Demand-Based Bluetooth Scheduling
`R. Rao, O. Baux and G. Kesidis
` EE and CS&E Depts
` Pennsylvania State University
`
`
`Abstract
`Bluetooth is a short-range wireless communication technology for ad-hoc communication
`between devices, each of which has a range of 10m. The master typically employs strict
`round-robin mechanism to poll the slaves in a piconet consisting of a master and up to
`seven slaves. In this paper we propose a flexible polling mechanism for the master that
`begins with common polling periods for all slaves and subsequently increases the polling
`period for slaves with less traffic load. We compare our flexible polling mechanism with
`strict round-robin polling mechanism and conclude that flexible polling will improve the
`throughput of the piconet.
`1 Introduction
`Bluetooth is a wireless communication technology that permits communication between
`bluetooth enabled devices. Bluetooth operates in the ISM band. This band can be very
`noisy, so bluetooth employs a frequency hopping scheme for modulation. The range of a
`bluetooth device is 10m. Bluetooth devices within range can set up an ad-hoc network
`called a “piconet” with one master that controls the piconet and a maximum of 7 slaves
`[1,5,12 ,16,18]. The master and a slave communicate using Time Division Duplex (TDD)
`slots. Typically, though not necessarily, the master talks to the slave in a time-slot (625
`s) and the slave replies to the master in the very next time-slot. The two consecutive
`slots where the master and one particular slave communicate with each other is called a
`“bluetooth frame”. We use the term “metaframe” to denote a number of consecutive
`bluetooth frames equal to that of the number of slaves in the piconet.
`Bluetooth supports two types of channels: asynchronous and synchronous. For
`asynchronous communication the master polls a slave and the slave responds in the next
`slot. This communication is asynchronous in nature and is initiated by the master. For
`synchronous communication the master and slave talk to each other at regular intervals of
`time. The interval of time is set up before the synchronous communication starts. This is
`mainly used for voice traffic.
`The total capacity for communication for a bluetooth piconet is about 1Mbps. Thus, it is
`highly undesirable to allocate two slots to (i.e., to poll) a master-slave “connection” only
`to have nothing to transmit in one or both slots, i.e., one or both slots are wasted.
`Bluetooth standards have two mechanisms in place to improve efficiency of usage of the
`available bandwidth:
`a) An inactive slave can enter a dormant state for reduced power consumption.
`Bluetooth supports three such states: sniff, hold and park [16]. We consider only
`the park state only for simplicity. Note the bluetooth device can communicate
`only in the active state.
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`b) The use of multi-slot packet formats (1:1), (1:3), (3:1), (1:5), (5:1). The notation
`(m:s) means that the master-to-slave “packet” is m time-slots long and the packet
`from slave-to-master is s time-slots long. Multislot packet formats improve
`throughput in situations where the master-slave connection has information
`flowing primarily in one direction, i.e., an asymmetric TDD connection. The use
`of multislot packets is mitigated by the condition that they cannot span slots
`reserved for “synchronous” (isochronous voice) traffic [13].
`This paper proposes a flexible piconet scheduling mechanism which is not incompatible
`with a) and b) and involves variable-rate polling; the master-slave connections with little
`(but not dormant) activity are polled less frequently. The goal is to maximize the
`throughput of data traffic and to reduce the power consumption of the piconet. The
`proposed scheduling mechanism also realizes a graceful transition from the active to the
`park state and clearly stipulates when slaves are to be parked.
`A bluetooth device can participate in either one or two piconets. When it participates in
`two piconets, it may act as a “bridge” between them. A system of interconnected piconets
`is called a scatternets. Inter-piconet communication within a scatternet is accomplished
`using the bridge nodes [4,17]. We will describe certain restrictions on piconet scheduling
`in the presence of bridge nodes and other shared slaves.
`This paper is organized as follows. In Section 2, we describe a two-level hierarchical
`round-robin piconet scheduling mechanism, which allows for the co-ordination of shared
`slaves in a scatternet. In Section 3, we propose a demand-based flexible polling
`mechanism for a piconet as an alternative to standard strict round-robin polling; this
`mechanism is better suited to handle asynchronous data traffic with “elastic” bandwidth
`needs. In Section 4, the throughput and access latencies of the piconet under flexible
`polling is compared by simulation to that under standard polling. Finally, conclusions are
`drawn in Section 5.
