Architecture and Algorithms for an IEEE 802.11-based
Multi-channel Wireless Mesh Network
Ashish RaniwalaIn collaboration with Prof Tzi-cker Chiueh
Experimental Computer Systems Lab
Stony Brook University
Wired Backbone
Motivation
Today’s Enterprise Wireless Networks• Examples: Bell Labs Holmdel, SUNY campus, Hilton• IEEE 802.11-based Wireless Access• Wired Backbone (deployment, management)
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
Motivation
Hyacinth Goals• High-capacity Wireless Mesh => Multiple channels• Off-the-shelf hardware => No MAC modifications
Multi-channelWireless Backbone
• Motivation• Hyacinth Architecture• Research Issues
– Load-balancing Routing– Traffic-aware Channel Assignment
• Throughput and Latency Gains• Hyacinth Prototype• Conclusions• Other Mesh Projects
Outline
Access Net
InternetNFSERP
Enterprise Resources
Network Model
Interconnection Network ?
Gateways
Access Net
InternetNFSERP
Enterprise Resources
Hyacinth Architecture
Gateways
Wireless Backbone
Access Net
InternetNFSERP
Enterprise Resources
Hyacinth Architecture
Gateways
Wireless Backbone
Hyacinth Architecture
Wired Network
Hyacinth Architecture
5
5
4
3
1
1
3
3 3
12
2
2
2
4
4
Wired Network
Virtual linkoperating onChannel 2Mesh router
operating onChannel 1 andChannel 3
Research Issues
• Interface Channel Assignment• Channel assignment => Bandwidth of virtual links
• Connectivity vs. radio spectrum utilization efficiency
• Workload awareness
Connectivity Optimal Capacity
Goal: Maximize network cross-section goodput
• Packet Routing• Routing => Traffic load on virtual links and gateways • Network-wide load balance • Interaction between routing and channel assignment
Load-Balancing Routing: Problem
40 3050
20 30
10
20
40
10
Ingress/Egress Traffic
• 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
• Metrics1. Hop-count
+ stable because mostly static
- load-imbalance
2. Gateway residual capacity
+ load balanced, adapts to traffic
- route flaps because dynamic
3. Path residual capacity
+ handles non-gateway bottlenecks
(1)
(2)
(3)
Traffic-Aware Channel Assignment: Problem
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4030
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20
60 70
40 40 30
• For each mesh node interface, assign channels such thatthe 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
10 40 30 Channel loadimbalance
10 40 30Channel loadbalanced
• 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
Traffic-Aware Channel Assignment: Solution
Channel Load Metrics- Contention group size- Aggregated channel usage- Weighted sum of the two
SOLUTION
A
B
CD
E
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 Setup60 nodes with 4 gateway nodes2 NICs/node, 12 channels30 random flows to wired netCross-section goodput X
ResultsBaseline: Single-channel netSingle-NIC Multi-channel: Marginal improvementsIdentical CA: 2x improvementCentralized CA: 6-7x gainsDistributed CA: 6-7x gains
Performance Evaluation: Latency Reductions
Simulation Setup64 nodes with 4 gateway nodes2 NICs/node, 12 channelsHTTP traffic requests/responseTraffic intensity:0, X, 2X, 3X, 4X
ResultsReduced average delaySaturation 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• 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 ?
Conclusions..
Project site: http://www.ecsl.cs.sunysb.edu/multichannel
• 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
Other Related Mesh Networking Projects..
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
Capacity Issue in Single-channel Mesh Network
NP-hardness Proof
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