1. RoamHBA ： Maintaining Group Connectivity In Sensor Networks Qing Fang Jie Liu Leonidas Guibas Feng Zhao Department of Electrical Engineering, Stanford University Department of Computer Science, Stanford University ACM IPSN (Information Processing in Sensor Networks) 2004 Speaker ： Jenchi

2. Outline Introduction Roamingcast Roaming Hub Based Architecture Roaming Hub Formation Roaming Hub Update Simulation

3. Introduction The routing algorithm and techniques for building an efficient, shared, many-to-many multicast routing structure A Steiner tree The optimal many-to-many multicast routing structure But each node require O(log n) global routing table Backbone-based routing Roaming agents

4. Introduction The collaboration group is useful to support collaborative information processing in sensor networks Maintaining connectivity among a group of roaming agents arises in applications such as collaborative exploration, pursuer- evader games, and identity management in multi-target tracking Maintaining connectivity among a group of mobile agents requires multicast routing support at the network layer.

5. Introduction An efficient mechanism that supports symmetric multicast routing in a fixed, densely deployed wireless sensor network A subnetwork is used to connect a group of processes that reside in sensor nodes End points of the subnetwork may hop from time to time Mobility is implemented

6. Roamingcast For the communication patterns necessary to support such collaborating agents Give a group of mobile agents G in a sensor network Roamingcast is a group communication scheme that enables communication among any subset S of group members G Roamingcast is different from multicast, as in multicast S=G

7. Roamingcast Assume that sensors are densely deployed in large scale in an ad hoc fashion Focus on Distributed construction and dynamic updates of a many-to-many communication tree, the roaming hub

8. Roaming Hub Based Architecture (RoamHBA) In roamingcast, destination locations change from time to time without acknowledging the senders

9. Roaming Hub Based Architecture (RoamHBA) (cont.) Architectural ant functional entities of RoamHBA Mobile agent tracks the moving physical phenomenon of interest that the sensor network is monitoring resides in the terminal node Junction agent representing a mobile agent on the backbone resides in a junction node  The node at which a terminal node connects to the backbone, possibly through a multi-hop path

10. Roaming Hub Based Architecture (RoamHBA) (cont.) Rib the path consisting of a set of nodes (rib nodes) between a mobile agent and its junction node established by a mobile agent connecting to the backbone via geographic forwarding Roaming Hub is the rendezvous entity that all mobile agents communicate with Lead node is a terminal node with its y-coordinate chosen to be close to the median of all the terminal nodes’ y-coordinates

11. Roaming Hub Based Architecture (RoamHBA) (cont.) The component of RoamHBA The logical and physical relations between agents and nodes

12. Roaming Hub Formation Environment Sensor nodes are deployed in the plane, each with a constant communication range Each node can get its location information Node density of the network is high enough so that greedy forwarding can always make progress We chose to use a backbone that is cross-shaped consisting of a horizontal part and a vertical part Horizontal backbone and vertical backbone

13. Roaming Hub Formation -Greedy forwarding in horizontal/vertical direction Terminal node L(x 0,y 0 ) (x 0 +R,y 0 ) (x 0 -R,y 0 ) A(x 1,y 1 ) B(x 2,y 2 )

14. Roaming Hub Formation -Greedy forwarding in horizontal/vertical direction (cont.) L(x 0,y 0 ) A(x 1,y 1 ) B(x 2,y 2 ) Terminal node

15. Roaming Hub Formation -Greedy forwarding in horizontal/vertical direction (cont.) L(x 0,y 0 ) Terminal node

16. Roaming Hub Formation -The backbone A bounding box A virtual rectangle defined by dual pairs of x(x min,x max ) and y(y min,y max ) k is the number of mobile agents x min ＝ min(x 1,x 2,…,x k ), x max ＝ max(x 1,x 2,…,x k ) y min ＝ min(y 1,y 2,…,y k ), y max ＝ max(y 1,y 2,…,y k ) The bounding box state Includes these dual pairs and the agent ID associated with each of them Is propagated along the backbone and every backbone node stores a copy

18. Roaming Hub Formation -The backbone (cont.) The active backbone The longer side of the bounding box Each backbone is associated with a backbone agent A backbone agent resides in the lead node The backbone agent decides when a migration is needed

19. Roaming Hub Formation -Mobile agents connecting to the backbone Mobile terminals connect to the active backbone A path to the backbone is constructed by an agent sending a path finding packet upstream towards the backbone The packet is forwarded at each hop using geographic greedy forwarding which carries agent state including its ID and location When the packet reaches the backbone, a junction agent is created at the junction node and agent state is stored there

