enabling large scale wireless broadband: the case for taps
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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. The Killer App is the Service. High bandwidth High availability Large-scale deployment - PowerPoint PPT PresentationTRANSCRIPT
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