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framework where we put the isochronous stream s to work
the cis structure and timings
when you design for digital audio you generally have the constraint that youre go ing to be
sampling the incoming audio at a standard consistent rate once the incoming audio is
sampled and encoded its sent down to the bluetooth transmitter to send to the receiving
device the system is repetitive the audio data is time bounded an d transmissions have a
constant interval between them which is called the isochronous interval or the isointerval
the start of each isochronous interval in a cis is called its anchor point as figure
shows the transmission starts off with an initiator sending a packet containing audio data d
to an acceptor when the acceptor receive s it it sends back an acknowledgement and that
process is repeated on a regular basis the third data packet in figure has no
acknowledgement so the initiator would presume it had not been received
chapter isochronous streams
figure simplified unidirectional audio transfer
most modern codecs are optimised to run at a frame rate of milliseconds ie they sample
milliseconds of audio at a time which provides a good compromise between audio quality
and latency thats the preferred setting for lc which is the mandat ory codec for bluetooth
le audio unless i specify otherwise well be assuming a millisecond sampling interval is
used throughout this book so the isochronous intervals will always be ms or multiple s of
ms
isochronous payloads
the structure of the data in the pdu of the air interface packet d shown i n figure which
is sent between devices is very simple and is shown in figure
figure bluetooth le link layer packet format
the encoded iso pdu is preceded by a preamble and access address and followed by a
crc these add or octets when transmitting over a n le m phy depending on
phy refers to the physical layer and specifically the choice of symbol rate which corresponds to
the number of bits which can be transmitted each second currently most bluetooth le products use
a symbol rate of million symbols per second in contrast most bluetooth le audio products will
use the enhanced symbol rate of million symbols per second as that allows higher quality codec
parameters to be used the trade off is a slight reduction in range
section connected isochronous streams
whether there is a m essage integrity check m ic included with the iso pdu payload and
or octets when using a le m phy
for a cis t he iso pdu is called the cis pdu and its structure is shown in figure
figure iso pdu format and header for a cis
the iso pdu has a header followed by a payload of up to octets if the audio needs to
be encrypted theres an optional mic at the end of that packet in the cis pdu header there
are five control elements
the llid link layer id which indicates whether it is framed or unframed
nesn and sn the next expected sequence number and sequence number
which are used for acknowledgments and flow control at the link layer level
cie the close isochronous event bit and
npi the null payload indicator which indicates the payload is a null pdu
identifying that there is no data to send
subevent s and retransmissions
well cover the control bits in the iso pdu header as they become relevant to the explanation
of how cises work b efore that we need to look at the structure of a connected isochronous
stream weve already talked about the isochronous interval which is the time between
successive anchor points of a c is the anchor point is the point where the first packet of a
cis is transmitted by the initiator and the start of each successive isochronous interval
within a cis we can retransmit the cis pdu if required as the cis structure supports
multiple subev ents each subevent starts with the transmission from the initiator and ends
with the final point of the expected response from an acceptor all of the subevent s within
a cis form a cis event which starts at the anchor point of the cis and finishe s at the
reception of the last transmitted bit received from the acceptor
message integrity check a value calculated from the payload contents to detect if it has been
corrupted
chapter isochronous streams
figure events subevent s and subintervals
the time between successive subevent s is defined as the subinterval spacing which is the
maximum duration of the subevent for that cis plus the inter frame spacing which is defined
as being µs the sub interval spacing is determined when the cis is configured and does
not ch ange for the lifetime of the cis
frequency hopping
an important reason for defining subevent s is that bluetooth le audio changes the
transmission channel on every subevent as illustrated in figure to protect against
interference
figure frequency hopping per subevent
the core specification introduced a new channel selection algorithm which is more
efficient than the one in core this is applied to each subevent if a subevent is not
transmitted then the channel hopping scheme assumes that it has been and moves to the next
frequency channel for the fo llowing subevent
closing a subevent
when we looked at the isochronous pdu header we saw that theres a close isochronous
event bit the cie thats used by the initiator to signify that it has received an
acknowledgmen t from an acceptor confirming that its packet was successfully received by the
acceptor so that it will stop further retransmission s of that particular pdu in figure
the initiator has sent out its first pdu and had an acknowledgment back so it then sends a
section connected isochronous streams
packet with the cie bit set to to tell the acceptor that there will be no further transmissions
as it is closing the event it would not normally bother to include the audio data in that
transmission so it would also set the null packet indicator bit allowing it to shorten the
transmitted packet figure shows the duration of the original cis event for
comparison
figure closing a cis event early with the cie bit