A Location-Based Management System for Enterprise Wireless LANs

1
A Location-Based Management System for Enterprise
Wireless LANs

• Ranveer Chandra, Jitendra Padhye, Alec Wolman and
  Brian Zill
• Microsoft Research

2
Wireless Network Woes

• Corporations spend lots of on WLAN
  infrastructure
• Worldwide enterprise WLAN business expected to
  grow from 1.1 billion this year to 3.5 billion
  in 2009
• Wireless networks perceived to be flaky, less
  secure than wired networks
• Users complain about
• Lack of coverage, performance, reliability
• Authentication problems (802.1X protocol issues)
• Network administrators worry about
• Providing adequate coverage, performance
• Security and unauthorized access

Better WLAN management system needed!
3
Typical Questions Asked by Network Administrators

• Are all areas of the building covered?
• Are there areas in the building where clients
  repeatedly switch between APs?
• Are there locations with very high loss rates?
• Where do most of the clients use the wireless
  network from?
• Conference rooms? Offices?

Many problems are location-specific
4
Two Key Requirements for WLAN Management Systems

• Integrated, accurate location system
• Dense array of sensors
• Complex, time-varying signal propagation indoor
  environments
• Many channels need to be monitored

5
State of the Art

• AP-based monitoring Aruba, AirDefense
• Pros Easy to deploy (APs are under central
  control)
• Cons
• Can not detect coverage problems using AP-based
  monitoring
• Single radio APs can not be effective monitors
• Specialized sensor boxes Jigsaw, WIT SIGCOMM
  06
• Pros Can provide detailed analysis
• Cons Expensive, not scalable
• Monitoring by mobile clients ClientConduit -
  Mobicom 04
• Pros Inexpensive, suitable for un-managed
  environments
• Cons Unpredictable coverage, client locations
  not known, battery power may become an issue

6
Observations

• Desktop PCs with good wired connectivity are
  ubiquitous in enterprises
• Outfitting a desktop PC with 802.11 wireless NIC
  is inexpensive
• Wireless USB dongles are cheap
• As low as 6.99 at online retailers
• PC motherboards are starting to appear with
  802.11 radios built-in

Combine to create a dense deployment of wireless
sensors DAIR Dense Array of Inexpensive Radios
Details HotNets05, MobiSys06
7
DAIR Architecture
AirMonitor
Summarized Data
Commands
Wired Network
Commands and Database Queries
Data from database
Data to inference engine
Summarized data from Monitors
Other data AP locations, Floor Map, AP BSSIDs
Inference Engine
Database
8
Advantages of DAIR Architecture

• Dense deployment of sensors
• Without excessive cost
• Robustness Can tolerate loss of a few sensors
• Can use very simple algorithms for analysis
• Stationary sensors
• Help build simple, yet accurate location system
• Permit historical analysis

9
Testbed
98 meters x 32 meters 150 offices and conference
rooms. Typical office size 3 meters x 3
meters Full-height walls. Solid wood doors 59
AirMonitors.
10
Example Application

• Estimate transmission rate obtained by clients at
  various locations on the floor
• Study impact of distance between AP and client on
  transmission rate
• Useful for detecting areas of poor coverage
• Design questions
• Which channels should the AirMonitors listen on?
• What information should each AirMonitor record,
  and how to analyze the information?
• How to locate clients?

11

• Which channels should the AirMonitors listen on?
• What information should each AirMonitor record,
  and how to analyze the information?
• How to locate clients?

12
Channel Assignment

• Six APs (Aruba)
• Known, fixed locations
• Known, fixed BSSIDs
• But not fixed channels
• APs change channels (roughly once or twice a day)
• Dynamic channel assignment by Arubas centralized
  controller
• Cant assign AirMonitors to listen on fixed
  channels

13
AP Tracking

• AirMonitors track AP nearest to them
• Start by scanning all channels
• Once AP is found, stay on that channel
• If no beacons are heard in 10 seconds, scan again
• Why nearest AP?
• Most of the traffic near an AP is likely to be on
  the channel that the AP is on
• Other schemes possible
• Strongest signal
• Scanning

14
Testbed Map with AP Assignment
15

• Which channels should the AirMonitors listen on?
• What information should each AirMonitor record,
  and how to analyze the information?
• How to locate clients?

16
Information Gathering

• Reporting every packet to database not scalable.
• Jigsaw and WIT SIGCOMM 06
• Can overwhelm wired network and database.
• Each AirMonitor submits summary information
• Aggregate packets for each ltsender, receivergt
  pair
• For each pair record aggregate statistics
• Average signal strength, total number of packets
  and bytes
• Submission intervals randomized to avoid load
  spikes
• 30-60 seconds.

17
Advantages and Disadvantages of Aggregation

• Advantage
• Scalability lt 10Kbps traffic per AirMonitor
• Disadvantage
• Cant perform packet-level analysis like
  Jigsaw/WIT
• Difficult to combine observations from multiple
  AirMonitors
• Problem solved to some degree by density of
  sensors

18
Collecting Transmission Rate Data
Sndr Rcvr Rate History
AP C (6, 300)
AM1
AP
1000 bytes
300 bytes
Client
1000 bytes
1000 bytes
AM2
AM3
Sndr Rcvr Rate History
C AP

Sndr Rcvr Rate History
AP C (54, 1000)
(54, 1000)
(54, 2000)
(6, 300)
AP
(6, 300)
C
19
Correlating the Data
AirMonitor Sender Receiver Rate History
AM1 Client AP (54, 2000)
AM1 AP Client (6, 300)
AM2 AP Client (54, 1000) (6, 300)
AM3 AP Client (6, 300)

• Each AirMonitor has an incomplete view of the
  reality
• Simple technique
• For each direction (uplink or downlink), use data
  from AirMonitor that heard the most packets

20
Advantages and Disadvantages

• Advantages
• Scalable
• Requires only coarse-grained time synchronization
• Accuracy improves with density of sensors
• Disadvantages
• Accuracy degrades at lower density
• Does not permit packet-level analysis

21

• Which channels should the AirMonitors listen on?
• What information should each AirMonitor record,
  and how to analyze the information?
• How to locate clients?

