self organizing wireless mesh networks microsoft research march 21, 2003 intel/microsoft quarterly...
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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
10
20
30
40
50
60
70
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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Why Topology Control?
u
v
u
v
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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: