presented by Hasan SOZER 1

A Pseudo Random Coordinated Scheduling Algorithm forBluetooth Scatternets

Andras Racz, Gyorgy Miklos, Ferenc Kubinszky, Andras Valko

Traffic Analysis and Network Performance Lab.,

Ericsson Research

MobiHOC 2001, Long Beach, CA, USA © ACM 2001

presented by Hasan SOZER 2

Outline Introduction PCSS Algorithm

Overview Operation Dynamic adjustment in responce to varying traffic

Simulation Reference algorithms Scenarios

Conclusion

presented by Hasan SOZER 3

Introduction Switching between piconets on a

time division basis Two important phenomenas

unnecessary polls & waiting nodes absence of nodes (slaves & masters)

presented by Hasan SOZER 4

Introduction (cont’d) Scatternet wide communication

problem Scheduling orders may lead to

conflicts Variation of traffic intensity Two approaches

Hard coordination schemes Soft coordination schemes

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Introduction (cont’d) Hard coordination schemes

Time slots are allocated in advance Complex, explicit signalling Reallocation in case of changes

Soft coordination schemes Based on local information Reduced complexity Cannot guarantee conflict-free

participation

presented by Hasan SOZER 6

Introduction (cont’d) Key components of the algorithm

Check Points (Meeting points): Specific to particular links Pseudo-random distribution for avoiding

systematic conflicts Dynamic adjustment of checking intensity

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PCSS Algorithm checkpoint -> Bluetooth frame

check: being present at the check point

Link becomes active at the check point

Link remains active until; there is no user data or, one leaves for another checkpoint

presented by Hasan SOZER 8

PCSS Algorithm (cont’d) Randomness in assigning

checkpoints avoids systematic collusions

Sequence is generated based on Bluetooth clock of the master MAC address of the slave

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PCSS Algorithm (cont’d) Base checking interval: T Checking intensity: 1/T

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PCSS Algorithm (cont’d) Checkpoints may collide or overlap

There may be non-present neighbours Each node performs traffic

measurements for each of its link Checking intensity for each link is

increased or decreased accordingly

presented by Hasan SOZER 11

PCSS Algorithm (cont’d) ??? Two nodes on a given link may

select different base checking periods

Pseudo random generation of checkpoint sequence guarantees; Set of checkpoint positions at a lower

checking intensity is a subset of checkpoint positions at any higher checking intensities

presented by Hasan SOZER 12

PCSS Algorithm - Operation Initialization

No specific procedure required Communication

Starts at the checkpoint If there is no data

nodes remain idle until next checkpoint One may leave because of another

checkpoint Other node would realize the situation worst case: 5+1 slots after

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PCSS Algorithm - Operation Inputs to pseudo random

generator Current value of the master’s clock Base checking period length MAC address of the slave

Base checking period T = 2n

2fmin >= T >= 2fmax

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Dynamic Adjustment of Checking Intensity

L: number of links of the node

Avgerage Number ofcheckpoints in Tmax

Avgerage time betweentwo consecutive checkpoints

Share of a link, j from total capacity

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Dynamic Adjustment of Checking Intensity (cont’d) Measurement of utilization of

checkpoints p(i) on the ith link Moving average method

If checkpoint is utilized;

If checkpoint is not utilized;

presented by Hasan SOZER 16

Dynamic Adjustment of Checking Intensity (cont’d) Total utilization p(node):

P(win): fraction of time slots in the past time window of length Nuti,win where the node has been active over the total number of time slots

If p(node) < plower, T(i) is doubled After increase or decrease, p(i) is reset

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Dynamic Adjustment of Checking Intensity (cont’d) Communication continues despite

the decrease in the checking intensity

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Dynamic Adjustment of Checking Intensity (cont’d) If p(i) > pupper, T(i) is divided by 2 Typical values for pupper and plower

for avoiding oscillation;

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Dynamic Adjustment of Checking Intensity (cont’d) Communication in case of a

checking intensity increase

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Simulation – Reference Algorithms Reference Algorithms

Ideal Coordinated Scatternet Scheduler (ICSS)

Uncoordinated Greedy Scatternet Scheduler (UGSS)

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Simulation – Reference Algorithms (cont’d) Scheduling list of a node pair in

case of the ICSS algorithm

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Simulation - Scenarios Simulation Scenarios

Network Access Point (NAP) Several forwarding hops Bridging node with varying bridging

degrees

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Simulation - NAP Checking intensities are fixed for

mouse delay requirement As laptops increase, sum of laptop

throughputs do notdecrease in case ofPCSS & ICSSalgorithms

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Simulation – Several Forwarding Hops Background traffic provided

PCSS lies between ICSS & UGSS

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Simulation – Varying bridging degrees Dynamic PCSS scheme matches

quite closely the throughput achieved by the ICSS algorithm

Perform much betterthan fixed PCSS

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Conclusion Key concepts

use of pseudo random sequences of meeting points

Dynamic meeting point intensity Especially dynamic PCSS scheme

performs quite well (close to ideal) A lightweight algorithm

? Numerous steps for random sequence generation