1. 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

2. 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?

3. Stony Brook Wireless Router
Wired Backbone
Access Points
Clients
Ethernet
Traditional Wireless LAN needs “wired” connectivity to access points.
Deployment slow and expensive, particularly for wide area.

4. Get rid of the wires! Use a mesh routing backbone.
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.

5. 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.

6. Routing Layer 2 handoff triggers routing updates.
Mesh network cloud of APs
Layer 2 handoff triggers routing updates.

7. Routing
Mesh network cloud of APs
Handoff delay with Prism2-based cards and HostAP driver = 240ms at L2 + 28ms per hop at L3.

8. Multihop Relaying Performance with Multiple Channels
TCP throughput
Setup: b 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

9. 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).

10. 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

11. 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

12. 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

13. 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

14. 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.

15. Can iterative approaches help in lieu of joint optimization?
Routing
Influences
interference
Influences
topology
Channel
Assignment
Convergence?
Practicality?

16. Random Graph-based Simulations
50 nodes. Dense network.
12 independent channels.

17. NS-2 Simulations 50 node. Dense network.
9.5 x
Several
orders of
magnitude
50 node. Dense network.
MAC layer capacity with Poisson traffic on each link.

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