Defining AP and antenna placement locations

  • The hardest part of physically performing a coverage analysis site survey is often finding where to place the first access point and determining the boundaries of the first RF cell
    1. Place an access point with a power setting of 30 mW (or the power level that you determine is ideal for your environment) in the corner of the building.
      Site Survey - 1st AP placement.png
    2. Walk diagonally away from the access point toward the center of the building until the received signal drops to –70 dBm, or the signal strength that you are planning for. This is the location where you place your first access point. (–70 dBm will be used as the desired signal level throughout the rest of this example. If you are using a different desired signal level, use it in place of this.)
    3. Temporarily mount the access point in the first location and begin walking throughout the facility to find the –70 dBm endpoints, 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.
  • After the first coverage cell and boundaries have been determined, the next question is where to place the next access point. The placement of the next access point is performed by using a technique that is similar to the one you used to place the first access point.
  • Think of the cell boundary of the first access point, where the signal is –70 dBm, as the initial starting point, similar to the way you used the corner of the building as your initial starting point, and do the following:
    1. From the first access point, walk parallel to the edge of the building, and place a temporary access point at the location where the received signal is –70 dBm
      Site Survey - 2nd AP placement.png
    2. Now walk away from this access point, parallel to the edge of the building, until the received signal drops to –70 dBm.
    3. Move to that location and temporarily mount the access point.
      The AP mounted at this location will provide for the first coverage
      cell.
    4. Begin walking throughout the facility to find the –70 dBm
      endpoints, or cell boundaries.
    5. 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.
  • It is important to avoid excessive overlap because it can cause frequent roaming and performance degradation
  • The shape and size of the building and the attenuation caused by the various materials of walls and obstacles will require you to change the distances between
    access points to ensure proper cell overlap.
  • After finding the proper placement of the second access point and all of its cell boundaries, repeat the procedure all over again.
  • The rest of a manual site survey like this one is basically repeating this procedure over and over again, effectively daisy-chaining throughout the building until all coverage needs are determined.
  • WLAN design guides and white papers from various WLAN vendors often reference 15 percent to 30 percent coverage cell overlap for roaming purposes. However, there is no way to measure coverage cell overlap. Coverage overlap is really duplicate coverage from the perspective of a Wi-Fi client station
  • A proper site survey should be conducted to make sure that a client always has proper duplicate coverage from multiple access points. In other words, each Wi-Fi
    client station needs to hear at least one access point at a specific received signal strength indicator (RSSI) and a backup or secondary access point at a different RSSI. Typically, vendor RSSI thresholds require a received signal of greater than –70 dBm for the higher data rate communications. Therefore, a client station needs to see at least two access points at the desired signal level so that the client can roam if necessary.
  • The following cell edge measurements are taken during the site
    survey:

    • Received signal strength (dBm), also known as received signal level (RSL)
    • Noise level (dBm)
    • Signal-to-noise ratio, or SNR (dB)
  • The received signal strength measurements that are recorded during a site survey typically depend on the intended use of the WLAN. If the intent of the WLAN is primarily to provide low density data service versus capacity, a lower received signal of –73 dBm might be used as the boundary for your overlapping cells. When throughput and capacity are a higher priority, using a received signal of –70 dBm or
    higher is recommended. When you are designing for WLANs with VoWiFi clients, a –67 dBm or stronger signal that is even higher above the noise is recommended.
  • The SNR is an important value because, if the background noise is too close to the received signal, data can be corrupted and retransmissions will increase.
  • Many vendors recommend a minimum SNR of 18 dB for data networks and a
    minimum of 25 dB for voice networks.
  • When a semidirectional antenna is used, recording the received signal strength, SNR, and noise level measurements is still necessary to find the coverage edges. Simply record the signal measurements along the directional path and the edges of the directional path where the antenna is providing coverage.

 

  • When the site survey is conducted, all the cell edge measurements will be recorded and written on a copy of the floor plan of the building.
  • An entry with the exact location of each access point must also be recorded.
  • Next to the entry of each access point should be the transmission power level of
    the AP’s radio when the survey was conducted
  • The location of all the wiring closets will also be noted on the floor plan, and care should be taken to ensure that the placement of any access point is within a
    100-meter (328-foot) cable run back to the wiring closet because of copper Ethernet cabling distance limitations.
  • Be sure to account for vertical cabling distances as well as horizontal runs.
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