Predictive Radio Frequency Planning
September 4, 2014
This chapter discusses predictive RF planning that should be undertaken after a pre-deployment RF site survey is completed and two tools to perform RF planning, the Cisco Prime Infrastructure RF Planner tool and the Ekahau Site Survey tool.
Note For smaller presence sites that involve only one or a few APs, a thorough predictive RF plan may not be required. However it is still necessary to do a limited site survey to understand the RF characteristic of the location before deploying APs.
After the physical site survey is done and analyzed, it is time to conduct an actual RF site survey to assess the location where CMX is to be deployed and determine an effective RF plan that requires minimum or no physical changes after it has been deployed. This is achieved through:
- Predictive planning through RF tools like Ekahau or Prime Infrastructure.
- Coverage check through a single AP in the location for RF characteristics.
- Post-deployment checks to ensure that RF coverage is indeed as determined.
Once a pre-site survey has been done and all the necessary information has been obtained, tools like the RF planning tool in Prime Infrastructure or Ekahau Wi-Fi planner should be used to do predictive RF planning. Perform as much background work as possible before doing predictive RF planning. Predictive RF planning does not guarantee that the network will behave exactly as the tool predicts, but it will generally have an error margin of 15-20% based on the background work done. For example, if a WLAN RF plan is done without any consideration given to an existing WLAN that already exists, then the RF plan, though theoretically accurate, might still not perform well. It is for this reason that after a deployment has been done, post RF site surveys are conducted to determine if coverage holes exist and whether they can be mitigated by either increasing power levels, adding access points, etc.
Note Cisco does not recommend a particular RF planning tool. Tool from other vendors might achieve the same results, but for the purposes of this CMX CVD we use the Ekahau Wi-Fi planner and RF planning tool discussed in detail below.
Before starting predictive RF site planning, have the following information available:
- Maps of the location where CMX is to be deployed.
- Points on maps where access points can or cannot be installed.
- Points and areas on maps where full coverage is expected.
- Type of walls and materials used in the walls.
- If using external antennas, the type of external antenna used.
Cisco Prime Infrastructure RF Planning Tool
The RF Planning tool can be used to add wall attenuation information to floor maps. Wall information added via the Map Editor does not affect access point placement or location designs, however it will be used by the planning tool when displaying predicted RF coverage maps for planned access points.
The planning tool operates purely on a hypothetical basis without the need to connect or deploy any access points or controllers. Since it is Cisco Prime Infrastructure feature, a Cisco Prime Infrastructure server must be installed somewhere in network before the planning tool can be used. If there are any existing access points that have been deployed and defined to Cisco Prime Infrastructure already, the planning tool allows for the configuration of those access points to be copied into the planning virtual environment, allowing you to safely model with a virtual copy of your production environment.
Before using the planning tool for RF coverage planning, ensure that an appropriate path loss model has been assigned to the floor upon which you wish to conduct your planning. Cisco Prime Infrastructure uses the coverage reference path losses and path loss exponents when it plots the predicted coverage heatmaps from each access point in the planning tool. Seasoned WLAN veteran designers have the option of using the planning tool in a manual mode to place access points on floor maps as they see fit and adjust several criteria to see their effect (such as transmit power, antenna type, and so on). Alternatively, the Cisco Prime Infrastructure planning tool also allows automated access point placement based on the type of deployment model desired. Those users and designers desiring that the system make an initial design suggestion can use the planning tool in an automated mode, thereby specifying the type of design they wish and allowing the planning tool to examine their requirements and make qualified suggestions. For designers wishing to combine voice and data designs meeting Cisco VoWLAN best practices with location tracking, it is recommended that the planning tool be first used to model voice and data designs separately from location tracking requirements. Once a satisfactory voice and data design has been created, any modifications necessary to provide for good location fidelity can then be manually incorporated.
The planning tool assumes a transmit power of +18dBm for 802.11bgn and +15dBm for 802.11anac, along with an antenna azimuth position of 180º, elevation height of ten feet and elevation angle of 0º. Transmit power, access point type, antenna type, and azimuth position can be changed individually for each access point. In addition, planning tool users can specify a several additional criteria to further fine tune.