`2 Scheduling for Bluetooth Scatternets
`
` A
`
` simple example of a bluetooth scatternet is depicted in Figure 1. Consider two piconets
`with a common node. One piconet has a total of three slaves and the other has a total of 7
`(the maximum) slaves. Each slave has a single bluetooth transceiver so that it can only
`transmit once per bluetooth slot. We assume that adjacent piconets are synchronized so
`that transmission epochs for common slaves are aligned [19]. Since 3 and 7 are co-prime,
`under strict round-robin scheduling both masters may occasionally poll the shared slave
`at the same time.
`We now describe a way to prevent simultaneous polling of shared slaves. Recall that a
`piconet with 7 slaves has a metaframe of 2x7=14 time-slots. We propose that each
`piconet have a metaframe of 14 slots (7 consecutive polling epochs (bluetooth frames))
`irrespective of the number of slaves. So, if a piconet has 3 slaves, each slave could be
`polled at least twice in a metaframe because, of course, 2x3<7<3x3. Consider the single
`(7-2x3) “pad” slot in this situation. Pad slots are never assigned to shared slaves. Pad
`slots can be assigned to dedicated slaves in a round-robin fashion. Thus, we create a two-
`layer hierarchical round-robin polling mechanism [3,12] where the first layer has 7
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`polling opportunities and only dedicated slaves are polled in the second layer. See Figure
`2 for an example in which slave 1 is assumed to be shared and slaves 0 and 2 are
`dedicated. In this figure, each square represents a two-slot bluetooth frame (a polling
`epoch); the slave labeled “1” is shared and, consequently, does not participate in the
`layer-two slots. In this example, the HRR mechanism works so that slaves 0 and 2 are
`served in the pad slot of alternate metaframes, i.e., they take turns using the pad slot.
`
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`5
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`1
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`2
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`3
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`9
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`10
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`11
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`8
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`7
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`6
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`13
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`14
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`12
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`15
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`16
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`17
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`4
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`18
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`19
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`is a slave that is shared by many master
`
`is a master.
`
`is a dedicated slave that is communicating
`with one master
`
`
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`Figure 1 - A bluetooth Scatternet.
`
`
`In Figure 3, we consider a piconet with four slaves: slave 3 is a synchronous (isochronous
`voice) slave, slave 1 is an asynchronous (data) slave shared with another master, and
`slaves 0 and 2 are dedicated asynchronous slaves. Note that neither the synchronous slave
`nor the shared slave participate in the pad slots and that the synchronous slave is polled
`only once per metaframe.
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`pad slot
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`0
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`1
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`2
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`0
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`1
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`2
`
`
`Layer 1 metaframe (7 polls)
`
`
`0
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`2
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`Layer 2 metaframe (dedicated frame)
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`Figure 2 - An example of a two-layer HRR structure
`
`pad slot
`
`
`0
`
`
`1
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`2
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`
`3
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`Layer one metaframe (7 polls)
`
`
`0
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`
`2
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`Layer two metaframe
`(dedicated nonisochronous slaves)
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`Figure 3 – An alternative two-level HRR scheduler
`
`
`When a slave that already belongs to a piconet joins a new piconet, it communicates to
`the new master the slots at which it is polled by its other existing master. Therefore, the
`new master can poll the slave (i.e., assign it to slots in its metaframe) so as to avoid the
`situation where the slave is polled simultaneously by two masters.
