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Grid: Scalable Ad-Hoc Wireless Networking

Grid: Scalable Ad-Hoc Wireless Networking. Douglas De Couto http://pdos.lcs.mit.edu/grid. A. F. D. B. E. C. G. J. I. H. Goal: Networks out of Chaos. Ad hoc Applications. Temporary, fast setup Emergencies & events Rooftop networks No wires, trenches, etc. Developing communities

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Grid: Scalable Ad-Hoc Wireless Networking

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  1. Grid: Scalable Ad-Hoc Wireless Networking Douglas De Couto http://pdos.lcs.mit.edu/grid

  2. A F D B E C G J I H Goal: Networks out of Chaos

  3. Ad hoc Applications • Temporary, fast setup • Emergencies & events • Rooftop networks • No wires, trenches, etc. • Developing communities • Cheap, incremental, automatic

  4. A F D B E C G J I H Direct Contact Scales Badly “Hello J!”

  5. A F D B E C G J I H Solution: Multi-hop Forwarding “A to J: Hello!”

  6. Design Challenges • Finding routes • Cope with mobile nodes • Conserving battery power • Coping with malicious/faulty nodes • Scaling to large networks

  7. Completed Research • Scalable routing: • Geographic forwarding • Distributed P2P location database • Low-power forwarding • Understanding capacity limits • Avoiding malicious nodes • Current research: link selection

  8. System Status • Software distributions for • Linux, BSD • PC, iPaq • Works with unmodified Internet software • Two Grid nets deployed • In-building network • Rooftop network

  9. LCS Grid Net 5 5 6 5 6 5 5 5 6 5 6 6 5 6 5 5 5 • 17 static nodes on 5th/6th floors • A dozen iPaq hand-helds wired gateway

  10. Roof-Top Grid Net 6 5 4 3 2 1 LCS

  11. Geographic forwarding (GF) C’s radio range A D F C G B E • Packets addressed to idG,locationG • Next hop is chosen from neighbors to move packet geographically closer to destination location • Per-node routing overhead constant as network size (nodes, area) grows • Requires location service, which adds overhead

  12. Grid Location Service (GLS) overview E H L B D J G A “D?” I F K C Each node has a few servers that know its location. 1. Node D sends location updates to its servers (B, H, K). 2. Node J sends a query for D to one of D’s close servers.

  13. GLS’s Spatial Hierarchy level-0 level-1 level-2 level-3 All nodes agree on the global origin of the grid hierarchy

  14. sibling level-0 squares s n s s s s sibling level-1 squares s s sibling level-2 squares s s 3 servers per node per level • s is n’s successorin that square. • (Successor is the node with “least ID greater than” n )

  15. location query path Queries search for destination’s successors s n s s s s Each query step: visit n’s successor at increasing levels, until location server found s s s1 x s2 s s3

  16. GF + GLS performs well Grid DSR Number of nodes Biggest network simulated: 600 nodes, 2900x2900m (4-level grid hierarchy) Fraction of data packets delivered successfully • Geographic forwarding is less fragile than source routing. • DSR queries use too much b/w with > 300 nodes.

  17. GLS properties • Spreads load evenly over all nodes • Degrades gracefully as nodes fail • Queries for nearby nodes stay local • Per-node storage and communication costs grow slowly as the network size grows: O(log n), n nodes • More details: Li et al, Mobicom 2000

  18. A F D B E C G J I H Does Grid Find Useful Paths?

  19. A F D B E C G J I H Mistake: Shortest-Path Routes A’s max range

  20. Link Quality Isn’t Bi-modal

  21. Route metrics • How to select good routes? • Compare metrics • Good metric: expected total packet transmissions • Want to mimimize • Route metric = sum of link metrics • Fight strong bias towards shortest paths • While penalizing longer paths

  22. Obstacles to Better Routing • Want to detect and avoid lossy links, but… • Loss rate masked by 802.11 re-sends • Changes quickly with time, motion

  23. How to find loss rate? • Signal strength?

  24. Current Work • Trying to directly measure loss rates • Route broadcast packets • Long time constants • 802.11 protocol beacons? • Requires driver integration

  25. Grid Summary • Grid routing protocols are • Self-configuring • Easy to deploy • Scalable http://www.pdos.lcs.mit.edu/grid

  26. End Of TalkDemo

  27. Access Point Application: Smart Devices Remote Control Print Share E-Mail Internet

  28. Application: Rooftop Nets School/Homework Server Internet Access Game server

  29. Application: Disaster Services • Disaster may have damaged phone system &c • Want to avoid N2 plans for N services to communicate

  30. A B C D F G Topology Distribution Scales Badly 1. “C can reach A and B.” 3. Data from F to B. 2. “D can reach A, B, and C.”

  31. A F D B E C G Geographic Forwarding Scales Well “Send towards latG / lonG.” Latitude Longitude

  32. A F D B E C G Location Database DB 2. “Where is G?” 1. “G is at latG / lonG” Latitude Longitude

  33. Distributed Location Database • Each node is DB for a few other nodes • How to find a node’s location server(s)? • Every node has an unchanging ID • hash(ID) maps ID to position in unit square

  34. G’s Location Server is a Point I H x hash(G) = 0.1,0.9 G (0,0)

  35. Spatial Grid Hierarchy All nodes agree on the global origin of the Grid hierarchy

  36. Multiple Servers per Node c b a G

  37. Lookups Expand in Scope c b a G ? A

  38. Grid Protocol Overhead Grows Slowly Protocol Overhead (packets per second) Number of nodes • Protocol packets include: Grid update, Grid query/reply.

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