I have put down my thoughts on wireless site survey and its best practices. Wireless site survey is a debatable activity as there is “no my way or your way”. As long as you have achieved success with planning, design and optimising you have done it the right way.
Having said that, each engineer may have his/her own way of performing site survey. it is always good to learn from fellow engineers as experience and unique environments and past success cannot replace theory.
The best thing about site survey is that each site can be different than the other and will need your mind to put in the right thoughts to design that particular venue.
Types of site survey :
- Active site survey : An active site survey allows the client device to connect to an access point to measure signal strength, throughput, noise and other signal characteristics. In this method, you can only associate to a single access point and take its measurement. This will show how client device will work with the access point once it is connected to it. This method will not allow you to scan the neighbour wireless network and will be unable to capture the interference. This method uses the data frames signal characteristics.
- Passive site survey: Passive site survey will allow the client device (Survey device) to scan the RF environment based on the channels enabled during the survey. If all the channels are enabled, then it captures all the available wireless signals and its characteristics . This method is very important to analyse the CCI/ACI/SNR/ etc. Passive site survey used the beacons form access point to measure its RSSI value and other elements of the wifi signal.
- Predictive site survey: This method of site survey is conducted by the RF software which performs data modelling based on the inputs provided by you. it is recommended to visit the site prior to performing this survey so that you can use the right obstruction and attenuation during predictive modelling. usually, this method gives 75-85% accuracy and cn go higher depending on the input provided by the engineer to simulate the actual site.
Phases to perform site survey: Site Survey is an essential activity for a successful WLAN deployment. Sometimes, we have no control on when this task will be performed.
If the engineer has control to plan this activity, it is recommended to know the the environment and ensure that both pre deployment (Active/Passive) and post deployment (Passive) site survey is performed (Including Spectrum Analysis)
Below are the common phases when site survey will be performed.
- Before the building is constructed (Predictive)
- Pre deployment (AP on a stick using active/passive )
- Post deployment ( Validation using passive survey)
Deciding on the type of site survey can be quite tricky. Quite often, engineers will not have the option to get engaged whenever they would like to and it depends on factors like cost, budget, project timelines, site access, SOW, cabling requirements etc etc.
When do you perform what kind of site survey?
This is not a one liner as there are many ways to perform site survey in different situations.
Wireless Engineers have their own way of perform ing this task and can differ significantly based on the above reasons.
Usually, if the building /floor is not constructed, we can opt for predictive site survey and perform a post deployment site survey (Passive site survey) once the building and installation is complete.
If the area to be surveyed is constructed and is challenging in nature like manufacturing floor/stadium/warehouse/outdoor/harbour/high density etc etc, then AP on a Stick survey is recommended ( We can perform both Active and passive site survey) on this method. It is recommended to perform passive unless the requirement for site survey is to measure throughput and client testing.
Active/Passive site survey can be performed either during pre deployment or post deployment phase.
Performing site survey prior to deployment and also post deployment is very crucial for the WLAN deployment as RF environments are very complex in nature.
Requirements prior to starting site survey and best practices:
- Understand the application requirements that will be used on the wireless network (Voice, data, location services etc), this will decide the kind of survey and signal strength, client adapters and transmit power, antenna gain you will be using during the survey.
- Understand the client device types (Vendor model, make, wifi capability) used in this network.
- Is redundancy required for access points ? This can aggressive change your cell overlap requirements.
- Understand the capacity requirements for that area.
- Design for the lowest capable client in mind.
- Define your coverage requirements ( Minimum Signal strength, SNR, coverege overlap, and channel planning)
- Understand what is currently there on the wifi network by performing passive scan of the venue.
- Arrange your site survey kit, site access and manage logistics.
- Have the right gear for site survey (AP, laptop, Licensed site survey software, many wi-fi adapters, battery pack, tripods, laser distance meter, angle meter, fabricated antenna brackets, spare survey device /battery for laptop/tablet), duct tapes, cable ties, nuts and bolts as required.
- Use seperate adapters to perform passive site survey on 5 GHz and 2.4 GHz as this will save time.
- Perform site survey on both the frequency bands 2.4 GHz and 5 GHz as nature of these frequencies is different and shows different behaviour when there is an attenuation.
- Set the right values for signal propagation assessment so that you get the right quality data . Usually this shows how wide your signal bleeds post your survey walk. Always keep an eye on the signal values on survey software as it can malfunction and stop collecting data. Ensure that you walk at least 10-15 dBm above the designed threshold towards the cell boundary to capture accurate results.
- Always survey on the access point and antenna that you plan to use in production network. It is recommended to survey on device that is least capable as signal receiver sensitivity differs between a 1 Spatial stream device and 3 Spatial stream device.
- Configure the access points with right data rates, lower transmit power level (below 14 dBm) so that it can have some buffer to increase or decrease and be similar to your planned production network. it is recommended to lower the transmit power on 2.4 slightly than 5 GHz during survey.
- Post site survey, create a static channel plan for 2.4 GHz radios and disable a few if required by ensuring that there is sufficient coverage form the non overlapping channels.
- Perform spectrum analysis to understand the non wifi interference (Like radar, microwave, jammers etc)
Hybrid Site survey : This is a new term to perform a predictive site survey after a AP on a stick site survey. This method saves time and also provided accurate data.
This method allows you to collect the right data from the venue, understand and measure the attenuation, validate the data and know the obstruction materials, onsite challenges and simulate this on the software.
AP on a stick (Passive/Active) provides right data form the site and you could compare these results with software and tweak it as required to ensure that you are getting the most accurate results.
It is recommended to perform a couple of survey on AP on a stick for a given area and replicate the solution if the other remaining area is identical.
Please note that this is recommended for pre-deployment and it should be followed up with a passive site survey in the post deployment phase.