Architecture and Algorithms for an IEEE 802 Architecture and Algorithms for an IEEE 802.11-based Multi-channel Wireless Mesh Network Ashish Raniwala In collaboration with Prof Tzi-cker Chiueh Experimental Computer Systems Lab Stony Brook University

Today’s Enterprise Wireless Networks Motivation Today’s Enterprise Wireless Networks Examples: Bell Labs Holmdel, SUNY campus, Hilton IEEE 802.11-based Wireless Access Wired Backbone (deployment, management) Wired Backbone

Potential Solution: Wireless Mesh Network Motivation Potential Solution: Wireless Mesh Network Multi-hop ad hoc network of wireless routers Based on “Mobile Ad hoc Network” Architecture Single-channel => Link interference => Low capacity Wireless Backbone

Hyacinth Goals Motivation High-capacity Wireless Mesh => Multiple channels Off-the-shelf hardware => No MAC modifications Multi-channel Wireless Backbone

Hyacinth Architecture Research Issues Outline Motivation Hyacinth Architecture Research Issues Load-balancing Routing Traffic-aware Channel Assignment Throughput and Latency Gains Hyacinth Prototype Conclusions Other Mesh Projects

Interconnection Network ? Network Model Internet NFS ERP Enterprise Resources Gateways Interconnection Network ? Access Net

Hyacinth Architecture Internet NFS ERP Enterprise Resources Gateways Wireless Backbone Access Net

Hyacinth Architecture Internet NFS ERP Enterprise Resources Gateways Wireless Backbone Access Net

Hyacinth Architecture Wired Network

Hyacinth Architecture Wired Network 4 3 3 1 2 5 3 1 2 4 5 Virtual link operating on Channel 2 Mesh router operating on Channel 1 and Channel 3

Connectivity Optimal Capacity Research Issues Interface Channel Assignment Channel assignment => Bandwidth of virtual links Connectivity vs. radio spectrum utilization efficiency Workload awareness Connectivity Optimal Capacity Packet Routing Routing => Traffic load on virtual links and gateways Network-wide load balance Interaction between routing and channel assignment Goal: Maximize network cross-section goodput

Load-Balancing Routing: Problem 30 20 40 10 30 10 40 50 Ingress/Egress Traffic 20 For each mesh node, find multi-hop path(s) to the wire such that (1) load on the gateway nodes is balanced (2) load on the intermediate nodes is also balanced

Load-Balancing Routing: Solution 802.1D-like Gateway Discovery Protocol Each node joins one (or more) gateways Protocol: ADVERTISE/JOIN Parent-child relationship between nodes Structure: Forest of trees rooted at gateway nodes. Cache extra advertisements for failure recovery Metrics Hop-count + stable because mostly static - load-imbalance Gateway residual capacity + load balanced, adapts to traffic - route flaps because dynamic Path residual capacity + handles non-gateway bottlenecks (1) (2) (3)

Traffic-Aware Channel Assignment: Problem 60 110 70 30 40 40 30 40 30 20 For each mesh node interface, assign channels such that the resulting capacity of virtual links matches their loads

Traffic-Aware Channel Assignment: Solution Workload-Awareness Why ? Need to distribute load uniformly across channels. How ? 1. Periodically construct a neighborhood channel-usage map 2. Re-assign channels to balance traffic load across channels 3. Coordinate with direct neighbors 10 40 30 Channel load imbalance 10 40 30 10 40 30 Channel load balanced

Traffic-Aware Channel Assignment: Solution Channel Dependency Issue Each node has a limited number of interfaces. Hence each interface is used to communicate with multiple neighbors. Control Channel - Physical: Extra NIC on dedicated channel - Virtual: Multi-hop connectivity to neighbors C E SOLUTION D B A Channel Load Metrics - Contention group size Aggregated channel usage Weighted sum of the two

Centralized Channel Assignment/Routing Channel Assignment is NP-hard Reduction from “Multiple subset sum” problem Greedy Channel Assignment Visit edges in order of “expected load” Greedily assign locally optimal channel Maintain previous channel assignments as constraints Centralized Routing Single-path iterative routing on residual graph Randomized multi-path load-balancing routing Overall Algorithm Start with single-channel routing for initial load estimation Iterate over channel assignment and routing until convergence

Performance Evaluation: Throughput Gains Simulation Setup 60 nodes with 4 gateway nodes 2 NICs/node, 12 channels 30 random flows to wired net Cross-section goodput X Results Baseline: Single-channel net Single-NIC Multi-channel: Marginal improvements Identical CA: 2x improvement Centralized CA: 6-7x gains Distributed CA: 6-7x gains

Performance Evaluation: Latency Reductions Simulation Setup 64 nodes with 4 gateway nodes 2 NICs/node, 12 channels HTTP traffic requests/response Traffic intensity:0, X, 2X, 3X, 4X Results Reduced average delay Saturation point: 4x users with multi-channel networking

Some Implementation Issues.. Interference Range Neighbor Discovery - Brute-force method - HELLO @ lowest encoding Inter-channel Interference - Antenna separation - Channel separation Coordinating Channel (and Route) Changes - Preventing (N1,C1)(N2,C1)  (N1,C2)(N2,C1) | (N1,C1)(N2,C2) - Exchange (Channel C1 Channel C2, Time T) Antenna orientations - Neighborhood information per antenna Channel Quality Estimation - Link errors, Channel encoding

Prototype Evaluation Configuration – 9 Win XP desktops, 2 gateway nodes Two 802.11a NICs / node User-level route/channel assignment daemon FTP Throughput – 5-times improvement in multi-channel mode Should be higher for larger testbed

IEEE 802.11 beyond AP—mobile communication Conclusions.. IEEE 802.11 beyond AP—mobile communication Multi-channel wireless mesh backbone Multiple commodity cards per node Workload-aware channel assignment Load-balancing routing Many-fold improvement with small increase in price Research problems to work on – (1) “Optimal” centralized load-balancing routing ? (2) Distributed channel assignment for general wireless mesh ? (3) Capacity of multi-radio wireless mesh networks ? (4) Applications to IEEE 802.16a mesh networks ? Project site: http://www.ecsl.cs.sunysb.edu/multichannel

Other Related Mesh Networking Projects.. Mesh Transport Protocol: that achieves much better and robust performance than TCP over multi-hop wireless networks Station-Transparent Mobility Management: that supports end-user mobility across a WMN without any software pre-installed on the stations Secure Routing Protocol: to protect a WMN from compromised routers. Directional Antenna Protocols: to reduce long-term interference and achieve better spatial reuse of channels. Miniaturized Mobile Wireless Network Testbed: to provide a manageable, reconfigurable, controllable multi-hop wireless experimentation platform

Capacity Issue in Single-channel Mesh Network Capacity Issues 802.11: MAC contention, PLCP header, ACK, bit errors Ad-hoc: Single-channel across the network => Inter-path and Intra-path interference Increasing Capacity Frequency: Multiple channels Spatial: Directional antennas, Transmit power control

NP-hardness Proof