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chapters that concept has now been extended with connected isochronous stream s so that
we can stream audio to multiple devices but those devices are still connected and every packet
that is correctly received is acknowledged with broadcast there is no connection any
device which is within range of a broadcast transmitter can pick up and render the broadcast
audio streams the big di fference from bluetooth classic audio profiles is that it is one way
there are no acknowledgements that means that it is technically possible to build a
broadcast source which does not contain a receiver but which only transmits those
broadcasts can be received by any and every broadcast sink which is within range
there is a practical advantage in having no acknowledgements which is that it generally results
in a much greater range that occurs because the link budget over a connection is often ve ry
asymmetric a broadcast transmitter is often mains powered so can easily transmit at the
maximum allowed power that gives it a good link budget to an earbud however an earbud
is unlikely to be transmitting acknowledgements back at anything more t han dbm mw
both to conserve its battery and because its small antenna is probably going to have a gain
below db that means that the link budget for the earbud to receive a broadcast transmission
may be db higher than for the return acknowled gement path it is the latter which
determines the maximum range for a cis for public coverage a broadcaster can cover a
considerable area far more than can be achieved with a connected isochronous stream
transmission
as we will see later on there can and often will be a connection but that enhances the way that
broadcast works it doesnt affect the structure of a bis or big so well ignore it for the time being
chapte r isochronous streams
the b is structure
the definition of the structure of broadcast isochronous streams and groups is very similar
to what weve just looked at with connected isochronous stream s
figure shows that broadcast isochronous streams have the same basic pdu structure of
header payload and an optional mic if you want encryption however the header for the bis
is a lot simpler as it does not need the sn and nesn flow control b its that are in the cis
pdu header it starts with the same two link layer id llid bits those indicate whether
the pdu is framed or unframed then come cssn and cstf which are used to signal the
presen ce of a control subevent cssn is the control subevent sequence number and cstf
is the control subevent transmission flag signalling that a control subevent is present in
that bis event these are new and dont exist in connecte d isochronous streams within a
big there is a control subevent which can be used to provide control information to every
acceptor we need these in broadcast as theres no acl to inform acceptors of things like
a change in the hopping sequence the o nly other information in the header is the length of
the payload
figure isochronous pdu and header for broadcast
if we look at the structure of a broadcast isochronous stream in figure its also very
familiar the fundamental difference between a bis and a cis is that there are no
acknowledgme nts a bis is composed purely of subevent s which contain data sent by the
initiator there is no bidirectional data everything is transmitted from the broadcast source
as before we see each subevent is transmitted on a different frequency channel
section broadcast isochronous streams
figure structure of a broadcast isochronous stream
a notable difference between a big a broadcast isochronous group made up of one or
more bises and a cig is the potential existence of a control subevent which is shown
dotted in figure when it occurs which in in bis events where the header of the iso
pdu contains the cstf flag the control subevent is transmitted after the final subevent of
the last bis and provides an opportunity for control information to be sent to every device
which is receiving a broadcast stream well cov er what that does in section
as figure shows a broadcast isochronous stream has the same basic elements of an
isochronous interval and anchor points for bis and big e vents as there is no
acknowledgement or return packet a subinterval time is defined which is the time between
the start of consecutive subevent s within a single bis it is also the time between the start of
the final subevent of the last bis and a control subevent if one is present
if the big contains more than one bis each bis is separated by a bisspacing by adjusting
the values of the subinterval and the bisspacing the bises can be arranged in either a
sequential or interleaved fashion which is illustrated in figure and figure if the
bisspacing is bigger than the subinterval they will be arranged sequentially if its smaller
they will be interleaved
chapter isochronous streams
figure sequential bises
both diagrams show two bises each of which have nse set to two ie two subevent s each
there are some important observations that can b e made from these figures the first is that
the control subevent is never counted in the nse it is a totally separate subevent the
second is that when the control subevent is present it does not affect any of the other packets
or the anchor points it is scheduled immediately after the last subevent of the last bis but
it does extend the big event for the isochronous intervals which contain a control subevent
figure interleaved bises
section broadcast isochronous streams
whereas a cis can stop transmitting audio packets once a device has received them a
broadcast source has no idea whether or not they have been received so it has to repeat every
transmission however a s soon as an acceptor has received a packet it can turn its radio off
and wait for the next scheduled bis this again highlights the asymmetry that we have within
bluetooth le because the broadcast source is always transmitting it typically has a much
higher power drain than the acceptors which are li kely to be earbuds in general that means
that broadcast source s need bigger batter ies or a permanent power source as they are
typically phones or infrastructure transmitters in public places thats net generally an issue
however it should be kept in mind if broadcast transmission is being embedded into small
devices like watches or wristbands
unlike cises all bises have the same timing structure whereas the structure of each
individual cises is normally tailored to its codec configuration optimising the subevent s for
the pdu size every bis subevent is the same length which must fit the largest pdu that is
being transmitted that constraint is because the overall bis timing structure is defined in the
biginfo which is inclu ded in the periodic advertisement s the biginfo structure is limited
in size so doesnt have the granularity to specify different timings for different bises it
specifies a one size fits all it means that if you want to broadcast one channel at a khz
sampling rate and a second one at khz the smaller khz channel would still b e allocated