Selecting the location planning option results in the planning mode access points being placed along the perimeter and in the corners of a floor, in addition to the interior of the floor as necessary. At least four access points are assumed to be present in every location design and access points are placed using a spacing of up to 70 feet. Note that when using the location planning option, the resulting design may meet best practice recommendations for voice and data, although the signal strength and overlap requirements of co-resident applications are not explicitly taken into account. Therefore in designs where location tracking is intended to co-reside with voice and high speed data, it is recommended that these application designs be addressed first according to Cisco-recommended best practices. Once a design satisfying application needs has been completed, the design can then be modified or augmented as necessary to meet location tracking requirements.
More complex designs containing totally enclosed interior voids (for example, a building with a fully enclosed interior atrium as shown in Figure 16-1 with the perimeter of the building shown by a red outline) may not lend themselves well to automatic access point placement. The planning tool does not currently allow the exclusion of zones into which access point placement should not occur. Note in Figure 16-1 the placement of access points 2, 4, 9, and 24 in the atrium area (indicated by the blue outline). The placement of these access points in this area is incorrect, since the floor map is for the building’s third floor. This should be corrected by manual intervention and moving the access points into correct locations or eliminating them entirely if not necessary.
Figure 16-1 Example of a Floor Plan with Fully Enclosed Interior Atrium
To perform predictive RF Planning through RF Planner tool through Prime Infrastructure:
Step 1 Create and import maps into Prime Infrastructure.
Step 2 Click Operate > Maps to go to the Maps View.
Figure 16-2 Maps View from PI
Step 3 Select the Floor for which you want to create an RF plan (this assumes that you have already uploaded a map to the floor).
Figure 16-3 Select Floor for RF Plan
Step 4 The Floor view should display. In the top right “Select a command” drop-down menu, select “Planning Mode”. A new window is displayed.
Figure 16-4 Select Planning Mode
Step 5 In the new window that opens for RF Planner, click Add APs to begin positioning the APs.
Figure 16-5 Position APs
Step 6 Follow the steps as outlined. Provide a name for the AP (optional), select the correct type of APs and the correct type of antennas, optimize for Data, Voice, and Location for CMX, and click Calculate. The Calculator then takes into account the various parameters that you input and suggests the number of APs that are required to obtain the desired coverage. Once satisfied, click Apply on Map to automatically place the APs on the maps.
Figure 16-6 Input AP Details
Step 7 Once an initial RF Plan has been made, APs are put on the map automatically. Notice that the placement is optimized for both perimeter and indoor coverage.
Figure 16-7 APs Placed on Maps via RF Planner
After the initial RF plan is completed, you can:
- Move APs around to meet your requirements.
- Delete APs by unchecking box on top left corner box for “Contributing APs”.
- If your deployment is an 802.11ac deployment, ensure that 802.11ac is selected.
- For RSSI threshold, the CMX solution recommends that the cut off be -67dbm to ensure that smartphones and tablets which have usually less powerful network cards than laptops are also clearly detected by the CMX solution.
- Click Apply to re-plan the RF plan.
Step 8 Antenna azimuth angles for each AP can be adjusted by clicking the Angle link in the tool. This is useful if you have directional antennas.
Figure 16-8 AP Antenna Details
Step 9 Adjust the RF plan until you obtain a plan that meets your requirements.
Note If you have walls and obstacles configured on the floor plan, then the RF plan tries to take that into account while planning. The type of floor (indoor high ceiling, office dry walls, etc.) also matters for RF planning. You may discover that you need fewer or more APs to get full coverage. Predictive RF Planner is a good tool to estimate and plan your network, but should not be used without a proper site survey.
Step 10 Generate a RF Plan report if you want a copy of the plan. This report contains all the details about the recommended plan.
Figure 16-9 Generate Report
Ekahau RF Planning
The Ekahau Predictive Site Survey tool offers way to simulate RF planning, however it is not a replacement for a physical site survey as the tool cannot determine actual interferences and RF characteristics that may impact a WLAN. However the RF tool can be used to plan for deployment and Ekahau can be used for RF planning in deployments where a Cisco Prime Infrastructure is not available.
Note To get started, download the latest version of the Ekahau Site Survey Planner from http://www.ekahau.com/ (there is a fee).
The basic steps to use Ekahau for RF planning are:
Step 1 Click Map > Add Map button to import the floor map into the tool.
Step 2 Click the scale icon and set scale (to change the scale unit between meters/feet, go to File > Preferences > Length Unit).
