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LeAudio

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the method for doing this is called extended advertising and was added to core
returning to basics for a moment figure show s the channels which are used for
bluetooth le for bluetooth le the ghz spectrum is divided into channels each
mhz wide three of these are reserved for advertising at fixed frequencies channels
and and are known as the primary advertising channels
chapter isochronous streams
figure bluetooth le channels
the position of these three channels was chosen to avoid the most commonly used part s of
the wi fi spectrum in betwee n the three advertising channels are general purpose
channels numbered from to
broadcaster s need to provide a fair amount of information if other devices are going to be
able to receive their transmissions not only do they need to tell the rece ivers where the
transmissions are located in terms of hopping channels but they also need to provide all of
the details about the structure of the big and its constituent bises which we have just
covered in many situations there are likely to be multi ple broadcast transmitters within range
of an acceptor particularly where theyre being used in public places and hence multiple
broadcast isochronous streams to choose from acceptors need to be able to differentiate
between them implying that the bro adcast advertisements contain information about the
content of each broadcast stream all of this requires a large amount of information to be
conveyed by the broadcaster it is too much to fit into the three primary advertising channels
ie channels and without the risk of overloading them
to overcome this limitation the extended advertising scheme of core allow advertising
information to be moved to the general purpose channels channels to to accomplish
this a broadcaster includes an auxiliary pointer auxptr in its primary advertisements which
informs devices that further advertising information is available in the general purpose
channels which now serve as secondary advertising channels the auxiliary pointer
provides the information that scanni ng devices need to determine where these secondary
advertisements are located at this point a scanner doesnt know whether the auxptr is for a
broadcast that only becomes apparent when it finds and reads the extended advertisement
figure illustrates the basic structure of this scheme
section broadcast isochronous streams
figure extended advertising for a big
the extended adve rtising process is built up using three types of advertising pdu
advextind extended advertising indications which use the primary
advertising channels each big requires its own advertising set and
advextind the headers of ea ch advextind include the advertising
address adva the adi which includes the set id for this set of extended
advertisement s and an auxiliary pointer to the
auxadvind auxiliary advertising indications these are transmitted on the
general purpose channels they contain the advertising data specific to a big
the auxadvind also include a syncinfo field which provides information
for scanners o n how to locate the
auxsyncind auxiliary synchronisation information these advertisements
contain two important pieces of information the first is the additional controller
advertising data field acad this is information w hich is required by the
controller of the receiving device describing the structure of the big and its
constituent bises the second is information f or the host of the receiving device
which is contained in the advdata field this contains the basic a udio
announcement servic e uuid and the base which contains a detailed definition
of the streams in the big including the codec configuration and metadata which
describes the use cases and content of the audio stre ams in a user readable format
the presence of the auxiliary pointer auxptr in an advextind packet indicates that
some or all of the advertisement data can be found in an extended advertisement however
the amount of information thats needed to describe and locate the broadcasting stream is still
bigger than what normally fits into an extended advertising packet to accommodate this
the extended advertisement could chain further packets by including an auxptr to an
auxiliary auxchainind pdu but it doesnt instead it sets up a periodic advertising
train which includes all of the information about a big and its bises within an
auxsyncind pdu the auxsy ncind packets in a periodic advertising train are
chapter isochronous streams
transmitted regularly so once a scanning device has found them it can continue track ing them
without having to refer back to the primary or extended advertisement s the broadcaster
can even turn off it s extended advertisement s but keep the periodic advertising train
running
the presence of this periodic advertising train is indicated by a syncinfo field in the
auxadvind pdu of the extended advertisement which provides information on
where to find it should the amount of information for the periodic advertising train be too
large to fit into a single pdu the auxadvind can also use an advptr to point to an
auxiliary auxchainind pdu in general the auxchainind pdus are not
needed
the data needed to locate and receive a broadcast audio stream starts in the extended
advertisement s the aux adv ind contains the broadcast audio announcement
service uuid which identifies the extended adv ertisement s as belonging to a specific
broadcast audio stream with a octet broadcastid which identifies it the broadcastid
remains static for the life of the big the auxextind may include additional service
uuids from top level profiles these allow a scanning device an early opportunity to filter
the different broadcasts it finds so that it doesnt need to synchronise with the pa and decode
the information it carries for broadcasts that are not of interest to it that feature is particularly
useful to help choose public broadcasts which are defined in the public broadcast profile
pbp which defines the public broadcast announcement uuid
having decided it like s the look of the big a scanner wi ll synchronise to the periodic
advertising train these advertisements use the general purpose channels to like
extended advertisement s they can be chained to include more data if required using another
auxptr for broadcast audio they contain two vital pieces of information
the first is the additional controller advertising data field the acad the acad contains
advertising data structures these are data structures containing an advertising data ad
type and its corresponding data all devices which are actively broadcasting will use the acad
to send the biginfo structure which provides all of the information needed by a receiving
device to detect the broadcast a nd understand the bis timings this is the information which
would be delivered to acceptors at the link layer in a cig but with no direct connection in
broadcast thats not possible hence this alternative method
section broadcast isochronous streams
figure the biginfo structure
figure shows the structure of the biginfo it starts with the big offset which inform s
devices exactly where they can find the big in relationship to this auxsyncind pdu
following that the big info describ es the structure of that big