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Hi all!

Can somebody tell me what is the diference between a Pasive and Active Site survey?

Thanks

Hi ZURC,
There are three main types of wireless site surveys. With two sub-types of manual.

Manual RF site Survey
- RF data is measured and accumulated on a client that passes through the RF
coverage areas
- There are two types of manual surveys

1) Passive
- monitors all RF activity
- In passive mode, the tool does not initiate any RF traffic to understand RF
behavior. Instead, it listens to the traffic that the client adapter hears and
displays the results.

2) Active
- associated to a single AP and test environment
- In active mode, the client adapter actively sends and receives low-level RF
packets to or from its associated access point and provides information on
the success rate. It also enables you to set parameters that govern how the
site survey is performed (such as the data rate).

- FROM: CWNA Study Guide (SYBEX)

Manual
The most common method of coverage analysis is the old-fashioned manual site survey. Manual coverage analysis involves the techniques described earlier, which are used to find the cell boundaries. There are two major types of manual coverage analysis surveys:

Passive During a passive manual survey, the radio card is collecting RF measurements, including received signal strength (dBm), noise level (dBm), signal-to-noise ratio (dB), and bandwidth data rates. The client adapter, however, is not associated to the access point during the survey, and all information is received from radio signals that exist at layer 1.

Active During an active manual survey, the radio card is associated to the access point and has layer 2 connectively, allowing for low-level frame transmissions. If layer 3 connectivity is also established, Internet Control Message Protocol (ICMP) ping traffic is sent in 802.11 data frame transmissions. RF measurements can also be recorded during the active survey, and additional information such as packet loss and retransmission percentages can be measured since the client card is associated to a single access point.
Most vendors recommend that both passive and active site surveys be conducted. The information from both manual surveys can then be compared, contrasted, and/or merged into one final coverage analysis report. So what measurement software tools can be used to collect the data required for both passive and active manual surveys? There are numerous freeware site survey utilities, including NetStumbler, which is a freeware utility that is included on the CD that accompanies this book. NetStumbler can be used for a passive coverage analysis survey. Most Wi-Fi vendors?¡é?€?? client card utility software at the very least comes with a passive survey tool that can be used to measure received signal strength and SNR. Many vendors?¡é?€?? software client utilities will also include active survey capabilities like the Cisco client software pictured in Figure 16.9. Some handheld devices such as VoWiFi phones or Wi-Fi bar code scanners may have site survey capabilities built into the internal software that runs on the handheld device. Many site survey professionals prefer working with the vendors?¡é?€?? client card site survey software
tools as opposed to newer dedicated coverage analysis applications. (end of quote)

(This says that Netstumbler can be used for a passive survey. Netstumbler is an active tool and I would think that any active survey tool can be used for passive surveys. Mike_grol)



Predictive RF Site Survey
- Uses predictive design and analysis
- Predictive Site Survey fully models your building (or outdoor RF environment)
using modeling and simulation software tools that use a floor plan of a building,
along with specifications of the type of construction, to create site survey maps.
These maps show where access points should be placed for optimal coverage.
In addition, the number of users in a space, and the bandwidth and throughput
required for their network activities, is integrated into the mathematical
simulation to assure that the network will provide the required level of service.

- FROM: CWNA Study Guide (SYBEX

Predictive
The last method of RF coverage analysis uses applications that provide RF simulations and modeling design capabilities. Predicted coverage analysis is accomplished using an application that creates visual models of RF coverage cells, bypassing the need for actually capturing RF measurements. Projected cell coverage zones are created using modeling algorithms and attenuation values. One major switch vendor touts these modeling capabilities; however, most predictive coverage analysis tools are stand-alone software design applications. Blueprints and floor plans often use vector graphic formats (.dwg, .dwf) and can contain layer information including the type of building materials that are used. Predictive analysis software supports both vector and raster graphics (.bmp, .jpg, .tif), allowing for the import of building floor plans. The WLAN design engineer will indicate in the software what materials are used in the floor plan. The predictive application already has attenuation values for various materials such as drywall, concrete, and glass programmed into the software. The software creates forecast models using the predictive algorithms and the attenuation information.
The modeling forecast can include the following:
_ Channel reuse patterns
_ Coverage cell boundaries
_ Access point placement
_ Access point power settings
_ Number of access points
_ Data rates
Virtual access points are created and overlaid on the floor plan graphic. Multiple ?¡é?€??what-if?¡é?€?? scenarios can be created by changing the power settings, channel settings, or antenna type of the virtual access points, which can also be moved to any location on the floor plan. Predictive capacity analysis software vendors proclaim 85 percent accuracy in their modeling capabilities. Predictive applications are an excellent tool to use with blueprints of buildings that have yet to be built. It should be noted that entering the initial data for a predictive survey can be very time consuming; however, many site survey engineers have begun using predictive modeling software to cut down on the time needed for the actual site survey. A forecast model is first created with the predictive application and then the site survey engineer conducts a manual site survey to validate the projections. Predictive coverage analysis software can be a valuable tool, but a manual site survey is still necessary to confirm the coverage simulations. (end of quote)



Assisted RF Site Survey
- Uses a centralized management application that receives RF data over a LAN, from
APs
- RF characteristics generated by manual client walk-through

- FROM: CWNA Study Guide (SYBEX)

Assisted
Some WLAN switches and some centralized wireless network management system (WNMS) applications have the capabilities to conduct assisted coverage analysis. After the installation of access points, a centralized solution such as a wireless network management system (WNMS) or a WLAN switch scans the access point radio cards and collects the RF information, which is then used for visualization of coverage cells and for optimizing AP configurations such as channel and power settings. Most assisted solutions use the information gathered from the access point radio cards, but some solutions can also use a client radio to report information back to the centralized device during a client ?¡é?€??walk-through?¡é?€?? of the building. Assisted site surveys typically are used as a starting point before final deployment and are often used as a calibration or planning stage tool with WLAN switches. An assisted calibration process configures and reconfigures the access
points based on analysis of all the collected RF data, as pictured in Figure 16.12. Most WLAN switch solutions that have assisted site survey capabilities also go to the next level and also offer dynamic Radio Frequency Spectrum Management (RFSM), which is discussed later in this chapter. Some system integrators bypass the site survey and install a grid pattern of thin access points. After the thin access points are installed, a wireless switch or wireless switches working together use some form of RFSM technology to dynamically adjust power and channel settings. The Wi-Fi switching architecture constantly monitors the environment and makes adjustments as needed. Although assisted site survey features are an excellent starting point prior to deployment, most professionals still recommend a manual site survey for validation. (end of quote)


I have also read of Fully Automatic site surveys but I do not have very much information in my notes on that.

Automatic
Automated site surveys include both assisted and totally automatic surveys. The assisted survey is really a mix between a manual and a totally automatic survey, providing benefits of both. Some techniques purport to be automatic?¡é?€?¡±implying that no actual "site" work has to be done and everything is fully automatic, being handled by a system that resides somewhere on the network.


mike_grol

wow thats great!! Thank you!!! i understand the differience. :D