`Methods of setting up (at once) a conflict-free group polling schedule for a bluetooth
`scatternet of M piconets with a total of L shared slaves have been explored in the context
`of bandwidth arbitration for switch fabrics. For example, let L=M and consider the MxM
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`matrix whose entry in the ith column jth row is the number of layer-one slots of the ith
`piconet that are dedicated to the jth shared slave of the scatternet. So, this is a doubly sub-
`stochastic matrix all of whose entries are multiples of 1/7. Note that, to determine this
`matrix, whether to poll a shared slave once or twice per metaframe (a choice illustrated in
`Figure 3) must be previously determined. Given the doubly stochastic matrix, the
`Slepian-Duguid [10,2] or Birkoff-von Neumann-Konig [7,6] approaches can be used to
`find a conflict-free scatternet schedule, i.e., to find a Mx7 matrix whose ith row is the
`layer-one frame of the ith piconet and the index of any shared slave appears at most once
`in any column (this latter condition is implies that no (shared) slave is simultaneously
`polled by two or more masters). For the case where L>M (resp., L<M), one can pad the
`MxL matrix with L-M all-zero row vectors (resp., M-L column vectors) to create an LxL
`(resp., MxM) doubly sub-stochastic matrix and proceed as above. Once the schedule for
`the shared slaves has been determined, the slot assignments for the dedicated slaves are
`added to the Mx7 matrix as described above. The following table is a complete, conflict-
`free schedule for the scatternet of Figure 1 wherein M=5, L=4 and the circled labels are
`those of the shared slaves. For that example, device no. 8 is a slave of device no. 6 and
`both are masters. Thus, device no. 8 cannot poll its slaves during the bluetooth frame in
`which it is polled by device no. 6.
`
`7
`
`43
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`19
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`6
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`2
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`7
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`15
`
`18
`
`1
`
`1
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`7
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`9
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`14
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`18
`
`2
`
`2
`
`4
`
`10
`
`15
`
`19
`
`3
`
`3
`
`8
`
`(6)
`
`13
`
`16
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`4
`
`4
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`7
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`11
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`14
`
`18
`
`5
`
`1
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`4
`
`13
`
`16
`
`19
`
`Bluetooth
`Frame
`
`Master
`Piconet
`
`5
`
`6
`
`8
`
`12
`
`17
`
`
`
` 3
`
` A Flexible Piconet Scheduling Mechanism
`In a bluetooth piconet, each slave has a FIFO queue for transmission to the master and
`the master has a FIFO queues for transmission to each of the slaves. Recall that, a TDD
`“connection” is associated with each slave; hereafter, the term “connection” will connote
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`the two associated queues depicted in Figure 4. Note that, within a single connection, the
`two “arrival” processes to the queues will be dependent.
`
`
`MASTER
`
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`Packets
`for
`Slave n
`
`
`queue
`n
`
`
`Wireless
`Bluetooth
`Channel
`
`Bluetooth
`scheduler
`
`
`SLAVE n
`
`
`Packets to
`the master
`
`
`Figure 4 - The nth TDD connection of a bluetooth piconet
`
`1
`
`
`3
`
`
`1
`
`
`Asynchronous Dedicated
`Slave (ADS) slots
`
`
`2
`1
`2
`1
`0
`0
`3
`
`
`
`
`
`
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`Layer one metaframe (7 polls) after ADS slots are filled. Slave 0 and
`Slave 2 have equal traffic load.
`
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`Figure 5 - Example of flexible polling mechanism
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`Bluetooth piconet scheduling mechanisms have been considered in [9,11,13,15,16]. In
`the following, we propose a new scheme with a flexible polling rate per slave. Our
`mechanism will be described using the example in Figure 5. The following table depicts
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`0
`0
`x
`
`1
`1
`
`
`2
`2
`x
`
`3
`3
`
`
`4
`0
`x
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`5
`1
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`
`6
`2
`x
`
`7
`0
`x
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`8
`1
`
`
`9
`2
`x
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`11
`0
`x
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`12
`1
`
`
`13
`2
`x
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`14
`0
`x
`
`10
`3
`
`
`the polling epochs of this example; those of the asynchronous dedicated slaves (ADSs),
`numbers 0 and 2, are checked with an “x” in the third row. In this table, discrete time is
`measured in multiples bluetooth frames (2 x 625 s). The focus of the rest of this paper is
`on the polling decisions for the ADS slots; those of the “standard” policy are indicated in
`middle (second) row of the following table for the example under consideration. Our
`flexible polling policy will give alternative polling decisions for the ADS slots. The
`flexible polling mechanism does not make any polling decision for the shared or
`synchronous slaves.
