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