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bis subevent timings for the larger khz packets which wastes airtime if that causes a
problem an alternative is to transmit the packets in separate bigs in this case that would
mean one big for the khz sampled packets and another for the khz sampled ones the
downside it that this adds complexity and doubles the am ount of advertising as each needs its
own advertising set implementers need to decide which works best for their specific
application
robustness in a bis
the way the parameters of a bis are defined is a little different as without a cknowledgments
we need to employ some different strategies to maximise the chance of a transmission being
received
a bis still has a burst number bn and a number of subevent s nse defined in the same
way as that for cises to compensate for th e lack of acknowledgements the core introduces
the concept of a group count within isochronous intervals which is the ratio of nse to
burst number nsebn group count is used to determine the way that retransmissions
are arranged in a bis allocating them to groups which are used to add diversity into the
transmission scheme these groups have nothing to do with broadcast isochronous groups
it is an unfortunate use of the same word for two totally different concepts
chapter isochronous streams
figure the effect of group count
figure shows two examples of a bis each of which contain s six subevent s so nse
in the first case we have set a burst number of that means that each of the three payloads
which are available for that bis event are sent twice in the second example on the right
there are the same number of subevent s but with a burst number of so we have three
retransmissions of the two payloads each individual subevent is given a group number g
that goes from up to group count so in the left hand diagram we have a group count
of comprising group and group in the right hand example there are three groups group
group and group
an important th ing to note is that although these two examples look the same they will have
different isochronous intervals assuming that our frames are ms the example on the left
will have an isochronous interval of ms as it contains two payloads p and p the
example on the right will have an isochronous interval of ms as there are three payloads
p p and p
group count works with two other new parameters
immediate repetition count irc which defines the number of groups which
carry data associated with the current event and
pretransmission offset pto the concept of pre transmission is to allow early
transmission of packets in the hope th at a receiving device can receive audio data
packets early allowing it to power down and then be ready to render them at the
point that the final transmission opportunity occurs
these parameters which are calculated by the scheduler in the controller replace flush
timeout for a cis flush timeout is essentially an end stop terminating transmissions of a
pdu in most cases its never needed because as soon as an initiator receives an
acknowledgment that its data has been received it can close th e event and stop transmitting
section broadcast isochronous streams
a broadcaster cant do that it has to keep on transmitting therefore its advantageous to
maximise the diversity of transmissions of packets to give every acceptor the best chance of
finding a packet the sooner they c an do that the sooner they can go to sleep until the next
one arrives
pretransmission offset works by introducing the concept of subevent s associated with a
current event as well as subevent s associated with future events this allocation of subevent s
to pdus is even more complex than the scheme for cis so the best was to explain it is to go
through a series of examples showing the effect of changing the values this is all worked out
by the controller and is not accessible to an application but it he lps to have an understanding
of it when you set the hci parameters to configure your streams
to illustrate the concept of future subevent s figure demonstrates a bis event where we
have an nse of eight subevent s the first five of which are associated with the current bis
event the last three of which are used for data from future bis events
figure the concept of future bis events
we can now put everything together with some more examples
these group numbers g which we first saw in figure along with the i mmediate
repetition count i rc determine which payloads are sent in each transmission slot according
to the following rules
if g irc group g shall contain the da ta associated with the current bis event
if g irc group g shall contain the data associated with the future bis event that
is pto g irc bis events after the current bis event
chapter isochronous streams
well now look at a few examples which illustrate how these different parameters result in a
variety of different retransmission schemes
figure pretransmissions with nse bn irc and pto
in figure we have four subevent s with an nse of the burst number of means
one new payload is provided within each bis event so the isochronous interval for this
example is ms the immediate repeat count is three which means that the data associated
with each event is transmitted in the first three slots the conditions shown above show that
the last transmission in that bis event comes from the next bis event with this scheme
within every bis event there are always three transmissions of the current data plus one
transmission of the data from the next event
in case it seems like weve invented time travel we havent weve just ben t our definitions
slightly event x cant happen until we have both the p and p pdus available so p is
really the current data in event x this demonstrates that as soon as pto is gr eater than
the latency starts to increase as the sync refence point cannot occur until after the last possible
transmission of data which for p is in event x
section broadcast isochronous streams
figure pretransmissions with nse bn irc and pto
figure shows what happens when we increase the number of subevent s to here the
irc remains at but as nse is that provides two subevent s which are associated with data
from future bis events these are used to pre transmit a packet from event x and event
x that means that transmissions are being spread across more isochronous interval s with
data coming from three consecutive bis e vents that helps to provide robustness against
bursts of wide band interference which may last more than one isochronous interval but it is
at the expense of greater latency which has grown by another m s
figure pretransmissions with nse bn irc and pto
chapter isochronous s treams
figure looks at the case where we are spreading even further by increasing the pre
transmission offset to this results in the inclusion of data from an x bis event and an