`
`Time
`Slave polled
`ADS slots
`
`In general, suppose that there are D asynchronous dedicated slaves. At any given time,
`the dth ADS is assigned a time-stamp td and a polling period nd by the master. Our
`flexible polling policy works as follows. For an ADS slot, the master polls the ADS slave
`with the smallest (current) time-stamp; ties are broken arbitrarily. When an ADS is
`polled, its time-stamp is increased by its polling period. More precisely, if the dth ADS is
`polled, the result is that td  td + nd. The following statement is easy to prove [8]:
`
`Lemma: The ADS polling decisions are periodic with period mp equal to the least
`common multiple of the current polling periods n1,n2,...,nD-1. Moreover, in every period
`of mp ADS slots, the dth ADS is polled mp/np times, for all d  {0,1,...,D-1}.
`
`Note that if all the polling periods are identical, the result is “standard” round-robin
`polling. Under flexible polling, the polling periods are periodically adjusted according to
`the following rule. Initially, all polling periods of the asynchronous dedicated slaves are
`set to D. At any given time, the polling period of an ADS belongs to the set of integers:
`N0  D  N1  N2….... NK-1 < NK
`We assume that the increase in the polling period, hereafter called the “step size,” is
`constant and denoted it by m = N k+1 - N k, i.e.,
`Nk = D + km.
`Suppose that the dth ADS is polled at a point where nd = Nk for some k  {1,...,K}. One
`or both queues associated with this ADS connection (Figure 4) could be prompted to
`transmit during the two-slot frame. There are two cases:
`1. If, as a result of this polling decision, there was nothing to transmit in the frame (no
`packet was transmitted by the ADS connection), the master would increase the
`polling period of this ADS; i.e., set nd = Nk+1 if k < K or, if k=K, the master would
`consider parking the ADS.
`2. If a packet is transmitted by the ADS connection, the polling period of the ADS is
`reset to D.
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`The quantities K and NK are chosen so that if the polling period is NK, the slave is
`inactive enough to be parked. Parking the slave not only saves valuable bandwidth but
`also helps in reducing the power consumption of the bluetooth device. More details on
`parking can be found in [15].
`4 Performance Evaluation of a Flexible Polling Strategy
`One simply stated objective of our flexible polling policy is to maximize the throughput
`of the bluetooth piconet over the ADS frames. A second issue is the access latency (or
`“response time”) of the piconet. By “access latency” we mean the delay experienced by
`the first packet of an arriving data burst to a master-slave connection.
`The simulation scenarios we considered assume all (ADS) slaves were active (slaves in a
`parked state neither transmit nor receive data and, hence, are not affected by the choice of
`polling strategy). Under the strict round-robin polling mechanism, the master polls each
`slave in turn and the polling period for each slave is simply the number of slaves in the
`piconet. Under a flexible polling policy, a range of access latencies for a data burst is
`possible depending on the current polling period of the master-slave connection. If a data
`burst arrives just after a polling epoch of a slave and the current polling period of that
`slave is large, the access latency of the burst will be large. The polling step size (m = N
`k+1 - N k) can be adapted to significant changes in the traffic conditions in order to reduce
`access latency. For example, if overflow of the transmit buffers is too frequent, the step
`size could be reduced accordingly.
`A simulation study was performed for the example given in Figure 5. Though not a
`comprehensive simulation of an actual piconet, our study was detailed enough the show
`the affect of our flexible polling algorithm on the throughput and access latency. Various
`traffic profiles were selected for the two ADS slaves from the ones shown below in
`Figure 6. The resolution in Figure 6 is at the level of a bluetooth frame (polling epoch),
`i.e., each arrival represents an entire bluetooth frame (two time-slots). We chose different
`combinations of two traffic profiles from those depicted. The access latency was
`calculated as the delay experienced by the first packet of a traffic burst, i.e., the shaded
`packet(s) in Figure 6.
