Grid: Scalable Ad-Hoc Wireless Networking
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Transcript of Grid: Scalable Ad-Hoc Wireless Networking
Grid: Scalable Ad-Hoc Wireless Networking
Douglas De Couto
http://pdos.lcs.mit.edu/grid
Goal: Networks out of Chaos
A
F
D
B
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GJ
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H
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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D
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GJ
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H
“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
5
43
12
6
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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CD F
C’s radio range
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G
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EH
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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
s
s
s
s
s
s
s
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
n
s
s
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s s3
xs2
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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.
Fra
cti o
n of
da t
a pa
c ket
s de
l iver
ed s
ucce
ssfu
l ly
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
Share
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
C
D
F
3. Data from F to B.
2. “D can reach A, B, and C.”
G
Geographic Forwarding Scales Well
Longitude
Latit
ude
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D
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G
“Send towards latG / lonG.”
Location Database
Longitude
Latit
ude
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G
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
G
hash(G) = 0.1,0.9
x
(0,0)
H I
Spatial Grid Hierarchy
All nodes agree on the global origin of the Grid hierarchy
Multiple Servers per Node
G
c
b a
Lookups Expand in Scope
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b a
A
?
Grid Protocol Overhead Grows Slowly
• Protocol packets include: Grid update, Grid query/reply.
Number of nodes
Pro
toco
l Ov e
rhea
d (p
acke
ts p
er s
econ
d)