
application and sends that along with a target latency and phy the format of parameters
for this operation are defined in table of ascs and summarized in table
section configuring an as e and a cig
parameter description
target latency i x low latency
x balanced latency and reliability
x high reliability
target phy i x m phy
x m phy
x coded phy
codecid i codec configuration id length and configuration
as described in section sink pac and source
pac characteristics codecspecificconfiguration i
these terms are generic and do not necessarily align with the configurations defined as low latency
or high reliability in bap
table state specific parameters for the config codec operation table of ascs
there are a number of points to highlight here the first is that these parameters do not need
to be the same for each audio stream with a bidirectional cis the parameters even the phy
and codec can be different for each direction in most cases th ey are not but if youre
streaming music in one direction and using the return direction for voice control they may
well be currently most profiles require that bluetooth le audio is run using the le m
phy
the second is that the first two of these parameters which are prefixed with target are
recommendations the acceptor will use these recommendations to select the qos
parameters it feels best satisfy the requirement and fit its current status then return those
choices in response to this co mmand
in contrast the codec id and codec specific configuration parameters are a statement of
fact and a directive to the ase to use specific values the codec configuration is generally
based on the top level profile being used which builds on the mandatory options specified in
bap we looked at mandatory and optional qos configurations defined by bap and other
profiles in chapter codec and qos in most cases the initiator will choose one of these
predefine d configurations although it could also use settings for an additional codec which is
defined in a top layer profile along with its recommended qos settings or a manufacturer
specific codec it could also make up its own configuration although that run s the risk of poor
interoperability the predefined configurations have been through a lot of testing to ensure
they perform well and should always be the preferred option
implementers should note that there are two very similar sounding codec specific l tv
structures the first the codecspecificcapabilities ltv structure is used in pac records
and normally indicates a range of capabilities the second the codecspecificconfiguration
ltv structure is used in the codec configuration operation described above and is for a
single specific configuration the syntax of th e individual parameters is subtly different
chapter setting up unicast audio streams
throughout this process there is a degree of the devices dancing around each other making
recommendations so that the other can provide information on what best suits its current
state of resources this isn t a true negotiation its better described as an informed
enumeration allowing the initiator to populate a command to send to its controller to
configure each cis even then some of the parameters in its hci command are also
recommendations with the controller having the final say over how the cig is configured
a third and important point to note is that whilst the acceptor notifies the range of codec
parameters it can support through its pac records the initiator writes specific values which
define how it will configure the encoded audio stream some of these are defined with
different values in the supported settings the acceptor sends and the actual settings the
initiator writes for example as we saw above the supportedsamplingfrequencie s ltv is
a bitfield whereas the samplingfrequency ltv is a specific value mapped to a sampling
frequency so if an ase supports only khz the acceptor will notify a value of x in
response the initiator will configure the ase codec to support khz sampling by writing
x but to return to the process
having obtained the information about the ases their capabilities and availability the
initiator can issue a single write without res ponse command with an opcode of x which
includes an arra y of all of the aseids that it wants to configure on an acceptor which can
cover both directions the acceptor will respond by updating and then notifying its ase
control point characteristic including an appropriate response code x if its ok t he
acceptor then transitions all of the specified ases to the codec configured state once it
has done that the acceptor will notify the new state of each ase including the following
statespecific parameters summarized in table which are described in table of
ascs
field description
framing support for framed or unframed isoal pdus
preferred phy a bitfield of values normally includes mbps
preferred retransmission
number rtn how many times packets should be retransmitted
maximum transmit latency the maximum allowable delay for bluetooth transport
presentation delay min max supported range of presentation delay
preferred presentation delay
min max preferred range of presentation delay
codec configuration the codec configuration for this ase
table notified parameters following a configcodec opcode table of ascs
that concludes the bap codec configuration procedure and moves us on the to the qos
configuration procedure bap
section configuring an ase and a cig
the bap qos configuration procedure
at the end of the codec configuration procedure the initiators host will compare the
configuration parameters that each acceptor notified at the end of the codec configuration
procedure with the requirements of its current application and decide what values it wants to
use within the limits of what it has been told
before issuing the config qos command it is good practice for the initiator to send these
parameters to its controller using the le set cig parameters hci command this will
confirm that the selected parameter set can be supported remember that these are use case
related recommendations which inform the scheduling algorithms in the controll er the
actual values the controller chooses may differ slightly the parameters used in this hci
command are shown in table full details of the command are in the core in vol part
e section
as we saw in chapter an application cannot force specific values for the link layer
reiterating what is happening here the initiator and acceptors have exchanged information
which allow ed the initiator to d etermine what to put into its hci command to instruct the
core to schedule cises that best meet the application requirements bluetooth
implementations use the hci commands for testing to check that they are compliant but they