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`Slave 1
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`Slave 2
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`Slave 3
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`Slave 4
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`1
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`2
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`11 12
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`21 22
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`31 32
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`41 42
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`51 52
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`61 62
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`71 72
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`81
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`82
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`1
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`5
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`11
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`15
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`21
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`25
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`31
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`61
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`65
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`71
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`81
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`1
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`27
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`31
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`61
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`67
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`71
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`81
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`87
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`1
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`2
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`13
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`14
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`26
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`27
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`39
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`40
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`52
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`53
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`65
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`66
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`78
`
`79
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`Figure 6 - The different traffic profiles of example slaves
`
`
`Note that Slaves 2 and 3 have high traffic load while Slaves 1 and 4 have low traffic
`load. The simulation results can be classified in two basic groups.
`Results when the slaves had similar traffic load:
`Here we chose the combination of Slave 2 and Slave 3 or Slave 1 and Slave 4. Clearly, in
`these cases, a flexible polling policy will make scheduling decisions similar to those of
`round-robin polling and this is reflected in the simulation results:
` The results for round-robin polling for Slave 2 and Slave 3:
`o Percentage of wasted frames: 0
`o The throughput of the piconet: 100%
` The results for flexible polling for Slave 2 and Slave 3:
`o Percentage of wasted slots: 0
`o The throughput of the piconet: 100%
` The results for round-robin polling for Slave 1 and Slave 4:
`o Percentage of wasted slots: 39%
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`o The throughput of the piconet: 61%
`o The average access latency: 3
` The results for the flexible polling for Slave 1 and Slave 4:
`o Percentage of wasted Slots: 39%
`o The throughput of the piconet: 61%
`o The average access latency: 3
`Note that the low throughput of Slave 1 and Slave 4 is due to the fact that their queues
`are under-loaded; thus, there are many unused slots because there is simply nothing to
`send.
`Results when the Slaves had different traffic load:
`In this scenario, we choose the two slaves such that they had different traffic load. We
`shall display the result for Slaves 2 and 4, a representative combination. Slave 2 has
`significantly higher traffic load compared to Slave 4. We noticed a clear improvement in
`the throughput for these selected traffic profiles for flexible polling. The advantage of
`flexible polling was clearly visible here. Flexible polling allocated more slots to the slave
`with greater traffic load, i.e., to Slave 2.
` The results for the round-robin polling for Slave 2 and Slave 4
`o Percentage of wasted slots: 20%
`o The throughput of the piconet: 80.0%
`o Average access latency: 3
` The results for the flexible polling for Slave 2 and Slave 4
`o Percentage of wasted slots: 8.75%
`o The throughput of the piconet: 91.25%
`o Average access latency: 4
`We varied the step size (m = Nk+1 – Nk) for different simulation cycles to find its affect
`on the access latency. The results for different values of step size m are graphically
`shown in Figure 7 below. In this graph, the percentage of wasted slots, throughput and
`burst access latency are plotted. Clearly, throughput of the piconet is an increasing
`function of m. However, burst access latency is also an increasing function of m.
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`Percentage of
`wasted Slots
`Througput of the
`Piconet
`Average access
`latency
`
`Average access latency
`
`0123456789
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`1 5 10 15 20 25 30 35 40 45 50 55 60 65 70
`step size - m
`
`100
`90
`80
`70
`60
`50
`40
`30
`20
`10
`0
`
`Throughput and Wasted
`
`Frames (percentage)
`
`
`
`
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`Figure 7 - Changes due to variation of polling step size
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`5 Conclusion
`In this paper, a strategy was described for scheduling of shared slaves in a bluetooth
`scatternet. Also, we compared strict round-robin polling to a proposed flexible demand-
`based polling mechanism for a bluetooth piconet. We are assuming here that the master
`can do a small amount of integer computation. The flexible polling mechanism resulted
`in greater throughput when the traffic load was unevenly distributed among the
`(dedicated) slaves. Also, there would be a corresponding improvement in overall power
`consumption as the slaves that have less traffic load were polled less frequently. Finally,
`the flexible polling mechanism gives a well-defined rule to determine when to park a
`slave. The drawback of flexible polling is a potential increase in the access latency of an
`arriving burst of data to be transmitted. This tradeoff can be made tolerable by adapting
`the polling period step-size to the traffic.
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