…another guest post from Ahmad Nassiri
Site Surveys play a critical role in the planning, design and delivery of a robust wireless network.
I have come across multiple instances where the wireless networks were deployed without any design consideration. Access points (APs) installed wherever deemed easy for installers, transmit power left at full and no channel planning done but yet high expectations for performance!
I was once called to an educational institution to troubleshoot wireless issues and I found students setting in the hallway to get wireless connectivity. I walked inside the lecture rooms and found no APs installed there. APs were only installed in the hallways, and this is a 120 years old sandstone building with high ceilings and wall thickness of 80cm! Wireless was working but not where needed most.
Be prepared for the arguments like why we need a design, use the same design from another site or in multi storey buildings, design only a single floor and copy it across all the other. Even today there are projects that do not allow enough budget for site surveys. I have been asked to drop site surveys due to cost constraints completely.
It is best practice to itemise all components of deploying a wireless network in the scope of works or quotation document, but in this instance I believe it would be a better option to sum it all up as one item and call it “Wireless Design”.
Educating the client about the importance of site surveys (design) is very important, explaining the long term issues and costs associated with fixing poor or no design outweighing the cost of a survey. Site surveys must be completed for all small and large deployments alike.
Site surveys provide estimate of AP numbers for budgeting and procurement purposes, assists in planning cable runs and above all validation of the wireless network performance against some sort of acceptance or success criteria.
There are a few survey types which are performed in different stages of deploying a wireless network. Each one is undertaken differently but they all complement each other in the common goal of delivering a good wireless design.
Wireless Surveys in Detail:
Physical Site Survey – This survey is undertaken mostly on an existing premises where clients are unable to provide floor plans or the maps are outdated. That said, it is highly recommended to undertake this survey at all sites just to make sure the supplied information is accurate.
The survey is carried out by a survey engineer attending site, taking measurements, drawing up the floor plan on paper and confirming the following:
- floor area
- building material (brick/concrete walls, steel, glass etc.)
- mobile areas (client roaming)
- high density
- void areas
- difficult areas (confined)
- outdoor areas (AP placement options like poles, awning type/material)
- heritage listed
- known sources of interference, what is seen on site (microwave oven and coreless phones)
- cabling paths and proximity to a LAN switch
The above information is very crucial for the design of a wireless network.
Once completed and if required, the information is forwarded to a CAD drawer who can create the floor plan and schematics with scales. This floorplan or map is required for our next survey called, “Predictive Survey”.
Predictive Survey – This survey is undertaken with a survey modelling tool like Ekahau or AirMagnet. A floor map and AP model performance characteristics (antenna pattern) is selected based on the AP/antenna type and vendor. The floor map is imported and calibrated in the survey tool with the scale information. Walls, windows, elevator shafts, and other building boundaries causing Radio Frequency (RF) attenuation are drawn. APs are placed on the map based on the application types (data, voice, RTLS), client type, throughput requirements, power, channel and frequency bands in scope.
The outcome of this survey is AP numbers and location for placement of them on the floor map as well as coverage heat maps. Predictive survey results are then provided to installers for planning and running cabling to each AP location. The installers will then advise if there are any limitation in proposed AP locations and if anything needs to be changed or updated.
This survey does not provide information regarding interference and is heavily dependent on the physical site survey or the information provided by the client in case of new sites. The predictive survey must be undertaken by professional wireless designers with sound knowledge of RF.
- Never use the automatic AP placement option offered by the survey tools as it results in very poor placement and causes many issues with coverage and co channel interference (CCI), always use manual option and take control of your design!
- Never use the generic AP model’s antenna pattern provided by survey tools for your design. This is the biggest mistake made and I have come across designs that suffered badly selecting this generic AP.
- In new sites where the building has not been built yet and the predictive survey is only used for budgetary, AP location and cabling cost purposes, it is very critical to zero in on a vendor’s product for the type of AP and antenna.
- While back I did a predictive design based on an agreed vendor products and by the time the building was completed client asked to switch to another vendor and use the same predictive design. It is only an AP and antenna right? Absolutely not, every vendor’s AP/Antenna propagation pattern is different and must be incorporated correctly in the predictive design.
