Grid scalable ad hoc wireless networking
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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

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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


Goal networks out of chaos

A

F

D

B

E

C

G

J

I

H

Goal: Networks out of Chaos


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


Roof top grid net

Roof-Top Grid Net

6

5

4

3

2

1

LCS


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


Does grid find useful paths

A

F

D

B

E

C

G

J

I

H

Does Grid Find Useful Paths?


Mistake shortest path routes

A

F

D

B

E

C

G

J

I

H

Mistake: Shortest-Path Routes

A’s max

range


Link quality isn t bi modal

Link Quality Isn’t Bi-modal


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


End of talk demo

End Of TalkDemo


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


Multiple servers per node

Multiple Servers per Node

c

b

a

G


Lookups expand in scope

Lookups Expand in Scope

c

b

a

G

?

A


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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