Low-cost 802.11 Wireless Infrastructure Networks

1
Low-cost 802.11 Wireless Infrastructure
Networks

• Stefan Savage and John Bellardo
• Department of Computer Science and Engineering
• University of California, San Diego

2
Motivation

• Large-scale 802.11 deployments are expensive
• Capital expenditures typically lt 35 (and
  hardware is on commodity price curve)
• Operational expenditures
• Site-survey
• Test and tuning
• Network wiring and provisioning
• Ongoing management (software update, rebalance,
  etc)
• Our goal make it cheap and trivial to provide
  building or campus-wide 802.11 APs (OpEx -gt 0)

3
Assumptions

• Radio hardware is cheap
• Multiple independent radios in a package is
  reasonable
• Antenna technology is not
• Omni antennas (low gain/directional separation)
• Intra/Internet access usage model
• Not point to point
• Largely homogenous administrative domain
• Not dealing with apartment building problem
  (initially)
• Indoor focus
• 3D, dense deployment, complex RF domain,
  significant spatial and temporal load shifts
• Must not require 802.11 client modifications
• Ok as optimization

4
Aside Why use 802.11?

• Bad experience with simulation
• Our wireless immigration project (USENIX Sec 03)
• Send CTS with large duration to freeze channel
  (devestating in simulation, then we built it)
• Have tried three wireless simulators (including
  ) cant find any that predict our
  measurements
• Multi-path, fading, variable noise, people (i.e.
  moving bags of water)
• Variable xmit pwr, receive sensitivity, power
  spectrum, MAC behavior on client NICs
• We want experience with real traffic driven by
  real users, hence we need to build real systems
• Not equipped/funded to build a lot of radios
• Although we do have some (CalRadio at end of
  talk)

5
Two elements of our work

• Radio Tomography and Frequency Management(RTFM)
• Measurement-based inference of RF domain capacity
  and contention
• Auto AP configuration to maximize system goodput
• Frequency, transmit power, CCA, coding
• Goal no site survey, no tuning, no manual
  configuration
• LessWire
• Simplified multi-hop routing (3 hop max)
• Best-exit routing wrt RF domain impact
• Goal opportunistic use of wiring, expand
  coverage/density

6
Radio Tomography

• Key questions
• If I send pkt x at rate r with power t on channel
  z, what is the distribution of delivery delay
  times?
• Why?
• Background interference
• Co-channel interference
• Clientlt-gtAP propagation (fading, multipath, etc)

7
Radio Tomography first try

• Naïve approach
• Model nodes as point transmitters with set xmit
  range r and channel z
• If two spheres overlap, delay is proportional to
  the sum over load
• Re-color, re-size to minimize delay
• Why this doesnt even vaguely work
• Non-uniform propagation
• Channel not exclusive
• Coding matters
• Channel conditions and clients change

8
Radio Tomography 2nd 3rd attempts

• Next idea
• Observe visible MACs and share among APs
• If two nodes share the same node then assumethey
  interfere
• Problems
• Incredibly conservative (ignores attenuation and
  RF capture)
• Next idea
• Measure RSSI and infer impact on xput
• Fine grained ground-truth measurements(sample
  over every 3x3 feet by hand)
• Problems
• RSSI is very very noisy and highly variable
• Hard to infer ground truth from few samples
• Very poor predictor of pkt delivery
• Happy to learn about any non-brittle modelshere
  that work

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Radio Tomography current approach

• Synchronized Co-Channel Interference Inference
• Idea create interference and see impact
  (analogies to slow start)
• APs send short burst on channel x at time t and
  rate r
• Other APs measure change in re-transmission
  probability and back-to-back xmit timing at same
  time (CCA indication)
• Infer same from client based on retry bit in
  header CRC failures
• Findings
• Rate sensitivity
• Both for data (makes sense)and interferer
  (unsure)
• Discontinuities (fastest rates -gt practically
  slower)
• Strong bimodality
• Good at characterizing interference
• 85 for sub-second samples
• Gotchas low S/N

10
RF Management

• RF Parameter optimization (work in progress)
• Minimize xmit power to maximally split offered
  load across APs
• Color frequency and set CCA to minimize
  interference effects
• Research questions
• NP-hard, Heuristic challenge ordering of
  power/frequency opt
• How often to re-optimize?
• Dont want to react to short workload dynamics
  (ftp transfer) or RF dynamics (jiffy pop
  time-scale)
• Client delay on reassociation
• Some NICs very bad
• Our APs support fast handoff (SyncScan, UCSD-TR)
  but requires client mods to take advantage
• Want to react quickly to AP failure
• Centralized vs distributed control?
• Impact if some nodes dont play?(e.g. static
  frequency inholding)

11
LessWire

• Idea use additional radios to provide multi-hop
  backhaul
• Research challenges
• Point-to-multipoint route optimization over RF
  domain (not ad hoc routing)
• Interaction with RF management
• Backhaul-only channels vs joint assignment
• Optimize freq/power assignment over opportunity
  cost of a route
• Simplicities from being short hop (2-3 hops max)
  network (very low state)

12
Where we are

• RTFM prototype limping along at UCSD
• Interference inference
• Background channel quality
• Co-channel impact on predicted delay on given
  frequency
• Extrapolate rate impact based on empirical curves
  
• Greedy channel assignment based on static
  threshold
• Lots of work left (CCA validation, TX power,
  better assignment, more features to classifier)
• LessWire
• In algorithmic stage no results to report today
• We are assuming that wired bandwidth is infinite

13
UCSD CSE Infrastructure

• 266Mhz Soekris w/40GB trace store
• Dual-radio Atheros 5212 miniPCIs
• Driver hacks for CCA adjust, per-packet TPCrate
  control
• Global time synced packet scheduling
• 5Ghz deployment on two floors-12APs(soon 2 more
  indoor/outdoor-40APs)

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UCSD Infrastructure CalRadio I

• Joint project of UCSD CalIT2, ECE and CSE
• Intersil baseband, 2.4Ghz RF, DSP-based MAC (TI
  C5471/ARM7, Symbol derived IP, about 4x4)
• Designed to allow L2 experimentation/innovation

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UCSD Infrastructure CalRadio II

• More aggressive physical layer innovation
• Several RF modules being constructed (2.4, 5Ghz
  WiFi, 2x2 MIMO 900Mhz, 3-10Ghz UWB)
• Modulation all in FPGA, Matlab/Simulink
  compatible