- It is always a best practice to allow at least 5m of additional cable length at the nominated AP locations to allow for AP relocation just in case. That said, the APs must be placed accurately in the first place otherwise the cost of bad AP placement and cabling is enormous.
On-Site Pre Deployment Survey or AP on a Stick Survey – This survey is undertaken in so called critical installs and is performed prior to full deployment usually on 20 – 30% of the floor area. It is usually undertaken to complement the predictive survey but it can be performed without a predictive survey too.
APs on tripods (stick) are placed in the same position as nominated by the predictive survey to simulate the final install and performance characteristics. The survey engineer then performs a passive or active survey (explained later). Measurement of the coverage, signal strength, SNR, interference and signal overlap are captured with the help of a survey tool.
Be prepared to carry the following kit with you:
- APs & antennas (the same make & model as intended for the final install), usually 3-5 APs is sufficient
- network switch (PoE preferred) or power injectors for APs
- a controller (easy if preconfigured along with the network switch)
- network (Cat5/Cat6) cables, 25-50 meters long (depending on indoor/outdoor coverage)
- power cables and power board
- cable ties or Velcro
- distance measuring tool
- floor plans (printed)
- laptops/tablets with survey tool installed
- PPE (personal protective equipment) if surveying a construction site
- Any other tool specific to site.
- While this survey is widely undertaken, the accuracy of reported RF coverage and interference is debatable especially in construction sites. This is due to limited access, presence of heavy machinery and other construction materials around the coverage area affecting the RF in a different manner as compared to a finished or existing site
- The heat maps may look all blue and green indicating good coverage but this does not mean the throughput, overlap, CCI etc. are satisfactory and within the scope when the final install is completed. For a site with 100s of APs it is not feasible and cost prohibitive to move the survey kit to all AP locations and perform the survey.
- Starting the survey and going all the way to the coverage boundary of each AP is needed for accurate data collection which in itself is very challenging.
- A big assumption is made with surveying 20 – 30% of the floor area and assuming the rest will likely be the same. This has to be a very well calculated and intelligent assumption!
- Timing of undertaking this survey in construction environment is very important, the closer to completion the better where all the main fixtures are in place. However this is not always the case, installers will finish all building works like cabling and AP installation first, then furniture, file cabinets, cupboards etc. are populated on the floor adding additional attenuation.
- It is very important to have a clear communication channel with the client or installers to be updated on changes that affects the wireless network in any shape or form.
- That is why a post install/validation survey (explained later) is crucial for capturing all RF affecting factors.
Types of on-site survey
The onsite survey is further broken in to passive and active surveys:
Passive – the term ‘passive’ is used a bit loosely here, although a laptop with a survey tool like Ekahau or AirMagnet is used to capture and take measurement of RF information. This information is then mapped to a Received Signal Strength Indicator (RSSI) level on a floor map and displayed in the form of a heat map. It can also show SNR, Noise, and CCI values for comparison with the predictive survey.
All this data is captured without the laptop associating to the wireless network, may be that is why the name ‘passive’. Yet, passive survey is predominately used!
Active – active survey is the same in capturing the RF information but the difference is in the client laptop associating to the test access point (AP) and running metrics to test for packet loss, jitter, delay, data rates etc.
In reality this survey is expensive for the time it takes to test each AP individually and at the mean time accurately. The amount of information obtained by this survey is much more detailed and comprehensive and must be checked by a professional wireless engineer for proper analysis and validation.
I have seen only a few instances of this survey performed, mainly where voice over wireless LAN (VoWLAN) services are required.
Post install Survey or Validation Survey – This survey is the most important and critical of all the surveys and must be performed to validate what was intended initially with the design. This survey is conducted passively and undertaken once the WLAN is operational.
Vendors suggest at least 24 -72 hours of operation before the post install survey is undertaken. This is primarily if you have Radio Resource Management (RRM) enabled for auto power/channel selection.
The same floor plan and survey tool should be used for this survey. A survey engineer walks with a laptop running the survey tool across the scope area and samples of RF energy is taken at regular intervals. The survey tool then calculates the RF information and displays the heat map with valuable information about the WLAN’s performance. Factors like signal quality, SNR, jitter, data rates, CCI etc. are measured by the validation survey. This survey then enables the stakeholders to either accept the results or make necessary changes to rectify any issues.