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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 samir@cs.sunysb.edu http://www.cs.sunysb.edu/~samir. A New Opportunity Has Arrived!.

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Stony Brook Mesh Router:Architecting a Multi-RadioMultihop Wireless LANSamir R. Das(Joint work with Vishnu Navda, Mahesh Marina and Anand Kashyap)Computer Science DepartmentSUNY at Stony Brooksamir@cs.sunysb.eduhttp://www.cs.sunysb.edu/~samir

a new opportunity has arrived
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?
stony brook wireless router

Access Points

Clients

Wired Backbone

Ethernet

Stony Brook Wireless Router
  • Traditional Wireless LAN needs “wired” connectivity to access points.
  • Deployment slow and expensive, particularly for wide area.
get rid of the wires
Get rid of the wires!

Access Points/ Mesh Routers

  • 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.

Clients

Wired Backbone

Ethernet

architectural choices
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.
routing
Routing
  • Layer 2 handoff triggers routing updates.

Mesh network cloud of APs

routing7
Routing
  • Handoff delay with Prism2-based cards and HostAP driver = 240ms at L2 + 28ms per hop at L3.

Mesh network cloud of APs

multihop relaying performance with multiple channels
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

channel assignment problem observations and approaches
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).
conflict graph based greedy algorithm
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

conflict graph based greedy algorithm11
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

conflict graph based greedy algorithm12
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

conflict graph based greedy algorithm13
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

the devil is in the model
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.
can iterative approaches help in lieu of joint optimization
Can iterative approaches helpin lieu of joint optimization?
  • Convergence?
  • Practicality?

Routing

Influences

interference

Influences

topology

Channel

Assignment

random graph based simulations
Random Graph-based Simulations
  • 50 nodes. Dense network.
  • 12 independent channels.
ns 2 simulations

Several

orders of

magnitude

9.5 x

NS-2 Simulations
  • 50 node. Dense network.
  • MAC layer capacity with Poisson traffic on each link.
summary
Summary
  • Extend infrastructure-mode WLAN to a mesh network.
  • Complete client transparency.
  • Handoff driven routing update.
  • Multiple radio on each router. Channel assignment problem.