Enabling Large Scale Wireless Broadband: The Case for TAPs

Roger Karrer, Ashu Sabharwal and Ed Knightly

ECE Department

Rice University

Joint project with

B. Aazhang, D. Johnson and J. P. Frantz

Ashu Sabharwal

The Killer App is the Service

High bandwidth High availability

– Large-scale deployment– High reliability– Nomadicity

Economic viability

Why?– Broadband to the

home and public places

– Enable new applications

Ashu Sabharwal

WiFi Hot Spots?

Why? poor economics– High costs of wired infrastructure ($10k + $500/month)– Pricing: U.S. $3 for 15 minutes– Dismal coverage averaging 0.6 km2 per 50 metro areas projected by

2005

11 Mb/sec, free spectrum, inexpensive APs/NICs

Carrier’s Backbone/Internet

T1

Medium bandwidth (wire), sparse, and expensive

Ashu Sabharwal

3G/Cellular?

Cellular towers are indeed ubiquitous– Coverage, mobility, …

High bandwidth is elusive– Aggregate bandwidths in Mb/sec range, per-user

bandwidths in 100s Kbs/s– Expensive: spectral fees and high infrastructure costs

High availability, but slow and expensive

Ashu Sabharwal

Ad Hoc Networks?

Availability– Problems: intermediate nodes can move, power off, routes

break, packets are dropped, TCP collapses, … Low bandwidth

– Poor capacity scaling

“Free” but low availability and low bandwidth

Ashu Sabharwal

TAPs: Multihop Wireless Infrastructure

Transit Access Points (TAPs) are APs with – beam forming antennas – multiple air interfaces– enhanced MAC/scheduling/routing

protocols Form wireless backbone with limited wired

gateways

Ashu Sabharwal

Multihop Wireless Infrastructure

Transit Access Points (TAPs) are APs with – beam forming antennas – multiple air interfaces– enhanced MAC/scheduling/routing protocols

Form wireless backbone with limited wired gateways

High bandwidth – High spatial reuse – Capacity scaling from multiple antennas

High availability– Non- mobile infrastructure – Redundant paths

Good economics– Unlicensed spectrum, few wires, exploit WiFi components– Deployable on demand

Ashu Sabharwal

Challenge 1a: Multi-Destination Routing

Most data sources or sinks at a wire The wireless backbone is multi-hop

Routing protocols for any wire abstraction Two distinct time-scales

– MU-MU, MU-TAP channels : fast variations– TAP-TAP channels : slow variations

Ashu Sabharwal

Challenge 1b: Multi-Destination Scheduling

Scheduling– At what time-scales, routes are chosen ?– At fast time scales, which path is best now (channels,

contention, …) ?– Fast time-scale information hard to propagate

Protocols should be– Decentralized – Opportunistic

Ashu Sabharwal

Challenge 2: Distributed Traffic Control

Distributed resource management: how to throttle flows to their system-wide fair rate?– TCP cannot achieve it (too slow)– Throttle traffic “near-the-wire” to ensure fairness and high

spatial reuse– Incorporate channel conditions as well as traffic demands

Ashu Sabharwal

Challenge 3: Distributed Medium Access

Challenges– Traffic and system dynamics preclude scheduled cycles– Others’ channel states, priority, & backlog unknown– Multiple air interfaces

Opportunism due to channel variations Modulate aggressiveness according to overheard information

Ashu Sabharwal

Challenge 4: TAP-TAP Physical Layer

TAPs carry traffic from many TAPs Data rates much higher than TAP-MU

Use MIMO, with target spectral efficiencies ~ 20+ bits/s/Hz – 802.11g ~2.5 bits/s/Hz 8X faster – 802.11b ~0.5 bits/s/Hz 40X faster

Ashu Sabharwal

TAP-TAP PHY Architecture

Spatial diversity: 4-6 antennas at each TAP. More power : FCC limit 1 Watt (802.11x uses 100mW)

Very high throughputs possible– Upto 440 Mb/s in one 802.11 channel– Large range for rates 50-150 Mb/s

Major challenges– None of current codes/modulations suffice– Low-power low-cost hardware architectures

Ashu Sabharwal

Challenge 5: Capacity Achieving Protocol Design

Traditional view of network capacity assumes zero protocol overhead (no routing overhead, contention, PHY training etc.)

Protocols themselves require capacity A new holistic system view: “the network is the channel”

– Incorporate overhead in discovering/measuring the resource– Explore capacity limits under real-world protocols– Shows PHY overhead no different from protocol overhead

Ashu Sabharwal

Prototype and Testbed Deployment

FPGA implementation of enhanced opportunistic, beamforming, multi-channel, QoS MAC

Build prototypes and deploy on Rice campus and nearby neighborhoods

Measurement study from channel conditions to traffic patterns

Ashu Sabharwal

Summary

Transit Access Points– WiFi “footprint” is dismal– 3G too slow and too expensive– Removing wires is the key for economic viability

Challenges– Multi-hop wireless architectures– Distributed control– Scalable protocols – High speed PHY