Reliable and Real-time Communication in Industrial Wireless Mesh Networks

1. Reliable and Real-time Communication in Industrial Wireless Mesh Networks Song Han, Xiuming Zhu, Al Mok University of Texas at Austin • Deji Chen, Mark Nixon • Emerson Process Management

2. Outline • Introduction • Network Management Techniques • Reliable graph routing • Schedule construction and channel management • Performance Evaluation • Implementation and Deployment • Future Work

3. Introduction • Previous work • Full-blown WirelessHART stack (RTAS’08) • Compliance test suite (RTAS’09) • Data quality maintenancetechniques in wireless network (RTSS’05, TC’08, RTSS’09) • WirelessHART network • Secure and TDMA-based wireless mesh networking technology • Centralized network architecture • Stringent timing and reliability requirements

4. Introduction (cont.) • Challenge • The complexity of network management is pushed to the centralized manager but engineering decisions can have large performance impact. • Goals • Achieve reliable graph routing in WirelessHART network • Achieve real-time communication by deterministic link and channel assignment • Evaluate their performance in industrial environments

5. Reliable Graph Routing • Reliable Broadcast Graph (GB) • GB is a graph connecting Gateway (GW) downward to all DEVs • Broadcasts common configuration and control messages • Each DEV has at least two parents in GB

6. Reliable Graph Routing (Cont.) • Reliable Uplink Graph (GU) • GU is a graph connecting all DEVs upward to the Gateway • DEVs propagate periodic process data • Each DEV has at least two children in GU • Both GB and GU have no fewer than 2 Access Points

7. Reliable Graph Routing (Cont.) To avoid infinite forwarding loop: Only one cycle of length 2 in Gv Each DEV on the cycle has direct edges to v • Reliable Downlink Graph (Gv) • The graph from the Gateway to DEV v • Transmit unicast messages from the GW and NM to v • Each intermediate DEV has at least two children in Gv • There exists at least one directed cycle in Gv

8. Constructing GB G A A 1 2 3 4 5 • Drop the links with low Receive Signal Strength (RSS) in the original network topology G

9. Constructing GB S G A A 1 2 3 4 5 • Drop the links with low RSS in the original network topology G • Maintain a set of explored node S, initially S = {G, APs}

10. Constructing GB S G A A 1 2 3 4 5 • Drop the links with low RSS in the original network topology G • Maintain a set of explored node S, initially S = {G, APs} • Grow S according to S = {G, Aps, 1}

11. Constructing GB S G A A 1 2 3 4 5 • Drop the links with low RSS in the original network topology G • Maintain a set of explored node S, initially S = {G, APs} • Grow S according to S = {G, Aps, 1, 2}

12. Constructing GB G S A A 1 2 3 4 5 • Drop the links with low RSS in the original network topology G • Maintain a set of explored node S, initially S = {G, APs} • Grow S according to S = {G, Aps, 1, 2, 4}

13. Constructing GB S G A A 1 2 3 4 5 • Drop the links with low RSS in the original network topology G • Maintain a set of explored node S, initially S = {G, APs} • Grow S according to S = {G, Aps, 1, 2, 4, 5}

14. Constructing GB S G A A 1 2 3 4 5 • Drop the links with low RSS in the original network topology G • Maintain a set of explored node S, initially S = {G, APs} • Grow S according to S = {G, Aps, 1, 2, 4, 5, 3}

15. Construct Gv • More complicated than GB and GU: • Only involves part of the nodes in G • The existence of cycle • Restrictions: One cycle (length 2) between the parents of destination node v • Standard Reliable Downlink Graph • Construct a completely new graph from GW to DEV v • Configuration in intermediate nodes cannot be reused • High configuration cost and poor scalability

16. Sequential Reliable Downlink Routing (SRDR) • Low configuration cost • High Scalability • High Reliability • Key Principles • Each node only keep a small local graph • Local graphs are reusable building blocks for constructing reliable downlink graph for multiple destinations

17. An example of SRDR Avoid node failure at DEV2 Local graph

18. SRDR vs. Standard Downlink Graph Configure cost is reduced by 3 links More significant improvement in large scale networks

19. Sequential Reliable Downlink Routing (SRDR) Extensions Network layer header extension:

20. SRDR Extensions • Routing module extension: • retrieve the earliest graph ID in the graph list and route the packet on this graph • If current node is the sink of the graph, remove this graph ID and route the packet on the next earliest graph. • If routing is failed, remove this graph ID and try the next earliest graph ID if it has the corresponding edges.

21. Optimization on SRDR • In SRDR, routing is performed strictly according to the sequence in the ordered graph list. • SRDR-OPT • Observation: each node can keep graph info to multiple destination. • Have chance to take the “shortcut” • Principle: Search the ordered graph list backward and route the packet on the first graph ID that is stored in its table

22. An example of the SRDR-OPT

23. Communication Schedule and Channel Management • Key Principles: • Spread out the channel usage in the network • Apply Fastest Sample Rate First policy (FSRF) • Allocate the links iteratively from Src to Dest • Split traffic (bandwidth) among all successors

24. Schedule Construction (An Example) Ch Offset 16 Global Channel-Time Slot Matrix . . . 2 1 0 Slot 200 300 400 100 Device Schedule G Dev 1 A B AP A 1 2 3 Dev 2 1 sec 2 sec 1 sec

25. Schedule Construction (An Example) Ch Offset 16 . . . 2 1 0 Slot 200 300 400 100 G Dev 2 A B AP A 1 2 3 AP B 1 sec 2 sec 1 sec

26. Schedule Construction (An Example) Ch Offset 16 . . . 2 1 0 Slot 200 300 400 100 G Dev 3 A B AP B 1 2 3 Dev 2 1 sec 2 sec 1 sec

27. Schedule Construction (An Example) Ch Offset 16 . . . 2 1 0 Slot 200 300 400 100 G Dev 2 A B AP A 1 2 3 AP B 1 sec 2 sec 1 sec

28. Schedule Construction (An Example) Ch Offset 16 Channel offset will be converted into practical channel number in the runtime . . . 2 1 0 Slot 200 300 400 100 G Dev 2 A B AP A 1 2 3 AP B 1 sec 2 sec 1 sec

29. Performance Evaluation Reachability in broadcast graphs Configuration overhead in broadcast graphs Reachability in downlink graph Recovery overhead to regain reliability

30. Performance Evaluation Average latency vs. Network size Configuration overhead in downlink graphs Reusable local graph makes the difference Shortcut makes the difference Success ratio vs. Sample rate Network utilization vs. Sample rate

31. WirelessHART Prototype System PC Side Embedded Side Major Components in the prototype : Network Manager Gateway Host Application Access Point Device Sniffer

32. Overall Design of the System

33. Overview of the Network Manager Network Topology Routing Graphs Device Configuration Global Schedule Device Bandwidth Device Schedule

34. Overview of the System 10 devices and 1 AP in the system Devices publish data to GW with different sampling rates (1sec – 8sec) Retry happens but no packet loss is detected

35. Deployment (Work-in-progress) Network Manager, Gateway and Access Point UT Pickle Research Center Petroleum Engineering Department

36. Future Work • A general and adjustable framework • Applications have different requirements on timing, security, … • Building an adjustable MAC state machine • Collaborative wireless system • Wifi, WirelessHART, ZigBee,… • Competition -> Collaboration