Step 3 Set the Regulatory Domain (File > Preferences> Regulatory Domain).
Step 4 Click the wall icon to draw the walls (concrete on the outer boundary/stairs). This information should be available from background work you performed.
Step 5 Click the simulated AP icon and select the AP; for the purpose of this guide, we selected AP 3700.
Step 6 Click the coverage area icon and select boundaries on the map.
Step 7 Click on the Auto Planner. In the Coverage Requirement section, selection a location where CMX needs to be deployed. Next, use the values below as reference for various RF characteristics that can be configured in the tool:
- Enter Signal Strength is -67 dBm
- SNR ratio is 20 dBm
- Data rate is 12
- Number of Access Point = 3
- Ping round Trip = 500
- Packet Loss = 10%
Step 8 Click Consider in Plan under Capacity Requirements if you know the type and quantity of devices specified in the requirements. Click Edit and add that information.
Step 9 Click Access Point type and select the correct AP. Sometimes this goes reverts to a default, so be aware of this behavior.
Step 10 Click Optimize Coverage for and select either 2.4 or 5 Ghz.
Step 11 Important: Click Advanced Settings and change transmit power to 10mW.
Note Regarding transmit power, even though APs can transmit maximum power up-to 18 mw (or 20mw in some cases), it is a good idea to base your RF plan on half power (i.e., 10 mw). This ensures that you are planning for location and mitigating effects of possible unknown and known interferences sources near the deployment. This also helps later when RRM is used to plan for channels.
Step 12 You can change Antenna height or leave as default if not known.
Step 13 If an 802.11ac AP is chosen, change Bandwidth (on 5Ghz) to 80 Mhz if required. This is not required for the CMX solution or in general for Wi-Fi design. Plan on using 80Mhz only if you are sure that all your clients also support 80Mhz.
Step 14 Click Create Plan.
The APs now appear on the heatmap.
Step 15 Click View and select Access Point Names to see the AP names on the map (recommended). You can unclick Radio Channels so they do not appear on the map.
The AP heat maps provide you with a rough estimate of how many APs might be needed for an installation. Note that this number may be less or more than what might be desired. Rearrange the APs to get a better idea of how many APs might be needed to satisfy the use case.
Calculate the area manually (e.g, 300 feet x 150 feet = 33,750 sq feet)
- Rule of thumb is 1 AP per:
– 5000 sq feet for Data
– 2500 sq feet for Voice/Location (CMX Deployment)
- So if we only do data then: 33750/5000 = need 7 APs!
You can re-arrange AP location (remember to click Arrow Pointer first, so we do not add more APs).
You can also manually add APs if needed for better coverage.
Step 16 If everything looks satisfactory with the heatmap, save the map:
Click File > Export Image (perhaps save in the same folder created for the case).
Click File > Report (you may leave all options selected for your reference).
If you have more maps to create, follow the steps above for each one. Remember that for each map, we need the Heatmap Image and the Report.
If there a multi-floor building, create a separate map for each floor.
For multiple AP Models, you need to manually click and select them. You cannot select two different AP Models on Auto planner.
Additional activities that can be performed to improve designs and design implementation include:
- Perform a walk-around of the site and verify that areas on the floor plan where access point mounting is desired can actually accommodate it. This is always a good idea, since floor plans and blueprints do not always indicate the precise conditions present at each location where an access point may be mounted. For example, you may find that certain locations that appear to be viable candidates on paper actually are inaccessible (such as an electrical closet), inappropriate (such as an outdoor balcony), or are otherwise not acceptable. In such cases, access points should be relocated close to the original location such that the impact on the overall design is minimal.
- Verify RF propagation and coverage assumptions by temporarily installing a few access points in various test areas of the floor and measuring actual RF signal strength and cell-to-cell overlap using a portable client device with appropriate site survey software tools. This is an excellent time to measure the ambient noise levels of the potential access point cells as well and determine whether the projected signal to noise ratio will be sufficient. Note that Cisco’s RRM feature also monitors client SNR and increases access point power if a number of clients are noticed to fall below a prescribed SNR threshold. For more information about RRM, refer to Radio Resource Management under Unified Wireless Networks at:
- Validate whether there are any radar users present in your locale that may interfere with the use of the additional 802.11a that are subject to DFS. If there are not, these channels can be made available for use by enabling DFS on your WLAN controllers.