22
Self-Configuring Location Service

• Distinguishing features
• Heuristics to automatically determine AirMonitor
  locations
• Automatic profiling of environment
• Can locate any Wi-Fi transmitter (including
  uncooperative ones)
• Office-level accuracy
• How it works
• AirMonitors locate themselves
• AirMonitors regularly profile the environment to
  determine radio propagation characteristics
• Inference engine uses profiles and observations
  from multiple AirMonitors to locate clients

23
How do AirMonitors Locate Themselves?

• Monitor machine activity to determine primary
  user
• Look up ActiveDirectory to determine office
  number
• Parse office map to determine coordinates of the
  office
• Assume AirMonitor to be located at the center of
  the office
• Verify and adjust coordinates by observing which
  AirMonitors are nearby

May not be available in all environments
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Profiling the Environment
AM3
AM2
AM1
From To Signal Strength




AM1
AM2
60
AM2
55
AM1
AM2
AM3
33
Inference Engine
Database
AM3
AM2
39
25
Profiling the Environment
60
y -1.4 x 35.7
50
y 60e-0.11x
40
Normalized Signal Strength
30
20
10
0
0
10
20
30
40
Distance
Profile is used to calculate expected signal
strength
26
Locating a Client
Observed 35
Observed RSSI 50
Distance 7.2, Expected RSSI 27
Distance 6, Expected RSSI 31
Distance 3, Expected RSSI 43
Distance 1.3, Expected RSSI 52
?
Observed 52
Observed 35
Distance 0, Expected RSSI 60
Distance 6.5, Expected RSSI 31
Distance 6.2, Expected RSSI 30
Distance 1.1, Expected RSSI 53
Adjust location to minimize error
27
Two Simpler Algorithms that Do Not Require
Profiling

• StrongestAM
• Client Location estimated as the location of
  AirMonitor that heard the strongest signal
• Can be used if there is one AirMonitor in every
  office
• Centroid
• Find AirMonitor that heard the strongest signal
• Find all AirMonitors that heard signal within 85
  of strongest signal strength
• Client location estimated as the centroid of this
  group
• Works well for our deployment

28
Accuracy of Location Estimation
21 locations, laptop client connected to
corporate network, 802.11b/g
29

• Which channels should the AirMonitors listen on?
• What information should each AirMonitor record,
  and how to analyze the information?
• How to locate clients?

Example application Study Impact of client/AP
distance on transmission rate
30
Bug!

• Downlink transmission rate was always 5.5Mbps
  regardless of client location
• Notified IT department
• Problem resolved after AP firmware was upgraded

31
Impact of Distance on Transmission Rate
Downlink
Uplink
Byte-averaged transmission rate (Mbps)
Byte-averaged transmission rate (Mbps)
Oct 2-6, 2006, 15 minute intervals 802.11g clients
32
Impact of distance on Loss Rate
Downlink
Uplink
Downlink loss rates substantially higher than
uplink loss rates
33
Area of Poor Coverage

• Median downlink frame loss rates 50
• Clients rapidly switch between 5 APs

34
System Scalability

• Additional load on desktops lt 2-3
• Wired network traffic per AirMonitor lt 10Kbps

35
How many AirMonitors are needed?

• Depends on environmental factors, AP placement
  etc.
• In our environment
• With 59 AirMonitors
• Median packet loss is 1.85
• Max packet loss is 7
• Results degraded significantly with less than 44
  AirMoniors

36
Conclusion

• Effective Wi-Fi monitoring systems need
• Integrated location service
• Dense deployment of Wi-Fi sensors
• DAIR architecture creates dense deployment of
  Wi-Fi sensors without excessive cost
• Built a practical Wi-Fi monitoring system using
  DAIR

37
Questions?
38
Backup slides
39
Monitor Architecture
40
Association vs. Distance

• Majority of the clients do not connect to the
  nearest AP
• Median distance between client and AP is 15 meters

41
Requirements for a WLAN Management System
Integrated location service
Mobile Clients Problems may be location-specific
Multiple monitors Dense deployment
Complex signal propagation in indoor
environment Many orthogonal channels Asymmetric
links
Scalable Self-configuring
Cope with incomplete data
42
Other analysis

• Correlation between loss rate and distance
• Calculating loss rate is complicated
• Requires each AirMonitor to perform address
  matching, as ACKs do not contain senders
  address
• Estimating downlink loss rate is especially
  challenging, since each AP talks to multiple
  clients
• Detection of RF holes
• Locations from where clients repeatedly sends
  probe requests, but get no probe response from
  corporate APs
• AP flapping
• Clients repeatedly switch between several APs
• Usually because they get poor service from all of
  them
• Indicative of bad AP placement

43
Sample results

• One week of data (October 2006)
• Monday to Friday, 8am to 8pm
• 59 AirMonitors
• System is currently operational, and our IT
  department uses the data .

44
Frame Loss Rates Downlink
Median loss rate 43 when distance between client
and AP gt 20 meters. (20 when distance lt 20
meters)