Enabling Large Scale Wireless Broadband: The Case for TAPs

1. 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

2. 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

3. WiFi Hot Spots? • 11 Mb/sec, free spectrum, inexpensive APs/NICs • 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 Carrier’s Backbone/Internet T1 Medium bandwidth (wire), sparse, and expensive

4. High availability, but slow and expensive 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

5. 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

6. 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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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