Chapter 16 Site Survey Systems and
Devices
• Site Survey defined
• Site Survey tools
• Coverage analysis
Exam Essentials
• Define spectrum, coverage, and application analysis.
– Understand why both spectrum and coverage analysis are
considered mandatory and application analysis is usually
optional.
• Identify sources of WLAN interference.
– Describe all of the various devices that are potential sources of
interference in both the 2.4 GHz ISM and the 5 GHz UNII bands.
• Explain RF measurements.
– Be able to explain the procedure used while conducting
coverage analysis and the different types of RF measurements
recorded, including received signal strength and signal-to-noise
ratio.
Exam Essentials
• Understand AP placement and configuration.
– Explain how AP placement, power, and channel
settings are part of coverage analysis.
• Identify all site survey tools.
– Understand the difference between an outdoor and
indoor site survey, and identify all the necessary tools.
• Explain the three major types of coverage
analysis.
– Describe the differences between manual, assisted,
and predictive site surveys, and explain selforganizing WLAN technology.
Site Survey
• Technical Requirements
– After you have completed interview and
gathered documents
• Spectrum Analysis
• Coverage Analysis
– Placement of APs
• Capacity testing
– Optional, and depends on needs
Mandatory Spectrum Analysis
• Somewhat high cost
– Custom equipment
– Some options
• Spectrum analyzers
– Can recognize sources
– More than just 802.11 frequencies
Mandatory Spectrum Analysis
• Want background noise less than -85dB
– Noise will corrupt transmissions
• Sources
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Microwave ovens
2.4 GHz cordless phones, DSSS and FHSS
Fluorescent bulbs
2.4 GHz video cameras
Elevator motors
Cauterizing devices
Plasma cutters
Bluetooth radios
Nearby 802.11, 802.11b, 802.11g, or 802.11n (2.4 GHz) WLANs
Wireless Internet Service Providers ( WISPs)
Mandatory Spectrum Analysis
Mandatory Spectrum Analysis
• 5 Ghz sources
– 5 Ghz Cordless Phones
– Radar
– Perimeter sensors
– Digital satellite
– Nearby 5 GHz WLANs
– Outdoor wireless 5 GHz bridges
• DFS and TPC are designed to prevent
Radar interference
Mandatory Coverage Analysis
• Capacity and coverage expectations are
reached during interview
• Need RF measurements to guarantee that
needs are met
– Checking received signal strength
Coverage Process
Reduce AP power output
1. Place an access point with a power setting of 30 mW in
the corner of the building.
2. Walk diagonally away from the access point toward the
center of the building until the received signal drops to –
65 dBm. This is the location where you place your first
access point.
3. Temporarily mount the access point in the first location
and begin walking throughout the facility to find the –65
dBm end points, also known as cell boundaries or cell
edges.
4. Depending on the shape and size of the first coverage
cell, you may want to change the power settings and/or
move the initial access point. A good portion of a proper
coverage analysis involves starting over and trying
again.
Coverage Process
Coverage Process
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Think of the cell boundary of the first access point, where the
signal is –65 dBm, as the initial starting point. From the first
access point, walk parallel to the edge of the building, and place
an access point at the location where the received signal is –65
dBm.
Now walk away from this access point, parallel to the edge of the
building, until the received signal drops to –65 dBm.
This is the farthest point to place the access point if you do not
want cell overlap.
Using the distance from the previous access point and this
location, the placement of this next access point should be about
15 to 20 percent (depending on cell overlap requirements) closer
to the previous access point.
Move to that location and temporarily mount the access point.
Begin walking throughout the facility to find the –65 dBm end
points, or cell boundaries.6. Again, depending on the shape and
size of the first coverage cell, you may want to change the power
settings and/or move this access point.
Coverage Process
Coverage Process
• Avoid too much overlap because of possible roaming
issues.
