Self Organizing WirelessMesh Networks

Microsoft Research

March 21, 2003

Intel/Microsoft Quarterly Strategic CTO Review

What is a Mesh Network?

e.g. MeshNetworks, Invisible Networks, Radiant Networks, Nokia’s Rooftop Network

Architecture affects design decisions onCapacity management, fairness, addressing & routing, mobility management, energy management, service levels, integration with the Internet, etc.

Internet

Gas Station

Bus Stop

Mesh Router 2

End Device(Guest to Router 1)

Mesh Router 1

Mesh End Device

EXIT

Mesh Zone

Mesh Router 3

(Internet TAP)

Mesh Router 5

Mesh Router 7

90

101

206

Neighborhood Mesh Network

Scoping out the Problem

• What is the achievable capacity in an ideal wireless mesh? How can we reach this optimal capacity?

• What is the best way to reach mesh nodes? That is, how should we assign addresses and route packets within the mesh and to the Internet?

• How should we ensure fairness and privacy for end-users and security for the network? How should we guard against malicious nodes?

• What are the applications that exploit the properties of the Mesh?

Mesh Formation: When does a viable mesh form?

The answer is a function of the environment, and business model, however if we leave out

the business model…

Problem Formulation

Question How many homes in the

neighborhood have to sign up before a viable mesh forms?

Answer depends on– Definition of “viable”– Neighborhood topology– Wireless range– Probability of participation

by a given houshold

Example ScenarioViable mesh: group of at least 25

houses that form a connected graph

Topology: A North Seattle Neighborhood. 8214 houses, 4Km x 4Km

Wireless range: 50, 100, 200 and 1000 meters

Houses decide to join at random, independent of each other. We consider 0.1% to 10% participation rates.

Mesh Formation: Simulation Results

• 5-10% subscription rate needed for suburban topologies with 200 m wireless ranges

• Once a mesh forms, it is usually well-connected

– i.e. number of outliers are few (most nodes have > 2 neighbors)

• Need to investigate other joining models

• Business model considerations will be important

Increasing range is key for viable mesh connectivity

Investigating current technologies

There are many problems with existing technology -- we cover only a few to make

some points

Background: The Hidden Terminal Problem

Consider the following scenario [Tobachi75]– B is in range of A & C; A & C are out of range of each other

• i.e. A & C are hidden from each other

– A sends a packet to B– C sends a packet to B– The packets collide at B

• results in reduction of throughput

CSMA doesn’t work– C can’t know that it has to wait

since it can’t hear A

Solution: RTS/CTS - with intended transmission duration [Karn90]

A

C

B

2 3 4 5 7 8 91 1110

RTS RTS

CTSCTS

6

Multihop Networks Case:Packets in Flight Example

4 nodes are active, 2 packets in flight

Microsoft Confidential

Backoff window doubles

Backoff algorithm hurts

RTS RTS RTS

Single Hop

Range and Hop Effect: 802.11a & 802.11b

802.11b versus 802.11a

1 wall / hop

Conclusions from our Studies

• Multihop with IEEE 802.11{a.b,g}– Severe throughput degradation as number of hops

increase– Poor fairness properties

• No guarantee that every user will get a fair share (equal) bandwidth

• Current software (firmware) for ad hoc 802.11 connectivity is immature– Frequent disconnects & network partitioning, loss of bcast

packets

Bottom Line: Current off-the-shelf WLAN technologies are not suitable for multihop

Overcoming Limitations, Innovating

A 15-Node Mesh Testbed in Building 113

• IEEE 802.11a 1st generation wireless NICs• Internally developed multihop routing protocol• Packet overhead is minimal when nodes are relatively

static• Use it for everyday tasks, email, web, etc.• On-going improvements in performance via intelligent

software

Increasing Capacity – Multiple Radios

t1

t2

Source Destination

Source Destination

Mesh Router

Mesh Router

Channel 1

Channel 1

Multihop wireless networks with single radio are inefficient, as a node can not transmit and receive simultaneously.

Network capacity can be significantly improved if a second radio, tuned to an orthogonal channel is available

Source DestinationMesh Router

t1

Channel 1

Channel 11

Multiple radios provide frequency diversity

• reduce contention • provide robustness

MultiRadio Unification Protocol (MUP)

• Allows systems to locally optimize use of available spectrum

• Use existing hardware• Support legacy applications• Interoperate with legacy hardware• Global information should not be required

Simulations with a Real Topology

252 houses in a Seattle neighborhood

Mesh formation among 35 randomly selected houses

Range is 250 metersRoutes via AODV (IETF)ITAP

Web surfer

Performance using Seattle Neighborhood

0

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30

40

50

60

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80

90

100

1 2 3 4 5 6 7 8 9 10 11 12

Object size in Kilobytes

Pe

rce

nta

ge

re

du

cti

on

in r

ep

on

se

tim

e

MUP with 2 radios Using realistic Web Traffic

40-50% reduction in delay compared to a one-radio network

How do Wireless Devices affect Mesh Performance

Do we need Spectrum Etiquettes?

In the presence of other 2.4 GHz devices

Panasonic 2.4GHz Spread Spectrum Phone 5m and 1 Wall from receiver

Phone on

Local behavior affects Global Performance!

0

20

40

60

80

100

120

Base One TCP 10% Drop rate

No

rmal

ized

Per

cen

tag

e

Node D Node E

Node A Node B Node C

200 meters200 meters

100 meters

Packets get dropped!

Doesn’t care

Summing it up

• We believe community networking will become increasingly important.– MSR has several technologies in the works that will make it attractive.

• Viable meshes (of 25 nodes or above) can be formed with as few as 10% of the homes participating - Need good range and capacity

• Current off-the-shelf WLAN technologies are not suitable for building reliable high capacity meshes

• Capacity can be improved by utilizing the entire available spectrum

• Local misbehaving wireless devices cause unacceptable performance reduction

• At this time, per packet channel switching is not a viable option.

Additional Notes:• Cross industry spectrum harmonization is important for this vision to

succeed.

• Mesh networking is an important area of research for MSR (researchers from Redmond, Cambridge & SVC Labs are involved).

Backup

Etiquette Proposal

• Transmit Power Control (TPC)– Reduce interference between neighbors, increase capacity through

increased spatial reuse

• Dynamic Frequency Selection (DFS)– Reduce destructive interference resulting from simultaneous

transmissions

• Listen Before Talk with Channel Wait Time (LBT-CWT)– Eliminate the possibility of devices being shut out from using the

spectrum

In addition….

Etiquette Proposal (cont.)

• TPC is applied to the entire unlicensed band• DFS is applied to x % of the unlicensed band• LBT-CWT is applied to (100-x) % of the unlicsensed band

5.0

5 GHz Unlicensed5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9 6.0

US

For example,

TPC, DFS TPC, LBT-CWT

.... to achieve serious capacity improvement…range, power and topology control are necessary

Microsoft confidential

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v

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Why Topology Control?

u

v

u

v

ww

Increased Interference! Reduced throughput!

V

Ensuring Connectivity while Decreasing Interference

Who should be my neighbor ?

What should be my transmission power?Power level influences rangePower level determines interferencePower level affects routes

Want to decide locally but want to guarantee connectivity globally

Cone Based Algorithm

Theorem: If 5/6 and we find a neighbor in the cone, then we are connected.

Transmit with minimum power within a cone till you hit a node -- that’s your power limit !

Before After

Cone Based Algorithm with Edge Removal

Performance: