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CWNA guide to wireless LANs 2nd ch07

CWNA Guide to Wireless
LANs, Second Edition
Chapter Seven
Conducting a Site Survey


Objectives
• Explain what a site survey is
• Describe the tools that are used in a site survey
• Describe how to perform a site survey

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What is a Site Survey?
• When installing a WLAN for an organization, areas
of dead space might not be tolerated
– Ensure blanket coverage, meet per-user bandwidth
requirements, minimize “bleeding” of signal


• Factors affecting wireless coverage goals:






Devices emitting RF signals
Building structure (walls, construction materials)
Open or closed office doors
Stationary versus mobile machinery/equipment
Movement of mobile walls (e.g., cubicles)

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What is a Site Survey? (continued)
• Factors affecting wireless coverage goals
(continued):
– Expansion of physical plant or growth of organization
– Existing WLANs
• Both inside organization, and within nearby
organizations

• Site survey: Process of planning a WLAN to meet
design goals
– Effectiveness of a WLAN often linked to
thoroughness of the site survey
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What is a Site Survey? (continued)
• Design goals for a site survey:






Achieve best possible performance from WLAN
Certify that installation will operate as promised
Determine best location for APs
Develop networks optimized for variety of
applications
– Ensure coverage will fulfill organization’s
requirements
– Locate unauthorized APs

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What is a Site Survey? (continued)
• Design goals for a site survey (continued):
– Map nearby wireless networks to determine existing
radio interference
– Reduce radio interference as much as possible
– Make wireless network secure

• Survey provides realistic understanding of
infrastructure required for proposed wireless link
– Assists in predicting network capability and
throughput
– Helps determine exact location of APs and power
levels required
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What is a Site Survey? (continued)
• When to perform a site survey:






Before installing a new wireless network
Before changing an existing wireless network
When there are significant changes in personnel
When there are changes in network needs
After making physical changes to a building

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Site Survey Tools: Wireless Tools
• Most basic tool is AP itself:
– Position AP in various locations, monitor signal as
you move
– APs should have ability to adjust output power
– APs should have external antenna connectors
• Test effectiveness of different antenna types in
different situations

– May need DC-to-AC converter for testing

• Notebook computer with wireless NIC also
essential for testing
– Previously configured and tested
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Site Survey Tools: Wireless Tools
(continued)

Figure 7-1: Adjusting AP power levels

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Site Survey Tools: Wireless Tools
(continued)

Figure 7-2: External antenna connectors

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Site Survey Tools: Measurement Tools
• Site Survey Analyzers: Specifically designed for
conducting WLAN site surveys
– Software often built into AP
– Receive Signal Strength Indicator (RSSI) value
– Full-featured site survey analyzer software settings:






Destination MAC Address
Continuous Link Test
Number of Packets
Packet Size
Data Retries

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Site Survey Tools: Measurement Tools
(continued)
• Site Survey Analyzers (continued):
– Full-featured site survey analyzer software settings
(continued):
• Data Rate
• Delay Between Packets
• Packet Tx Type
– Unicast or multicast
• Percent Success Threshold

– Basic survey analyzer software contains far fewer
features
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Site Survey Tools: Measurement Tools
(continued)

Figure 7-3: Full-featured site survey analyzer software setup

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Site Survey Tools: Measurement Tools
(continued)

Figure 7-4: Full-featured site survey analyzer software results

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Site Survey Tools: Measurement Tools
(continued)

Figure 7-6: Basic site survey analyzer software results

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Site Survey Tools: Measurement Tools
(continued)
• Spectrum Analyzers: Scan radio frequency
spectrum and provides graphical display of results
– Typically measure signal-to-noise ratio
– Single-frequency analyzers measure signal-to-noise
ratio at specified frequency
– Helpful in identifying interference problems
• Thus, helps properly position/orient AP

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Site Survey Tools: Measurement Tools
(continued)

Figure 7-7: Single-frequency analyzer

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Site Survey Tools: Measurement Tools
(continued)

Figure 7-8: Spectrum analyzer

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Site Survey Tools: Measurement Tools
(continued)
• Network Analyzers: Can be used to pick up
packets being transmitted by other WLANs in area
– Provide additional information on transmissions
– Packet sniffers or protocol analyzers
– Not used in placement of AP

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Site Survey Tools: Documentation
Tools
• Create a “hard copy” of site survey results
– Make available for future reference
– No industry-standard form for site survey
documentation

• Site survey report should include:






Purpose of report
Survey methods
RF coverage details (frequency and channel plan)
Throughput findings
Sources of interference

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Site Survey Tools: Documentation
Tools (continued)
• Site survey report should include (continued):
– Problem zones
– Marked-up facility drawings with access point
placement
– Access point configuration

• Use plain paper and building layout blueprints as
tools
• Advisable to create database to store site survey
information and generate reports
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Site Survey Tools: Documentation
Tools (continued)

Figure 7-9: Sample site survey form

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Performing a Site Survey: Gathering
Data
• Obtaining Business Requirements: Determine
business reasons why WLAN being proposed or
extended
– If this step skipped, almost impossible to properly
design and implement the network
– Primary data gathering method is interviewing
– Must determine type of mobility required within
organization
– Must determine per-user bandwidth requirements
• May be different “types” of users with different
bandwidth requirements

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Performing a Site Survey: Gathering
Data (continued)
• Defining Security Requirements: Consider type
of data encryption and type of authentication that
will take place across WLAN
– Consider existing security policies and procedures

• Gathering Site-Specific Documentation:
– Blueprints, facility drawings, and other documents
• Show specific building infrastructure components

– Inspecting the site
• Document changes to blueprints and get visual
perspective

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Performing a Site Survey: Gathering
Data (continued)
• Gathering Site-Specific Documentation
(continued):
– Behind-the-scenes site inspection
• May require ladder, flashlight, and an escort

• Documenting Existing Network Characteristics:
New or expanded WLAN will “dovetail” into network
already in place
– Determine degree to which WLAN will interact with
other wired networks
– Legacy systems may require additional equipment to
support WLAN
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