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Enabling Large Scale Wireless Broadband: The Case for TAPs Roger Karrer, Ashu Sabharwal and Ed...
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Transcript of Enabling Large Scale Wireless Broadband: The Case for TAPs Roger Karrer, Ashu Sabharwal and Ed...
![Page 1: Enabling Large Scale Wireless Broadband: The Case for TAPs Roger Karrer, Ashu Sabharwal and Ed Knightly ECE Department Rice University Joint project with.](https://reader036.fdocuments.net/reader036/viewer/2022062516/56649da05503460f94a8b72f/html5/thumbnails/1.jpg)
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
![Page 2: Enabling Large Scale Wireless Broadband: The Case for TAPs Roger Karrer, Ashu Sabharwal and Ed Knightly ECE Department Rice University Joint project with.](https://reader036.fdocuments.net/reader036/viewer/2022062516/56649da05503460f94a8b72f/html5/thumbnails/2.jpg)
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
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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
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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
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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
![Page 6: Enabling Large Scale Wireless Broadband: The Case for TAPs Roger Karrer, Ashu Sabharwal and Ed Knightly ECE Department Rice University Joint project with.](https://reader036.fdocuments.net/reader036/viewer/2022062516/56649da05503460f94a8b72f/html5/thumbnails/6.jpg)
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
![Page 7: Enabling Large Scale Wireless Broadband: The Case for TAPs Roger Karrer, Ashu Sabharwal and Ed Knightly ECE Department Rice University Joint project with.](https://reader036.fdocuments.net/reader036/viewer/2022062516/56649da05503460f94a8b72f/html5/thumbnails/7.jpg)
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
![Page 8: Enabling Large Scale Wireless Broadband: The Case for TAPs Roger Karrer, Ashu Sabharwal and Ed Knightly ECE Department Rice University Joint project with.](https://reader036.fdocuments.net/reader036/viewer/2022062516/56649da05503460f94a8b72f/html5/thumbnails/8.jpg)
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
![Page 9: Enabling Large Scale Wireless Broadband: The Case for TAPs Roger Karrer, Ashu Sabharwal and Ed Knightly ECE Department Rice University Joint project with.](https://reader036.fdocuments.net/reader036/viewer/2022062516/56649da05503460f94a8b72f/html5/thumbnails/9.jpg)
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
![Page 10: Enabling Large Scale Wireless Broadband: The Case for TAPs Roger Karrer, Ashu Sabharwal and Ed Knightly ECE Department Rice University Joint project with.](https://reader036.fdocuments.net/reader036/viewer/2022062516/56649da05503460f94a8b72f/html5/thumbnails/10.jpg)
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
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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
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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
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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
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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
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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
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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