1. Reliable, Robust Data Collection in Sensor Networks Murali Rangan Russell Sears Fall 2005 – Sensornet

2. Motivation Data collection apps like habitat monitoring  All of the logged data is valuable  Real time / in order delivery not important  In-network aggregation not required Current data collection mechanisms  Lose data during temporary network partitions  Or, store all data in each node Our goal  Reliable, robust scheme to collect ALL the data in a steady stream, without in order guarantees

3. Our approach Reliability  Link level, hop-by-hop acknowledgements  Introduce redundancy with k replicas of data  Replicas ensure their data reaches the base station, or pass responsibility to a parent. Take advantage of broadcast medium Take advantage of existing routing algorithms (like MintRoute) for parent selection Keep basic mechanism simple, add optimization on top

4. Algorithm overview Node with new data  Broadcasts request for k volunteers (for replicas)  Parent(s) agree to volunteer  Broadcasts data to volunteer(s)  Volunteer(s) broadcast hop ACK (HACK)  Once k volunteers are found, deletes data Volunteer transfers responsibility upstream  Repeats above process, seeking 1 volunteer  Once responsibility is transferred, deletes data Can volunteer for same data again!

5. Status and Future Work Basic algorithm details are fleshed out Implement and simulate using TOSSIM  Work in progress! Address the following challenges  How many replicas? Depends upon network density, reliability  Base station broadcasts ACK to remove replicas of collected data Use bloom filters to summarize? Limit broadcast to ‘interested’ nodes?  How to bound k replicas? Eavesdropping?  How do we deal with fan-in near the base station? Few parents / replicas share paths

7. Prerequisites Use any routing protocol that  Assigns ‘parent(s)’ to each mote  Adapts to changes in network topology Reliable link level delivery  Make use of broadcast medium  Transfer data from a child node to its parents  ACK’s transfer responsibility for a piece of data. (Dropped ACK’s lead to more replicas) Broadcast ACK from base station  Guarantees replicas are eventually deleted  Efficient, reliable broadcast? Bloom filters? Reverse path routing + eavesdropping?

8. Sketch of algorithm Inserting data into the network  Broadcast request for k volunteers  Broadcast data, volunteers (parents) ACK  Once k volunteers are found, node deletes data. Volunteers  request enough volunteers to create k replicas in network. (Usually, just create 1 volunteer)  Pass ACK’s to children after creating > 1 replica. Challenges  How many replicas?  What about fan-in? Replicas share < k parents Shared paths?  How can we optimize propagation?