A New Opportunity Has Arrived!

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! • 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?

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! 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 • 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 • Layer 2 handoff triggers routing updates. Mesh network cloud of APs

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 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 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 • 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 • 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 helpin lieu of joint optimization? • Convergence? • Practicality? Routing Influences interference Influences topology Channel Assignment

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

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

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

A New Opportunity Has Arrived!