Transcript Self Organizing Wireless Mesh Networks

Self Organizing Wireless
Mesh Networks
Microsoft Research
March 21, 2003
Intel/Microsoft Quarterly Strategic CTO Review
What is a Mesh Network?
Neighborhood Mesh Network
Internet
101
Bus Stop
206
Gas Station
(Internet TAP)
Mesh Router 7
EXIT
Mesh Router 5
Mesh Router 2
Mesh Router 3
Mesh Zone
90
e.g. MeshNetworks, Invisible
Networks, Radiant Networks,
Nokia’s Rooftop Network
Mesh Router 1
Mesh End Device
End Device
(Guest to Router 1)
Architecture affects design decisions on
Capacity management, fairness, addressing & routing, mobility management,
energy management, service levels, integration with the Internet, etc.
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 endusers 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 Scenario
Viable 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
B
A
C
Solution: RTS/CTS - with intended transmission
duration [Karn90]
Multihop Networks Case:
Packets in Flight Example
RTS
RTS
RTS
RTS
1
2
3
RTS
4
5
6
7
8
9
CTS
CTS
Backoff window doubles
4 nodes are active, 2 packets in flight
Backoff algorithm hurts
Microsoft Confidential
10
11
Range and Hop Effect: 802.11a & 802.11b
Single Hop
1 wall / hop
802.11b versus 802.11a
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
Multihop wireless networks with single radio are inefficient,
as a node can not transmit and receive simultaneously.
Channel 1
t1
Source
Mesh Router
t2
Destination
Channel 1
Source
Mesh Router
Destination
Network capacity can be significantly improved if a
second radio, tuned to an orthogonal channel is available
Channel 1
Channel 11
t1
Source
Mesh Router
Destination
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
Web surfer
ITAP
Range is 250 meters
Routes via AODV (IETF)
Performance using Seattle Neighborhood
Percentage reduction in reponse time
100
90
MUP with 2 radios
80
Using realistic Web Traffic
70
60
50
40
30
20
10
0
1
2
3
4
5
6
7
8
9
10
11
12
Object size in Kilobytes
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
Phone on
Panasonic 2.4GHz Spread Spectrum Phone 5m and 1 Wall from receiver
Local behavior affects Global Performance!
Doesn’t care
Node D
Node E
100 meters
Node A
Node B
200 meters
Node C
200 meters
120
Packets get dropped!
Normalized Percentage
100
80
60
40
20
0
Base
One TCP
10% Drop rate
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
For example,
5 GHz Unlicensed
5.0
5.1
5.2
5.3
5.4
5.5
5.6
5.7
5.8
5.9
US
TPC, DFS
TPC, LBT-CWT
6.0
.... to achieve serious capacity
improvement…range, power and topology
control are necessary
Microsoft confidential
Why Topology Control?
u
u
w
w
v
V
Increased Interference!
Reduced throughput!
u
u
w
w
v
v
Ensuring Connectivity while Decreasing
Interference
Who should be my neighbor ?
What should be my transmission power?
Power level influences range
Power level determines interference
Power level affects routes
Want to decide locally but want to guarantee connectivity globally
Cone Based Algorithm
Theorem: If a  5p/6 and we find a neighbor in the cone,
then we are connected.
a
Transmit with minimum power within a cone till you
hit a node -- that’s your power limit !
Cone Based Algorithm with Edge Removal
Performance:
Before
After