by alex garcia scheduling in bluetooth networks. introduction bluetooth used to interconnect mobile...
TRANSCRIPT
BY ALEX GARCIA
Scheduling in Bluetooth Networks
Introduction
Bluetooth Used to interconnect mobile devices into a Personal area ad hoc
network (PAN). Short-range radio technology operating in an industrial-
scientific-medical (ISM) band at 2.45 GHzPiconet
Two or more nodes sharing the same channel where 1 unit acts as a master and the other units up to seven act as slaves
Scatternet Established by linking several piconets together to yield a global
wireless ad hoc network.Bridge
Delivers interpiconet messages between two neighboring piconets
Piconet Scheduling Schemes
Pure Round Robin (PRR) A fixed cyclic order is defined, and a single chance to
transmit is given to each master-slave queue pair.Exhaustive Round Robin (ERR)
A fixed order is defined like PRR, but the master does not switch to the next pair until the master and slave queues are empty
Exhaustive Pseudocyclic Master queue length (EPM) A dynamic cyclic order is defined at the beginning of
each cycle according to a decreasing master-to-slave queue length order.
Piconet Scheduling Schemes
Limited Round Robin (LRR) Similar to ERR except that the maximum number of
transmission per cycle allows to get a limit on the cycle length and to avoid the unbounded cycle length.
Limited and Weighted Round Robin (LWRR) Each slave is assigned a weight. Each time a slave is polled without data exchange
between master and slave, the weight of the slave is reduced by 1.
Otherwise it is increased by 1 up to the maximum priority
Results for Scheduling Schemes
Pure round robin shows the worst performance when the traffic is low and but a lower delay at higher traffic
Exhaustive round robin is the exact opposite when it comes to traffic load
Both LRR and LWRR can perform well in both conditions
A better scheme Fair Exhaustive Polling (FEP) which surpassed
Fair Exhaustive Polling
Main idea is to poll slaves that probably have nothing to send as seldom as possible
There is an active state and an inactive state Initially all slaves are placed in the active state During a polling, if the slave has no information to send and
the master has no information to send to that slave, it is placed in the inactive state
Whenever the master has something to send, the slave is placed back into the active state
If a slave’s maximum polling interval timer has expired, then the slave is moved back into the active state
FEP acts as an exhaustive scheduler at low loads and as a round robin scheduler at high loads
Causes of Reduction in Efficiencyin Polling-based Communication
Slaves that have no data to transmit may be unnecessarily polled, while other slaves with data to transmit may have to wait to be polled
At the time of an expected poll, one of the nodes of a master-slave node pair may not be present in the piconet The slave that is being polled is not listening or the
master that is expected to poll is not polling
Soft Coordination Scheme
Nodes decide their presence based on local information
Cannot guarantee conflict-free participation of bridging nodes, however, they have a significantly reduced complexity
Pseudorandom Coordinated Scatternet Scheduling (PCSS) algorithm Coordination is achieved by implicit rules in the
communication without the need of exchanging explicit control information
Pseudorandom Coordinated Scatternet Scheduling (PCSS) Algorithm
Consists of two key componentsCheckpoints mechanism
A designed Bluetooth frame Master node actively checks its slave by sending a
packet to the slave at corresponding checkpoint and waiting for a response from the slave
The slave nodes passively checks its master by listening to the master at the checkpoint and sending a response packet
Nodes remain active on each checkpoint until there is user data in the master-to-slave or slave-to-master directions or until it has to leave for a next checkpoint on one of its other links
Pseudorandom Coordinated Scatternet Scheduling (PCSS) Algorithm
Dynamic Adjustment The positions of checkpoints are assigned on given
link following a pseudorandom sequence that is generated based on the Bluetooth clock of the master and the MAC address of the slave
This guarantees the same sequence will by generated by both nodes in a master-slave pair, while sequences belonging to different node pairs will be different
This makes sure that different links of a node will collide only occasionally
Ideal Coordinated Scatternet Scheduler (ICSS)
Used to evaluate the PCSS schemeNode has the following extra information about its
neighbors A node is aware of the already prescheduled transmission of its
neighbors A node is aware of the content of the transmission buffers of its
neighborsAfter each package exchange on a given link the two
nodes schedule the next package exchange, provided there is user data to be transmitted
With the traffic dependent meeting point intensity adjustments, the throuput of PCSS quite closely match the results of the ideal ICSS algorithm
Uncoordinated Greedy Scatternet Scheduler (UGSS)
Another scheme used to evaluate against PCSS
Bluetooth nodes do not attempt to coordinate their meeting points; instead each node visits its neighbors in a random order
Nodes switch continuously among their Bluetooth links in a greedy manner
The greedy nature of the algorithm results in high power consumption of Bluetooth
PCSS achieves higher throughput than the uncoordinated UGSS algorithm
Hard Coordinated Central Scheduling Scheme
3 design goals: The computation time must be polynomial in the umber of nodes and
links The solution must be perfect to assign any feasible traffic load
request successfully Each individual link load is evenly distributed along the frame
interval so that QoS delay and jitter are minimizedSlots
Basic communication between a master and its slaves consists of two consecutive slots, polling and response
There are 4 types of nodes in a scatternet: pure master - - Belongs to one exclusive piconet pure slave - - Belongs to one exclusive piconet master bridges - - Master in one piconet, slave in another slave bridges - - is a slave in all piconets its in
Perfect Scheduling Problem for Scatternet
Input: A feasible load matrix LOutput: A feasible F assigning load L
perfectly, i.e.
Perfect Scheduling Algorithm for Bipartite Scatternet
Uses divide and conquer techniqueTwo matrices and are generated from the load
matrix LFor each entry in L(i, j), let (i, j) = (i, j) = L(i, j)/2For each row in, group the nonzero entries into pairs
with at most one possible unpaired entry; do the same for each column in A
Color the nonzero entries of A with black and white (b & w) so that no two directly linked entries have the same color
For each nonzero entry A(i, j), if it is black, increment else increment by 1.
Example
Results
Since the algorithm is reservation-based, they are suitable for multimedia and real-time traffic to obtain the guaranteed bandwidths along the communication routes
Once the algorithm generates a feasible schedule from the set of link load requests, the assigned bandwidths are exclusively used for the requesting applications
With bipartite scatternets that have no master bridge, the algorithm yields a perfect, feasible schedule which guarantees the exact amounts of bandwidth to the requesting applications
Scheduling Algorithm for General Scatternet
When a scatternet allows master bridge as an integral part, the perfect scheduling problem has no solution
Here is an example of a general scatternet that requires a subperfect scheduling algorithm
Piconet Scheduling Schemes
Step 2 in load partition is not sufficient because of the existence of a master bridge that creates some odd cycle in the scatternet while even partitioning of link and nodal load is necessary for the perfect schedule
Introduction of inclined links are necessary in order to solve this which pairs one unpaired entry on the row with another unpaired entry on the column of the master bridge
Example
Load_Partition_General
The main difference to this procedure compared to its original is that entries that unpaired with any other node must be paired with a nonzero entry.
For each master bridge whose load is presented by one row as a piconet master and another column as bridging slave interconnected two piconets, group an unpaired nonzero row entry with another unpaired nonzero column entry.
Other than these two differences, the rest of the subperfect scheduling algorithm is similar to the perfect scheduling algorithm
QoS Perfomance Analysis
Terms regarding Quality of Service (QoS): Bandwidth – the amount of data transferred per unit
time Delay – deliver within a bounded time Jitter – range of arrival time
Scatternet Traffic Model
There are two classes of connection-oriented traffic Constant bit rate (CBR) – guaranteed bandwidth,
bounded delay, bounded jitter Variable bit rate (VBR) – variable bandwidth, bounded
delay, bounded jitterExamples of CBR
PCM, G.721, EVRC Networked sensors and actuators
Examples of VBR MPEG4, H.263, and PNG
Scatternet Traffic Model
CBR traffic schedule should satisfy two QoS conditions The schedule contains a proper number of slots providing the
required bandwidth Scheduled slots are distributed evenly to meet delay and jitter
requirementsVBR traffic requirements are the same as CBR except
they also: The minimum bandwidth required by the application should be
statically scheduled as a guarantee and the additional bandwidth is dynamically assigned by another scheme
There is also statistical bursty rate (SBR) traffic which handles random, bursty phase and large delay/jitter One example of SBR is file transfer since the due time does not
exist,
Traffic-model Based Global Scheduling Scheme
Rule 1: Bandwidth division Total bandwidth for CBR and VBR traffic should be
upper bounded to leave remaining bandwidth for SBR, protocal messages, and local traffic
Rule 2: Instant rescheduling The actual traffic is statically distributed because of
mutual interference among independent traffic flows along routing paths and local delay in task scheduling
When data to be transferred on a link is less than the size of one logical slot, the unused slots should be rescheduled for other purposes such as local traffic
Traffic-model Based Global Scheduling Scheme
Rule 3: Resolution for bandwidth competition Messages with highest priorities are selected up to
the available number of slots and transferred The management protocol message has the highest
priority followed by time-critical, real-time messages and so forth
Rule 4: Dynamic slot assignment Free slots can be assigned on-the-fly by the consent of
both global and local traffic
Conclusion
There are several ways to handle Bluetooth scheduling
Each scheme used to handle the scheduling has different strengths and weaknesses depending on the amount of masters and slaves are being used