There are three survey types: Passive, Active, and Predictive.
Passive Survey
Passive surveys are performed with a listen-only mode. The survey client never associates to the access point (AP).
Such surveys can be helpful when you look for rogue devices or want a good gauge of downlink RF coverage from the infrastructure devices.
These can be accomplished with a passive survey:
- Identify rogues
- Locate RF trouble zones quickly
- Validate final RF setting
- Perform initial surveys
The most significant loss of information with passive surveys is uplink information, Physical (PHY) rate boundaries, and retransmission.
PHY rates are generally based on RF signal and noise levels. A passive survey only reports signal propagation for beacons measured by particular clients.
PHY rates can only be measured by actual data that is sent to and from an AP.
Active Survey
Active surveys are performed with the survey client associated to the APs used throughout the survey.
When a client is associated, it performs all the tasks that a typical 802.11 client performs, which includes rate-shift data rates as the RF condition changes and performs retransmissions.
Active surveys are commonly used for new WLAN deployments because they provide the most details upon which to base a design.
There are two main methods used in active surveys:
- Basic Service Set Identifier (BSSID) Method: This method locks a client into an AP’s radio MAC address and does not allow the client to roam.
- Service Set Identifier (SSID) Method: This is more commonly used for post-deployment scenarios and used to survey multiple APs. It enables the survey client to associate to an SSID where the client roams between multiple APs.
Predictive Surveys
Predictive surveys are performed with a software program. The program uses the information about the coverage area to perform AP placements based on RF algorithms.
These surveys are typically void of any type of field measurements.
The best times to incorporate a predictive survey are:
- when the deployment environment has not yet been built.
- when there is a need to obtain a budgetary environment for WLAN-related hardware.
- when roam requirements are less stringent.
Prepare for the Survey
Use this checklist to help you prepare your survey.
Identify Primary Requirements:
- RF application needs (voice, data, location…)
- Type of facility
- Type of client devices
- Industry vertical
- User questionnaire
Identify Facility Requirements
- Single-floor
- Multi-floor
- Campus (indoor and outdoor)
- Warehouse
- Obtain digital floor plans
Identify Requirements for Type of Client Devices
- Minimum Received Signal Strength Indicator (RSSI)
- Minimum Signal-to-Noise Ratio (SNR)
- Delay and Jitter tolerance
- Maximum transmit (Tx) power
Plan for the Survey
Use this checklist to plan for your survey.
Initial Walkthrough
- Access building type
- Anticipate difficult zones
- Confirm surveyed areas
- Check details of area not mentioned on the main coverage map
- Check unexpected roam path
Select Proper Survey Model
- Data
- Voice
- Location
Determine Proper Deployment Characteristics
- Dense deployment
- Highly mobile (Many cells; high-reliability; fast-moving clients) versus nomadic (temporary position)
- Predictive versus Actual
Specify the tools to complete the survey
- Obtain digital floor plans
- Spectrum analysis tool like Cisco Spectrum Expert
- Professional survey tool such as Air magnet Survey or Ekahau Site Survey
Define Client Devices to be Deployed
- Maximum Tx Power level
- Receiver Sensitivity
- Antenna
Determine Physical requirements:
- Power
- Understand cable considerations
- Mount and fastener considerations
- Outdoor grounding and atmospheric electrical discharge protection
- Consider placement of additional APs for monitoring and sniffing