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Issues Relating to a Device existing on Multiple Piconets
Original Post: masters and slaves (eGroups Msg.) Date:
2000-07-24
3 common question that who arise if a device serves as both master on one
piconet and slave on another. Note the answers given to these questions
are tentive and not verifiable as such.
1
- Question
- How does the device schedule itself (figure out which piconet to
service/listen)? .For example ,there may be slots in which the device has
obligations on BOTH piconets.
- Answer 1 : eGroups forum
- The spec is somewhat vague about BT scheduling. But if the slave is in
active mode, it will definitely get its share of bandwidth, according to quality
of service requested by slave. If the slave has a lot of data to send, it may
request higher token rate. If the data is bursty, it may request large token
bucket size. Thus the slave will be allotted more slots(by being polled more
frequently, if required). Note: Not too sure where this quality of
service who be implemented, on the L2CAP, LMP etc. ?
-
- Answer 2 : My perspective
- The specs say that slaves can participate in different piconets on a
time-division multiplex basis, but don't elaborate. I would say depending on
whether a slave is active/parked etc. if would spend different amounts of time
with different masters, with the highest priority (amount of time) on a
particular piconet if it is an active slave on that piconet. If it was a parked
slave on a different piconet it would only switch to that piconet on a beacon
-slot. Hold & sniff modes would have higher priority that park, but lower
than active. Obviously some beacon slots, slave-to-master slots etc. will be
missed but this should be expected
2
- Question:
- Does the device need to save any internal state information regarding the
piconets when switching from one to another? Is this a function of the device
hardware (a finite save stack)? what exactly (beside change the hop sequence
parameters) must a device do to switch to another piconet on which it is an
active member?
Answer:
- The unit has to save all the important parameters like FLOW, ARQN, SEQN, the
state of the unit before it switched etc. You can do whatever you want to
implement it, but I think software (like an data structure in C to store) would
be easier.
3
- Question:
- Does the performance of the one piconet (on which the device serves as
master) simply suffer (i.e., no transmits) because the device is busy responding
as slave on the other piconet?
-
- Answer:
- Yes the performance will be hit. First of all when it is in active mode as
slave in one piconet, it has to listen to the master of that piconet as it cant
predict when it may be polled. If a slave is participating in multiple piconets,
it could remain in active mode in only one piconet at a time and have to be in
HOLD(preferably, as it could retain the AM address) or PARKED mode in other
piconets. In HOLD mode a SCO link, if at all present, still remain supported. If
a slave has a SCO link in one piconet, it may not be able to form a SCO link in
another piconet (I think so) as the slots are pre allotted, the slots may
overlap and the slots may not be aligned. But if a device wants to have SCO
links with 2 different devices, it may become master to both so that it could
decide which slots to allot
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