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section broadcast isochronous streams
if the market develops in this way users will often find themselves within range of multiple
broadcast source s which will mean they need to choose which to receive a first level of
selection can be provided by profile uuids in auxadvind packets but devices will still
need to detect and parse the base information for each big to make a decision in the early
days it may be possible to select the first big you find then press a button to move to the
next but t hat is not a scalable user experience for the long term this presents product
designers with a problem as devices like earbuds have no room for a display and often barely
room for any buttons in addition the process of scanning is relatively power h ungry
constant scanning would have a noticeable impact to an earbud or hearing aids battery life
to get around this limitation the bluetooth le audio specifications have introduced the
concept of a commander a role which performs the scanning operat ion allows a user to
select a broadcast and then instruct a receiving device to synchronise to that selected bis or
bises the commander role can be implemented as part of an app in a mobile phone in a
smart watch an earbud case or a dedicated remote control device in fact any bluetooth le
device which has a connection with a broadcast sink can act as a commander devices which
implement the commander role are called broadcast assistant s and are defined in the
broadcast audio scann ing service bass they can be integrated into devices like tvs to
help automate the connection to earbuds and headphones
periodic advertising synchroni sation transfer past
a broadcast assistant scans for advertisements which indicate the presence of extended
advertisement s in exactly the same way as any other scanning device once it discover s them
it can synchronise to the associated periodic advertising train which contai ns a broadcast
audio announcement service uuid and then discover the accompanying biginfo and
base structure it may apply filters to its scan before reading and parsing the base metadata
of those it finds after which it provid es the list of available br oadcast streams to the user
generally by displaying the human readable programinfo
once the user has made their selection the broadcast assistant will use the past procedure
to provide the broadcast receiver with the information it needs to find the relevant periodic
advertising train this allows the broadcast receiver to jump straight to those advertising
packets acquire the biginfo and base and synchronise to the appropriate bises without
having to expend energy in scanning this process is calle d periodic advertising
synchronization transfer or past
the level of information provided by a broadcast assistant to the user is entirely down to the
implementation a phone app could list every broadcaster within range it could limit the
the core does not use the acronym past for periodic advertising synchronisation transfer so if
youre searching the core you need to use the full name the past acronym is introduced in bap
chapter isochronous streams
amount of i nformation displayed based on user preferences or use preconfigured settings
which it had read from a set of earbuds the broadcast assistant could equally be a button
on a watch or fitness band which selects the last known synchronised stream from the l ist of
broadcast sources it has found broadcast assistant s can also include applications to obtain
the required broadcast code to decrypt private encrypted broadcasts either by making a
bluetooth connection to the broadcaster or a proxy or by using an out of band oob
method
broadcastcode
encrypted streams are not new in bluetooth technology security and confidentiality have
always been an important part of the specifications however implementing encryption when
there is no connection between the transmitting and receiving devices as is the case with the
broadcast streams in bluetooth le audio introduce s a new problem which is how to cope
with encryption
many broadcast streams will not be encr ypted they will be broadcast openly so that anyone
can pick them up however that generates a few issues firstly anyone who is within range
can receive them as bluetooth technology is quite efficient in penetrating walls that can be
an issue in m eeting rooms hotel rooms and even homes where someone in a neighbouring
room could inadvertently pick up the audio from your tv that could range from annoying
to embarrassing depending on what the content is in a business environment it may well be
confidential so adding encryption is vital basic confidentiality is inherent within telecoil
systems as the audio transmission can only be picked up within the confines of the induction
coil to provide the same level of authentication in bluetooth le audio broadcasts the audio
data needs to be encrypted which requires the receiver to obtain the decryption key which is
known as the broadcastcode
devices looking for broadcasts can detect whether a broadcast audio stream is encrypted by
examining t he length of the biginfo in the periodic advertising chain if the stream is
encrypted the packet will include an additional octets containing a group initialisation
vector giv and a group session key diversifier gskd scanners will detect their
presence and in conjunction with other metadata determine whether or not to synchronise
with such a stream depending on whether they are able to retrieve the broadcastcode
although broadcast does no t need a broadcast source and sink to be paired in many cases
there will be an acl connection present that may sound strange but its an arrangement
that allows the number of encrypted connections to be scaled far beyond what is possible with
unicast without hitting an airtime limit this is expected to be the way most domestic tvs
will work so that neighbours cant hear what you are listening to in this case the users would
be pair ed to the tv using the features of bass to obtain the broadcastcode the
broadcastcode is a property of the host application the host provides it to the controller
when it is setting up the bises and it can equally supply it to trusted devices in public spaces
conference rooms and hotels it is likely that an out ofband method would be used probably
section broadcast isochronous streams
similar to the printed information which is used to connect to a wi fi access point
broadcastcodes can also be obtained through othe r out of band methods such as scanning
qr codes tapping an nfc terminal or even being included with a downloadable theatre
ticket well explore these configurations in more detail in chapter
note that the metadata used to describe broadcasts within advertising packets is not encrypted
accordingly care should be taken to ensure that it does not contain confidential or potentially
embarrassing information
broadcast topologies
the features included in bluetooth le audio provide a wide range of options for finding
broadcasts we will look at them in more detail in later chapters but t he figures below show
some of the common ones in most cases they show a single broadcast information
stream which encompasses all of the advertising information
figure direct synchronisation from headphones
the simplest case where a pair of headphones does its own scanning to find and select a
broadcast audio stream is shown in figure
figure using a phone as a broadcast sink
figure is essentially the same but points out the broadcast sink could also be a phone
here it can scan for broadcaster s display the available choice of broadcast audio streams to