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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

Grid: Scalable Ad-Hoc Wireless Networking

Douglas De Couto

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

ad hoc applications
Ad hoc Applications
  • Temporary, fast setup
    • Emergencies & events
  • Rooftop networks
    • No wires, trenches, etc.
  • Developing communities
    • Cheap, incremental, automatic
direct contact scales badly

A

F

D

B

E

C

G

J

I

H

Direct Contact Scales Badly

“Hello J!”

solution multi hop forwarding

A

F

D

B

E

C

G

J

I

H

Solution: Multi-hop Forwarding

“A to J: Hello!”

design challenges
Design Challenges
  • Finding routes
  • Cope with mobile nodes
  • Conserving battery power
  • Coping with malicious/faulty nodes
  • Scaling to large networks
completed research
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
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
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

geographic forwarding gf
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
grid location service gls overview
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.

gls s spatial hierarchy
GLS’s Spatial Hierarchy

level-0

level-1

level-2

level-3

All nodes agree on the global origin of the grid hierarchy

3 servers per node per level

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 )
queries search for destination s successors

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

gf gls performs well
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.
gls properties
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
mistake shortest path routes

A

F

D

B

E

C

G

J

I

H

Mistake: Shortest-Path Routes

A’s max

range

route metrics
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
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
How to find loss rate?
  • Signal strength?
current work
Current Work
  • Trying to directly measure loss rates
  • Route broadcast packets
    • Long time constants
  • 802.11 protocol beacons?
    • Requires driver integration
grid summary
Grid Summary
  • Grid routing protocols are
    • Self-configuring
    • Easy to deploy
    • Scalable

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

application smart devices

Access

Point

Application: Smart Devices

Remote

Control

Print

Share

E-Mail

Internet

application rooftop nets
Application: Rooftop Nets

School/Homework

Server

Internet

Access

Game server

application disaster services
Application: Disaster Services
  • Disaster may have damaged phone system &c
  • Want to avoid N2 plans for N services to communicate
topology distribution scales badly

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.”

geographic forwarding scales well

A

F

D

B

E

C

G

Geographic Forwarding Scales Well

“Send towards latG / lonG.”

Latitude

Longitude

location database

A

F

D

B

E

C

G

Location Database

DB

2. “Where is G?”

1. “G is at latG / lonG”

Latitude

Longitude

distributed location database
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’s Location Server is a Point

I

H

x

hash(G) = 0.1,0.9

G

(0,0)

spatial grid hierarchy
Spatial Grid Hierarchy

All nodes agree on the global origin of the Grid hierarchy

grid protocol overhead grows slowly
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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