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 Department SUNY at Stony Brook [e-mail] http://www.cs.sunysb.edu/~samir

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Stony Brook Mesh RouterArchitecting a
Multi-RadioMultihop Wireless LANSamir R.
Das(Joint work with Vishnu Navda, Mahesh
Marina and Anand Kashyap)Computer Science
DepartmentSUNY at Stony Brooksamir_at_cs.sunysb.edu
http//www.cs.sunysb.edu/samir
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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.

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Get rid of the wires!
Access Points/ Mesh Routers
Clients
Wired Backbone
Ethernet

• 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.

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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
Mesh network cloud of APs

• Layer 2 handoff triggers routing updates.

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Routing
Mesh network cloud of APs

• Handoff delay with Prism2-based cards and HostAP
  driver 240ms at L2 28ms per hop at L3.

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Multihop Relaying Performance with Multiple
Channels
TCP throughput

• Setup 802.11b prism2-based cards. HostAP driver.
  Relaying on WDS links.
• Gains over single channel not always spectacular.
• Suspect radio leakage.

Base case 1 hop throughput 5.5 Mbps
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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 nodes 2 radios/node 3 non-overlapping channels
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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 nodes 2 radios/node 3 non-overlapping channels
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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 nodes 2 radios/node 3 non-overlapping channels
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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 nodes 2 radios/node 3 non-overlapping channels
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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?
Routing
Influences interference
Influences topology
Channel Assignment

• Convergence?
• Practicality?

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Random Graph-based Simulations

• 50 nodes. Dense network.
• 12 independent channels.

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NS-2 Simulations

• 50 node. Dense network.
• MAC layer capacity with Poisson traffic on each
  link.

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Summary

• Extend infrastructure-mode WLAN to a mesh
  network.
• Complete client transparency.
• Handoff driven routing update.
• Multiple radio on each router. Channel assignment
  problem.