presented by Hasan SOZER1 A Pseudo Random Coordinated Scheduling Algorithm for Bluetooth 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 SOZER2 Outline Introduction PCSS Algorithm Overview Operation Dynamic adjustment in responce to varying traffic Simulation Reference algorithms Scenarios Conclusion

presented by Hasan SOZER3 Introduction Switching between piconets on a time division basis Two important phenomenas unnecessary polls & waiting nodes absence of nodes (slaves & masters)

presented by Hasan SOZER4 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

presented by Hasan SOZER5 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 SOZER6 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

presented by Hasan SOZER7 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 SOZER8 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

presented by Hasan SOZER9 PCSS Algorithm (cont’d) Base checking interval: T Checking intensity: 1/T

presented by Hasan SOZER10 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 SOZER11 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 SOZER12 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

presented by Hasan SOZER13 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 = 2 n 2 fmin >= T >= 2 fmax

presented by Hasan SOZER14 Dynamic Adjustment of Checking Intensity L: number of links of the node Avgerage Number of checkpoints in Tmax Avgerage time between two consecutive checkpoints Share of a link, j from total capacity

presented by Hasan SOZER15 Dynamic Adjustment of Checking Intensity (cont’d) Measurement of utilization of checkpoints p (i) on the i th link Moving average method If checkpoint is utilized; If checkpoint is not utilized;

presented by Hasan SOZER16 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) < p lower, T (i) is doubled After increase or decrease, p (i) is reset

presented by Hasan SOZER17 Dynamic Adjustment of Checking Intensity (cont’d) Communication continues despite the decrease in the checking intensity

presented by Hasan SOZER18 Dynamic Adjustment of Checking Intensity (cont’d) If p (i) > p upper, T (i) is divided by 2 Typical values for p upper and p lower for avoiding oscillation;

presented by Hasan SOZER19 Dynamic Adjustment of Checking Intensity (cont’d) Communication in case of a checking intensity increase

presented by Hasan SOZER20 Simulation – Reference Algorithms Reference Algorithms Ideal Coordinated Scatternet Scheduler (ICSS) Uncoordinated Greedy Scatternet Scheduler (UGSS)

presented by Hasan SOZER21 Simulation – Reference Algorithms (cont’d) Scheduling list of a node pair in case of the ICSS algorithm

presented by Hasan SOZER22 Simulation - Scenarios Simulation Scenarios Network Access Point (NAP) Several forwarding hops Bridging node with varying bridging degrees

presented by Hasan SOZER23 Simulation - NAP Checking intensities are fixed for mouse delay requirement As laptops increase, sum of laptop throughputs do not decrease in case of PCSS & ICSS algorithms

presented by Hasan SOZER24 Simulation – Several Forwarding Hops Background traffic provided PCSS lies between ICSS & UGSS

presented by Hasan SOZER25 Simulation – Varying bridging degrees Dynamic PCSS scheme matches quite closely the throughput achieved by the ICSS algorithm Perform much better than fixed PCSS

presented by Hasan SOZER26 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