Houston TIC

Technology For All Wireless:Deployment, Measurements, and New Applications: 

Technology For All Wireless: Deployment, Measurements, and New Applications Ed Knightly Rice University http://www.ece.rice.edu/~knightly

The Digital Divide Challenge: 

The Digital Divide Challenge Southeast Houston 37% of children below poverty 56% have < $25,000/year household income Goal: pervasive wireless and transformational applications

Technology For All/Rice Mesh Deployment: 

Technology For All/Rice Mesh Deployment “Empower low income communities through technology” Pilot neighborhood: Houston’s East End (Pecan Park) Status: approximately 3 km2 of coverage and 1,000 users Operational since late 2004 Applications Internet access, education, work-at-home, health care

Outline: 

Outline Digital divide objectives Network architecture and platform Network planning, deployment, and measurements New applications and future work Challenges for Houston

Two-Tier Mesh Architecture: 

Two-Tier Mesh Architecture Limited gateway nodes wired to Internet Mesh nodes wirelessly forward data Backhaul tier - mesh node to mesh node Access tier - mesh node to client node

Design Objectives/Constraints: 

Design Objectives/Constraints Single wireline gateway (burstable to 100 Mb/sec) $15k per square km ($100k typical for mesh) 99% coverage for entire neighborhood contrasts with single-tier “community nets” 1 Mb/sec minimum access rate Programmable platform for protocol design and measurement

Commercial Technologies: 

Commercial Technologies No programmability as required for research Wide range of cost and performance Source: Network World

Backhaul/Mesh Node Hardware: 

Backhaul/Mesh Node Hardware 200 mW 802.11b LocustWorld Mesh SW VIA C3 1Ghz 5 GB Hard Drives 4 GB Flash to run Linux HostAP driver 15 dBi Omni-directional Antenna Programmable, single-radio mesh node with storage

Mesh Antennas: 

Mesh Antennas Access and Backhaul links High-gain 15 dBi omni-directional antenna at 10 meters Serves access and backhaul Attenuation primarily due to tree canopy Long distance links Directional antennas as wire replacement

Access Node Hardware: 

Access Node Hardware Ethernet Bridge Access inside homes is limited Users must understand this is not like cellular Expect to need a bridge, repeater, or directional or high-gain antenna near a window ($20 to $100 price) USB WiFi Directional antenna

Outline: 

Outline Digital divide objectives Network architecture and platform Network planning, deployment, and measurements New applications and future work Challenges for Houston

Network Planning Issues: 

Network Planning Issues Density of mesh nodes If large inter-node spacing… reduces # nodes (costs) per square km yet, results in coverage gaps and, long distance links reduce throughput Number and placement of wires If few wired gateways… reduces costly wireline access and deployment fees yet, throughput decreases with the number of wireless hops What is the price-performance tradeoff?

Background in RF Propagation: 

Background in RF Propagation Pathloss Average or large-scale signal attenuation Exponential decay (pathloss exponent, ) Typically 2 to 5 in urban environments Shadowing Variation between points with similar pathloss Typically 8 dB in urban environments

Translation: 

Translation Links get much slower (and eventually break) as distance increases The key parameter is the “path loss exponent” A particular environment is stuck with its exponent (can’t change physics) Typical range: near 2 for near line-of-sight to 5 for numerous obstructions Shadowing: expect variations, even at one distance

Access Links: Throughput: 

Access Links: Throughput Shannon Capacity Note: 1 Mbps at -86 dBm Target throughput for access links DSL and Cable Speed Manufacturer specification severely optimistic target Manufacturer specification

Access Links: Pathloss: 

Access Links: Pathloss 150-200 meters Mesh-client distance For 1 Mbps/ -86 dBm deployment Pathloss  = 3.7 Urban pathloss 2 to 5 [Rappaport] Dense trees Wooden framed homes Shadowing = 4.1 Given the path loss exponent and the node profiles, the distance-throughput tradeoff is revealed

Backhaul Link Experiments: 

Backhaul Link Experiments Experiments yielded lower path-loss exponent of 3.3 Due to both antennas being at 10 meters and high-gain Permissible node spacing 200m to 250m for 3 Mb/sec links

Single Hop Measurement Findings: 

Single Hop Measurement Findings Accurate baseline physical measurements critical for effective deployment (measured  = 3.3, models suggest 2 to 5) 2 yields completely disconnected network 3.5 yields overprovision factor of 55% 4 yields overprovision factor of 330% 5 yields 9 times overprovisioning Accurate throughput-signal-strength function critical manufacturer values over-estimate link range by 3 times yielding disconnected network Requires small number of measurements 15 random measurements = std. dev. 3% about average 50 random measurements = std. dev. 1.5% about average

Multihop Experiments : 

Multihop Experiments Issue: How does the number of wireless hops affect performance? The answer controls the required number of wired gateways Ideally, throughput is independent of spatial location

Bad News: 

Bad News Scenario: large file uploads via FTP/TCP Nodes farther away nearly starve contend more times for more resources encounter asymmetric disadvantages in contention

Starvation Solution I: Rate Limiting: 

Starvation Solution I: Rate Limiting Need to throttle dominating flows Statically (as in current deployment) or dynamically according to congestion (via IEEE 802.11s)

Starvation Solution II: Exploit Statistical Multiplexing: 

Starvation Solution II: Exploit Statistical Multiplexing Bursty traffic yields gaps in demand on-off vs. greedy alleviates spatial bias Can support approximately 30 web browsers per mesh node with minimal spatial bias

Multihop Measurement Findings: 

Multihop Measurement Findings Imperative to consider multiple multi-hop flows Cannot “extrapolate” from link measurements as in wired nets Starvation in fully backlogged upload Without additional mechanisms, severe problem with p2p-like traffic Proper rate limiting of flows alleviates starvation Static or dynamic Web traffic and provisioning allows statistical multiplexing to alleviate starvation Even without rate limiting

Healthcare Applications: 

Healthcare Applications Pervasive health monitoring with body-worn health sensors Health information delivery through body-worn user interfaces Initial focus on obesity management and cardiovascular diseases Collaboration with health researchers Baylor College of Medicine Methodist Hospital UT Health Science Center at Houston User and field studies in Houston neighborhood with TFA wireless coverage

Current Prototype (Lin Zhong): 

Current Prototype (Lin Zhong) Left: Bluetooth wearable sensors for mobile system to connect health information: debugging and mini versions Right: Wrist-worn Bluetooth display for mobile system to deliver health promoting messages

Challenges for Houston: 

Challenges for Houston Tempered expectations, especially indoors Avoid Tempe-style complaints Heterogeneous propagation and usage environments Downtown vs. treed urban vs. sparce Evolvable architecture 802.11s will standardize, 802.16 will mature, MIMO will advance (802.11n), we will learn, etc. Balancing cost ($$/km2) and performance (Mb/sec/km2, %-coverage) Lowest cost solution may sacrifice throughput and coverage Incorporating cost and performance implications of the number of wired gateway nodes Innovative applications beyond “access”

Conclusions: 

Conclusions Multi-hop wireless technology is cutting edge Most experience is not in public access Deployment and operational challenges ahead Opportunities for innovative applications More information TFA website http://www.techforall.org Rice website http://www.ece.rice.edu/networks