samir r. das 1 stony brook mesh router: architecting a multi-radio multihop wireless lan samir r....
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1Samir R. Das
Stony Brook Mesh Router:Architecting a Multi-Radio
Multihop Wireless LAN
Samir R. Das
(Joint work with Vishnu Navda, Mahesh Marina and Anand Kashyap)
Computer Science DepartmentSUNY at Stony [email protected]
http://www.cs.sunysb.edu/~samir
2Samir R. Das
A New Opportunity Has Arrived!
Linksys WRT54G access point/router runs Linux. User programmable. Decent processor and memory. Costs $70.
Several router platforms provide multiple PC/mini-PCI/PCI card interfaces. Decent processor and memory. Can run FreeBSD/Linux. Costs $250-$400.
What a systems researcher can do with all these?
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Stony Brook Wireless Router
Traditional Wireless LAN needs “wired” connectivity to access points.
Deployment slow and expensive, particularly for wide area.
Wired Backbone
Access Points
Clients
Ethernet
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Get rid of the wires!
Use a mesh routing backbone. Clients can associate with any access point/router. Complete transparency. Multiple radio interfaces on each router assigned to different bands/channels.
Wired Backbone
Access Points/ Mesh Routers
Clients
Ethernet
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Architectural Choices Clients run on infrastructure mode.
Associate to a nearby AP. Unaware of the wireless backbone.
Use WDS (wireless distribution system) for inter-AP communication.
Use a routing protocol for inter-AP routing. Link state-based routing. Choice of link cost metric?
Multiple radios on each AP Channel assignment problem.
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Routing
Handoff delay with Prism2-based cards and HostAP driver = 240ms at L2 + 28ms per hop at L3.
Mesh network cloud of APs
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Multihop Relaying Performance with Multiple Channels
Setup: 802.11b prism2-based cards. HostAP driver. Relaying on WDS links.
Gains over single channel not always spectacular.
Suspect radio leakage.
0
0.5
1
1.5
2
2.5
3
3.5
2 Hops, 1 Channel
2 Hops, 2
Channels
3 Hops, 1 Channel
3 Hops, 2
Channels
3 Hops, 3
Channels
Mbps
Base case: 1 hop throughput 5.5 Mbps
TCP throughput
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Channel Assignment Problem: Observations and Approaches
Channel switching takes time (~100ms) in COTS hardware
Rule out dynamic approaches. Statically? Semi-dynamically?
Channel assignment is a topology control problem. Two neighboring node can talk only when they have a radio
on a common channel. Ideally, one should jointly solve channel assignment and
routing. Our approach: Assign channels to radios to minimize
interference (objective), but preserve original topology (constraint).
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Conflict Graph-based Greedy Algorithm
Visits nodes in a certain order and assigns channels to radios such that all links from this node gets a channel.
Channel selection based on a greedy heuristic.
Maintain a conflict graph on the side to model interference. Compute the heuristic on this graph.
Centralized; but can be distributed.
3 nodes2 radios/node3 non-overlappingchannels
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Visits nodes in a certain order and assigns channels to radios such that all links from this node gets a channel.
Channel selection based on a greedy heuristic.
Maintain a conflict graph on the side to model interference. Compute the heuristic on this graph.
Centralized; but can be distributed.
3 nodes2 radios/node3 non-overlappingchannels
Conflict Graph-based Greedy Algorithm
12Samir R. Das
3 nodes2 radios/node3 non-overlappingchannels
Conflict Graph-based Greedy Algorithm Visits nodes in a certain
order and assigns channels to radios such that all links from this node gets a channel.
Channel selection based on a greedy heuristic.
Maintain a conflict graph on the side to model interference. Compute the heuristic on this graph.
Centralized; but can be distributed.
13Samir R. Das
Conflict Graph-based Greedy Algorithm
3 nodes2 radios/node3 non-overlappingchannels
Visits nodes in a certain order and assigns channels to radios such that all links from this node gets a channel.
Channel selection based on a greedy heuristic.
Maintain a conflict graph on the side to model interference. Compute the heuristic on this graph.
Centralized; but can be distributed.
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The Devil is in the Model Interference model (used in objective)
Current model: Two links on the same channel with a common node interferes. Nothing else interferes.
Future: Model overlapping channels and radio leakage. Model interference beyond one hop. Factor in load?
What to optimize? Minimize max interference. Maximize no. of concurrent transmissions.
Topology (used as a constraint) Current model: Preserve the original topology. Future: Use the sub-topology actually used by
routing.
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Can iterative approaches helpin lieu of joint optimization?
Convergence? Practicality?
Routing
Channel Assignment
Influencesinterference
Influences topology
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NS-2 Simulations
50 node. Dense network. MAC layer capacity with Poisson traffic on each link.
9.5 xSeveral orders of magnitude