Architecture and Algorithms for an IEEE 802.11-based Multi-channel Wireless Mesh Network Ashish Raniwala In collaboration with Prof Tzi-cker Chiueh Experimental.

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

110

4030

30

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