• Placement will depend on building layout and materials
– they affect RF transmission and change the shape of cells
• Also, take measurements:
– Received signal strength (dBm), also known
as received signal level (RSL)
– Noise level (dBm)
– Signal-to-noise ratio, or SNR (dB)
Coverage Process
• Received Signal needs vary based on type of needs
– -85dB is good enough for “coverage”
– -65 is better for capacity and throughput
• SNR must be large enough for signal to be differentiated
from noise
Coverage Process
• Can also use data rate measurement
– Rates will depend on manufacturer, signal
and SNR
– Using Received Signal Level (RSL) allows
you to compare different vendors
Coverage Process
VoWiFi Coverage
• Many manufacturers require received
signal of -65dB
– Use -60 dB in VoWiFi
• Fade margin buffer
• SNR for VoWiFi is 25 dB or higher
• 15 to 20 % cell overlap
• Separation of same channel cells is 20 dB
Coverage
AP Placement
• Coverage analysis will help choose placement
• Mark locations on floor plan with power levels
• Also, semidirectional antennas may be used
– Most analysis use dipole/omnidirectional
• 2.14dB is standard dipole
• Using semidirectionals can help avoid reflections
in confined spaces
• A good kit has multiple antenna types
AP Placement
Optional Application Analysis
• Not normally a part of site survey
– Can give important information
• Software tools to stress test a network
– Simulate multiple clients and lots of traffic
Site Survey Tools
• Kit of equipment and software needed for
successful survey
• Can also buy pre-packaged kits
Indoor Kit
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Spectrum Analyzer
Blueprints
Signal Strength Measurement Software
802.11 client card
Access Point (external antenna)
WLAN Controller
Battery Pack
Binoculars
Walkie-Talkie or cell phone
Antennas
Temporary Mounting Gear
Digital Camera
Measureing Wheel/Meter
Colored electrical tape
Ladder or lift
Outdoor Kit
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Topographic Map
Link Analysis software
Calculators
Maximum Tree growth Data
Binoculars
Walkie-Talkie or cell phone
Signal generator or wattmeter
Variable loss attenuator
Inclineometer
GPS
Digital Camera
Spectrum Analyzer
Spotlight or sunlight reflector
Coverage Analysis
• Different ways to do survey
• Manual
– Active
– Passive
• Assisted
• Predictive
Manual Site Surveys
• Passive
– Radio card collects RF (Signal strength, noise level,
SNR) but doesn’t associated to AP
– Netstumbler
• Active
– Associate to AP for layer 2 or 3 connectivity
– Measure packet loss and retransmission
• Cards sometimes come with basic survey tools
• Commercial Tools are also excellent
Manual Site Surveys
Manual Site Surveys
• Commercial Tools can import
blueprint/floor layout
• Take measurements and display on image
• Allow for what if
Assisted Site Surveys
• WNMS and WLAN controllers can often
take RF measurements
– Assisted coverage analysis
• After installing APs, WNMS server can
gather data
• Often used to figure final AP placement
• WLAN controllers offer radio frequency
spectrum management (RFSM)
Predictive Site Surveys
• Simulated coverage and information
– Software takes information about floor plan, building,
etc and plots AP locations and expected coverage
• Can Model:
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Channel reuse patterns
Coverage cell boundaries
Access point placement
Access point power settings
Number of access points
Data rates
Predictive Site Surveys
• Can play out “what if” scenarios
• Playing with options takes time
• Use as a good first step and then verify
during manual site survey
Self Organizing Wireless LANs
• Next step from RFSM
• Centralized device manages power levels to provide for
coverage and correct overlap
• Based on the accumulated RF information, the
centralized device controls the access points and adjusts
their power and channel settings, dynamically changing
the RF coverage cells. WLAN capacity needs can also
be addressed with RFSM, which utilizes dynamic load
balancing of clients between the access points.
• When implemented, RFSM provides automatic cell
sizing, automatic monitoring, troubleshooting, and
optimization of the RF environment, which can best be
described as a self-organizing wireless LAN.
Exam Essentials
• Define spectrum, coverage, and application analysis.
– Understand why both spectrum and coverage analysis are
considered mandatory and application analysis is usually
optional.
• Identify sources of WLAN interference.
– Describe all of the various devices that are potential sources of
interference in both the 2.4 GHz ISM and the 5 GHz UNII bands.
• Explain RF measurements.
– Be able to explain the procedure used while conducting
coverage analysis and the different types of RF measurements
recorded, including received signal strength and signal-to-noise
ratio.
Exam Essentials
• Understand AP placement and configuration.
– Explain how AP placement, power, and channel
settings are part of coverage analysis.
• Identify all site survey tools.
– Understand the difference between an outdoor and
indoor site survey, and identify all the necessary tools.
• Explain the three major types of coverage
analysis.
– Describe the differences between manual, assisted,
and predictive site surveys, and explain selforganizing WLAN technology.