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 self-organizing 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– 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 ProcessReduce AP power output1. 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 Process1. 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.

2. Now walk away from this access point, parallel to the edge of the building, until the received signal drops to –65 dBm.

3. This is the farthest point to place the access point if you do not want cell overlap.

4. 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.

5. 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• 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• 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:– 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 self-organizing WLAN technology.