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section connected isochronous streams
events it results in a much greater gap after their transmissions freeing up airtime which can
be used for other purposes
bidirectional cises
the multiple unidirection al connected isochronous stream s weve defined above replicate the
use case of adp for earbuds and many other audio applications we want to get data back
as well requiring bidirectionality one approach would be to set up a second cis in the
opposite direction so that we would use one cis to transmit from phone to earbud and the
other to transmit from earbud to the phone however thats inefficient the core
specification provides an optimis ation by adding return data into the acknowledgement
packet s its still a single cis b ut its now being used for two separate audio streams
figure example of a bidirectional cis for the left earbud and a unidirectional cis for a right earbud
in the example shown in figure the initiator has set up individual cises with a left
acceptor and a right acceptor both receive data from the initiator but the left one also
sends data from its microphone back to the phone the same principles weve seen before
apply data is sent by the initiator the acceptor immediately responds to acknowledge receipt
or not and if it has data includes it in the return cis pdu if that ret urn pdu gets back
with a good crc the initiator will acknowledge it close the event and transmit data to the
right acceptor cis
the acknowledgements in both directions use the nesn and sn bits in the iso pdu header
flipping the bits when a packet is acknowledged in figure all of the transmitted packets
have the same iso pdu format those marked ack contain no audio data so would have
the npi bit set to one to indicate they have a null cis pdu the data packets from the left
acceptor will be an identical format to the initiators l and r packets but will contain
data from the acceptors microphone the initiators ack of the left acceptors data would
include the cie bit occurs before it transmits the data to the right acceptor shows that it is
transmitting the cises in sequential mode these ack s would also have the cie bit set
chapter isochronous streams
the value of nse applies to both directions in a cis as the same subevent s are used for
transporting pdus both to and from the acceptor once the payload in one direction has
been acknowledged future transmissions can replace the payload of that pdu with a null
payload that has no effect on the ti ming of the subevent s but saves a small amount of
power
only the initiator can set the cie bit in the iso pdu header in a bidirectional cis to
indicate that they are not ging to transmit any further payload data in the current cis event
it should not close the cis event unless the payloads for both directions have been
acknowledged if an initiator has had its packet acknowledged but has not received an
acceptors packet it should continue to transmit null pdu packets in every available
subevent to give the acc eptor opportunities to retransmit
the two directions in a bidirectional cis can have different properties such as codec and
phy and may even be used by different applications even some of the structural parameters
can be different for the two directions ft and bn can be different as can the maxpdu
and maxsdu values which also means that the sduintervals can be different although
one needs to be an integer multiple of the other however the isointerval packing
framing and nse m ust be the same
synchronisation in a cis
once we add a second acceptor both acceptors act independent ly of each other but under
the initiators control if they are a coordinated set they will know the other one exists
because they know how many members there are in the coordinated set however your left
ear bud and right ear bud dont necessarily know the other one is present turned on or
receiving data a user may take one out to share with a friend or the batte ry in one may die
even if they have a means of communicating there is no guarantee that they will always be in
contact with each other to cope with this and enable them to render or capture audio streams
at precisely the same time the core isochronou s channels design includes a synchronisation
method which allows microsecond level accuracy of rendering without an acceptor needing
to have any knowledge of the presence or otherwise of any other acceptors figure
illustrates how this is done
within a cig cis events are scheduled for each of the cises in that cig for a pair of
earbuds that will be two cis events one for the left earbud o ne for the right earbud it could
be more such as with multi channel sound systems these cis events make up a cig event
in figure for the sak e of simplicity the multiple cises are shown as sequential they
could equally be interleaved
section connected isochronous streams
figure synchronisation of multiple cises
the core defines a cig reference point which may be coincident with but cannot be later
than the anchor point of the first cis typically it will occur slightly before that it also
defines a cig synchronisation point which occurs after the last possible receive event fo r the
last cis event within that cig at that point the initiator knows that every acceptor will
have had every possible opportunity to receive every pdu that has been sent to it and time to
return any data for the initiator
in order to reconstruct th is common point every device is informed of its individual cis sync
delay for every cis which it supports which is calculated from the instant for the acl link
associated with that cis this allows it to calculate the cig synchronisation point which is a
common timestamp across every device with this knowledge each device can determine
when it should start to decode the audio bap adds a feature called presentation delay which
tells the acceptor when to render th e decoded stream as we move up the layers and start to
dig into the detail of the basic concepts of qos and latency well see how presentation delay
is used to add flexibility to the rendering time
if you have devices with microphones the same synch ronisation problem exists but in this
case you need to coordinate the point at which audio is captured by an acceptor if it uses
the uplink of a bidirectional cis it will use the same cig synchronization point but is likely
to require a different valu e for presentation delay to the one used to render the downlink
an insta nt is a timing reference normally the start of a transmitted packet which is used for
synchronisation its use is defined in the core vol part b sect
chapter isochronous streams
audio data from the initiator as it needs extra time to capture and encode the audio stream
the synchronisation timing is defined with a microsecond accuracy this means that the
controlle rs of all of the acceptors within a cig have a timestamp assigned which will be
within a few microseconds of each other that needs to be conveyed to the application layer
that renders the audio streams although the method of doing that is down to the
implementation however the specification means that left and right earbuds can present the
two audio streams to the ears around an order of magnitude closer together than the brain can
discern
the ci g state machine
having cover ed all of the features that make up connected isochronous stream s we can look
at how to put them together to configure and establish a connected isochronous group