Real-Time Scheduling for WirelessHART Networks by Abusayeed Saifullah, You Xu, Chenyang Lu, and Yixin Chen A Presentation of Findings for CSE5095 Joshua Ivaldi Dept of Electrical and Computer Engineering Center for Clean Energy Engineering University of Connecticut 1

Introduction to WirelessHART Wireless Highway Addressable Remote Transducer, or WirelessHART is a network standard tailored for sensors and actuators – or rather, the “internet of things”. In wireless automated system applications, the level of human interaction must be reduced as much as possible! So why do we care? – Feedback based control loops in industrial environments require strict adherence to end-to-end latency requirements on data exchange. – Networks failing to meet feedback data transmission requirements may result in production line inefficiency, destruction of equipment, and highly impactful financial or environmental damage. 2

Introduction to WirelessHART For the previously stated goals of the communication standard, the protocol exhibits the following key features: – Centralized Network Management architecture (controlled through the gateway). – Multi-Channel Time Division Multiple Access (TDMA) Transmission (for predictable latencies in RT Comm.) – Redundant Routes (for self-healing networks). – Real Time operation. – Avoidance of spatial reuse of channels for enhanced performance and reliability. 3

Features: Route and Spectrum Diversity Spatial diversity of routes allows messages to pass through many paths. This increases the reliability of the network! This is a useful feature in the event that there is interference, physical obstacles, or broken links. Spectrum diversity allows the network to fully utilize 16 physical layer channels. Time slotted channel hopping mitigates jamming and interference. 4

Features: Handling Internal Interference Detection of interference between nodes is challenging. To avoid this, WirelessHART allows only one transmission in each channel per time slot, across the entire network. Therefore, the maximum number of concurrent transmissions cannot exceed the number of channels in the network! 5

Developing a Mathematical Model 6

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Developing a Mathematical Model 11

Theorem 1 Given an instance of the real-time scheduling problem for a WirelessHART network with N flows, it is NP- complete to decide whether it is schedulable or not. This is verifiable in O(n 2 ) time. (proven in paper) 12

Conditions for Scheduling In a WirelessHART network, simultaneous transmissions are a major cause of communication delays and difficulty in scheduling signal flows. For a given set of flows, let T be the hyper period, or least common multiple of the periods of flows. 13

Conditions for Scheduling 14

Conditions for Scheduling 15

Conditions for Scheduling 16

Conditions for Scheduling 17

Conditions for Scheduling 18

Theorem 2 For a set of flows, F, let Γ s be the set of unscheduled transmissions at slot s. If these transmissions are schedulable, then there exist a finite number of transmissions that are elements of Γ s. (proven in paper) 19

Optimal Branch-and-Bound Scheduling 20

Optimal Branch-and-Bound Scheduling The following is the developed scheduling algorithm, 21

Conflict-aware Least Laxity First The gateway and the nodes with high connectivity in the network tend to experience significantly higher degrees of conflicts, and WirelessHART must be aware of these conflicts. Based upon these occurrences, the authors develop Conflict-aware Least Laxity First (C-LLF) which considers the length of time windows that allow transmission scheduling, as well as any potential conflicts that the transmission may experience. 22

Conflict-aware Least Laxity First 23

Conflict-aware Least Laxity First 24

Conflict-aware Least Laxity First 25

Evaluation The proposed algorithms are compared with several real-time scheduling algorithms: Deadline Monotonic (DM), Earliest Deadline First (EDF), Proportional Deadline Monotonic (PD), Earliest Proportional Deadline First (EPD) and Least Laxity First (LLF) 26

Evaluation Using a C code implementation on a Mac OS X with 2.4 GHz Intel Core 2 Duo, an evaluation of each algorithm is carried out with the following parameters, 27

Evaluation – Branch and Bound Scheduling vs Scheduling Upper Bound (UP) and Optimal Algorithm 28

C-LLF vs Other Algorithms 29

C-LLF vs Number of Nodes 30

C-LLF vs Number of Routes 31

C-LLF + B&B vs Other Algorithms 32

Test-Bed Washington University in St. Louis 33

Summary WirelessHART is applicable in sensor/actuator networks. Communication is TDMA and controlled/scheduled by the gateway using various algorithms. Because of the TDMA operation, we must carefully schedule transmissions based on expected delivery times for each transmission while considering the traffic of transmissions passing through a node (Branch and Bound + Conflict Aware Least-Laxity First) The proposed algorithm was compared with several common real- time scheduling algorithms, and in general, exhibited improved performance. 34

End of Presentation Questions? 35