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combinations of ft and nse parameters the location of the flush points can be calculated
from the equation s in the core vol part b
section connected isochronous streams
returning to figure we see the initiator sending packet p in the first subevent which
is acknowledged so the controller immediately moves on to transmit packet p transmitting
it three times before it is acknowledged
in the second isochronous interval packet p is transmitted but the acceptor sends nack s
to indicate error s in the received packet as the flush timeout is set to and bn the
flush point for packet p is in the same isochronous interval coming after two further
transmission attempts neither of which have been successfully received by the acceptor
at that point the initiator starts transmitting p although once again in this example t he
acceptor responds with nacks to indicate a problem with the packets it received after two
attempts p is flushed by the initiator
in the final isochronous interval of figure p is successfully transmitted leaving three
opportunities for p all of which are unsuccessful in each case the initiator will attempt to
transmit the pdus in every available subevent before that pdus flush point after each
acknowledged transmission it will move on to the next available packet or if there are no
further packets available close the isochronous event
in this example p p and p would be discarded in real life wed expect a much better
rate of acknowledgement this is just an example to illustrate the principle
as a final example figure shows the eff ect of increasing the flush timeout to with a
burst number of which gives the opportunity to transmit in two consecutive isochronous
intervals in this example no packets are flushed
figure example of nse bn and ft
using flush timeout and burst number can be very useful to provide more retransmission
opportunities span ning multiple isochronous intervals they are particularly useful if youre
chapter isochronous streams
in a no isy environment however they have an effect on latency every increment of flush
timeout increases latency as the retransmissions are spread across more isochronous
intervals whilst burst number increases the duration of the isochronous intervals note that
burst number is confined to multiple payloads arriving in an isochronous interval it cant
be sued to solve the problem of a single payload hogging transmission opportunities which
we saw in figure
these parameters cannot by set directly by the host it is limited to setting values for the
maximum transport latency the maximum sdu size and the sdu interval bn nse and
ft are then calculated in the contr oller which takes into account any other radio requirements
in the chip it is however very useful to have an understanding of the potential effects these
parameters have on the isochronous channel structure table of tmap provides
examples of how a controller might interpret the host values for a cis to suit different
operating conditions such as prioritising airtime for coexistence or minimising latency the
exact allocation of parameters is always d own the algorithms in the scheduler which will be
set by the chip supplier
framing
the other parameter in a cis pdu header which needs to be understood is the pair of llid
link layer id bits which indicates whethe r the cis is framed or unframed unframed refers
to the case where a pdu consists of one or more complete codec frames it is used where
the isochronous interval is an integer multiple of the codec frame length in contrast framed
is where you have a mi smatch between the codec frame length and the isochronous interval
which results in a codec frame being segmented across multiple sdus this starts to get
complex but is important where an initiator may need to support bluetooth connections
which have d ifferent timings well revisit that when we look at a feature called isoal which
is the isochronous adaptation layer which has been designed to cope with this mismatch
the llid bits also indicate when t here is no iso pdu which is diff erent from the npi in
the iso pdu header
multiple cises
having cover ed all of the features of a single unidirectional cis the next step is to add more
of them the most common application for this is when an initiator is sending audio to a left
and a right earbud in this case the initiator will set up separate isochronous stream s with
two different acceptors in figure we can see that its a straightforward extension of
what weve seen before the initiator transmits and receives an acknowledgment from the
first acceptor then repeats this for the second ac ceptor
an important point to note is that although the left and right audio channels are sampled at
the same time the iso pdus are sent serially
section connected isochrono us streams
figure timings for cises to two acceptors
note that where we have more than one cis they always have the same isochronous interval
their anchor points are different as data is sent serially but for each cis their anchor points
are the same iso interval apart each anchor poin t represents the point of the first
transmission from an initiator to an acceptor for that cis as figure shows each cis
has an associated acl li nk the acl link always needs to be present because it is used to set
up the cis and control it if there are multiple cises between an initiator and an acceptor
they can share the same acl if the acl is lost for any reason any associated cises are
terminated the application then needs to decide what to do with any remaining cises
established with other acceptors within that cig
figure multiple cises and associated acl links
when an initiator is scheduling two or more audio channels there are two options for how
they are transmitted regardless of whether they are connected to one or multiple acceptors
the obvious approach is to tra nsmit them sequentially so that you send all of the data on cis
chapter isochronous streams
and then all the data on cis continuing until the initiator has worked through all of the
packets for both cises this is illustrated in figure where we have an nse of it
shows all of the subevent s within cis being transmitted before the subevent s for cis
figure sequential arrangement of two cises
the disadvantage of this approach is that if you have a reliable connection where the first
transmission of the packet is acknowledged you end up with gaps between each cis in
devices like phones which are often sharing the bluetooth and wi fi radios thats wasted
airtime that could be used for something else to address this the controller can choose to
interleave cises as an alternative approach as shown in figure
figure interleaved arrangement of two cises
in this case each subevent for cis is followed by a subevent for cis and so on for the
remaining cises the diagram repeats the example of figure having an nse of and
shows cis transmitting and receiving its first subevent followed by cis if both
acceptors receive those first transmissions and acknowledge them they can close their cis