Grid: Scalable Ad-Hoc Wireless Networking

Douglas De Couto

http://pdos.lcs.mit.edu/grid

Goal: Networks out of Chaos

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Ad hoc Applications• Temporary, fast setup

• Emergencies & events

• Rooftop networks• No wires, trenches, etc.

• Developing communities• Cheap, incremental, automatic

Direct Contact Scales Badly

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“Hello J!”

Solution: Multi-hop Forwarding

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“A to J: Hello!”

Design Challenges

• Finding routes

• Cope with mobile nodes

• Conserving battery power

• Coping with malicious/faulty nodes

• Scaling to large networks

Completed Research

• Scalable routing:• Geographic forwarding• Distributed P2P location database

• Low-power forwarding

• Understanding capacity limits

• Avoiding malicious nodes

• Current research: link selection

System Status

• Software distributions for• Linux, BSD• PC, iPaq

• Works with unmodified Internet software

• Two Grid nets deployed• In-building network• Rooftop network

LCS Grid Net

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• 17 static nodes on 5th/6th floors• A dozen iPaq hand-helds

wiredgateway

Roof-Top Grid Net

LCS

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43

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Geographic forwarding (GF)

• 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

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C’s radio range

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FC

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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.

“D?”

Grid Location Service (GLS) overview

level-0

level-1

level-2

level-3

All nodes agree on the global origin of the grid hierarchy

GLS’s Spatial Hierarchy

3 servers per node per leveln

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s

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s s

• s is n’s successor in that square. (Successor is the node with “least ID greater than” n )

sibling level-0squares

sibling level-1squares

sibling level-2squares

Queries search for destination’s successors

Each query step: visit n’s successor at increasing levels, untillocation server found

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location query path

• Geographic forwarding is less fragile than source routing.• DSR queries use too much b/w with > 300 nodes.

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Number of nodes

DSR

Grid

GF + GLS performs well

Biggest network simulated:600 nodes, 2900x2900m(4-level grid hierarchy)

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

Does Grid Find Useful Paths?

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Mistake: Shortest-Path Routes

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HA’s maxrange

Link Quality Isn’t Bi-modal

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

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

How to find loss rate?

• Signal strength?

Current Work

• Trying to directly measure loss rates

• Route broadcast packets• Long time constants

• 802.11 protocol beacons?• Requires driver integration

Grid Summary

• Grid routing protocols are• Self-configuring• Easy to deploy• Scalable

http://www.pdos.lcs.mit.edu/grid

End Of Talk

Demo

Application: Smart Devices

Internet

AccessPoint

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E-Mail

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RemoteControl

Application: Rooftop Nets

Game server School/HomeworkServer

InternetAccess

Application: Disaster Services

• Disaster may have damaged phone system &c• Want to avoid N2 plans for N services to communicate

Topology Distribution Scales Badly

1. “C can reach A and B.”

AB

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3. Data from F to B.

2. “D can reach A, B, and C.”

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Geographic Forwarding Scales Well

Longitude

Latit

ude

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“Send towards latG / lonG.”

Location Database

Longitude

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ude

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DB

1. “G is at latG / lonG”

2. “Where is G?”

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

G’s Location Server is a Point

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hash(G) = 0.1,0.9

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(0,0)

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Spatial Grid Hierarchy

All nodes agree on the global origin of the Grid hierarchy

Multiple Servers per Node

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Lookups Expand in Scope

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Grid Protocol Overhead Grows Slowly

• Protocol packets include: Grid update, Grid query/reply.

Number of nodes

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