20. Roaming Hub Formation -Greedy forwarding in horizontal/vertical direction (cont.) L(x 0,y 0 ) Terminal node

21. Roaming Hub Updates Due to agent mobility, routes provided by the roaming hub degenerate over time Perform such updates in a distributed fashion Dynamic updates by logic network entities Mobile agent Junction agents Backbone agent

22. Roaming Hub Updates -Mobile agent Maintaining agent connectivity to the backbone

23. Roaming Hub Updates -Mobile agent (cont.) Route updates using distance based rule s ： the node that the agent previous connection to the backbone t ： the node that the agent currently resides r ： the distance between s and t d ： the distance of node s from the backbone r/d <ρ(ρ<1) ： free ratio The farther away an agent is from the backbone, the less frequently it updates its path to the backbone

24. Roaming Hub Updates -Mobile agent (cont.) Determining and adjusting the backbone span The length of a horizontal backbone is approximately the length of the bounding box in x direction plus an additional length x c because agent makes a new connection to the backbone via geographic forwarding, the connecting path will be guaranteed to cross the backbone  d ＝ the largest y offset among all the mobile agents If x c ＝ d ×ρ, a direct connection from any of the mobile agents to the backbone will be guaranteed to cross the backbone

25. Roaming Hub Updates -Junction agents 1. The new junction agent sends out a packet along the backbone to invalidate the old junction agent that belongs to the same mobile agent

26. Roaming Hub Updates -Junction agents (cont.) 2. Before the old junction agent expires, it sends out an invalidation packet to invalidate the old junction rib associated with itself

27. Roaming Hub Updates -Junction agents (cont.) 3. The new junction agent decides if the existing bounding box state needs an update

28. Roaming Hub Updates -Junction agents (cont.) Updating the bounding box The junction node needs to decide if the bounding box state needs an update The criterion is whether or not the existing bounding box is still the smallest rectangle that covers all the terminal nodes  If ‘yes’, no update  If ‘no’, the junction node computes the new bounding box state and broadcasts it over the backbone

29. Roaming Hub Updates -Backbone agents A backbone agent resides in the lead node of the backbone is responsible for monitoring the number of mobile agents on each side and deciding when a backbone migration is needed Backbone will be migrated when the number of agents on one side is a fraction, γ, of the total number of agents the value γ reflects the tradeoff point between path length and roaming hub update overhead

30. Roaming Hub Updates -Backbone agents (cont.) When the backbone agent decides to migrated it polls all the junction agents to get their mobile agents’ locations It identifies the new lead node and creates a new backbone agent at the lead node The new backbone agent initiates a new backbone construction, and notifies all backbone nodes on the old backbone of the migration A death timer is set at each backbone node on the old backbone All junction node request their terminal nodes to connect to the new backbone

31. Simulation -Simulation setup RoamHBA is implemented at the network layer using NS-2 Geographic forwarding is used in constructing the roaming hub Each node has the same computation, communication and storage capabilities maintains a neighbor list storing coordinates of its neighbors learns its one-hop neighborhood and store this information locally A control packet header includes the direction the packet travels

32. Simulation -Soft states Rib nodes maintain a dynamic array of ports A port is created for one agent that has its rib through the node One port is associated with one agent’s connection to the backbone at the node At each port, upstream and downstream neighbors are recorded A port takes 3 integers of memory Backbone nodes store bounding box parameters(4 doubles) Mobile agents store backbone locations(2 doubles) Junction agent store their mobile agent’s location(2 doubles) Backbone agents store numbers of agents on each side(2 integers)

33. Simulation -Evaluation 1800 sensors are placed on a perturbed grid in a 250m by 250m 2-D space Each sensor node has transmission radius of 15 meters There are 7 mobile agents scattered across an area of about one sixth of the 2-D space

34. Sumulation -Tradeoffs between the data packet rate and the frequency of roaming hub updates How the free ratio ρ affects the total data packet and the control packet transmissions The more frequent the route update are, the less transmissions are necessary to deliver data to destinations, but at a price of more control packets

35. Sumulation -Tradeoffs between the data packet rate and the frequency of roaming hub updates How the criteria γ for migrating backbone affect total data packet and control packet transmissions Frequent backbone location adjustments help shortening total data path but drastically increase control overhead

36. Sumulation -Tradeoffs between the data packet rate and the frequency of roaming hub updates Number of control packet vs. agent mobility

37. Conclusions The Roamingcast, a new way of networking, which arises from collaborative information processing in wireless sensor networks RoamHBA one of the solutions to support group connectivity For dense network where mobile agent’s mobility has some locality, RoamHBA is both effective and practical