Predictive vs Passive Wireless Survey
As briefly discussed in the ultimate wireless network guide. We will delve further into what a predictive and passive wireless survey is and how to perform a predictive survey. We'll also see which point of the wireless deployment either survey is relevant. Which tools can be used. And what are the best practises.
What is a predictive wifi survey?
Predictive wifi surveys are in Networks Revived opinion and experience the bare minimum needed for a pre wireless deployment of any size. It is impossible to accurately estimate how many access points or where the placement of said access points will need to be mounted. Some very experienced wireless engineers can estimate but not known for absolute sure.
Predictive surveys are created on virtual emulation software what mimics a real life environment.
Before predictive designs can be perform, An up to date building diagram with dimensions will be requested from the customer. The wireless design will be overlayed on top the diagram.
The survey will highlight coverage, gignal to noise ratio, RF attenuation which types of access points are needed.
How to perform a predictive wifi survey?
Below we will go through the typical steps involved in producing a survey using a wifi predictive survey tool. These steps follow modern wifi survey best practises.
Define the scale
The first step after receiving the building plan is to import the building plan into the wifi survey tool. Before we start to build the wireless design we need to give the emulation software some dimensions to measure off. As you can see in our example. We have drawn a line (circled) using the dimensions on the building plan.
Draw the walls
The second stage is to draw the external and internal walls over the top of the building diagram. The emulation software has a variety of building materials. Each material affect RF attenuation an wireless interference. The wifi predictive survey tool has some pre set values what am be changed.
Ceiling Height
It is important to specific the ceiling height of the building as this can range massively dependant on the building. Majority of access points are ceiling mounted. At this point you can align multiple floors if needed.
AP Placement and selection
This is one of the most critical steps within a predictive design as its costly to change later on. We need to consider our density and capacity. If you speak to us at Networks Revived we can advise what the best access points and antenna are for your scenario. Then we also need to look where to place the access point. Take your time and play around. Here you can set your wireless ap placement. In this instance we are placing a Meraki MR36 and setting the 2.4 & 5ghz power levels. This access point is omni-directional and ceiling mounted.
Wireless Coverage
This step is obvious and self explanatory. Now we need to ensure that we have sufficient wireless coverage on our 5ghz spectrum. We aim to have nothing worse than -70dbm within working areas. Some customers ask how to increase wifi coverage? One thing to remember is not to have your AP power to high to increase range. This can become problematic with clients as they have smaller antenna than access points. Just because the the AP can reach the client doesn't mean the client has the power to return.
Wireless channel and noise
Wireless channels and signal to noise ratio needs to be considered.. This is especially important with busy urban environment's when lots of interfering and neighbouring wirless networks are broadcasting on the same or similar channels. Co channel interference is a real concern. This is where passive wifi surveys are more accurate in measuring the environment. The emulation software at least lets you plan your channels and estimate what noise can be caused due to objects.
Testing
Finally we can simulate client devices as they roam around the building. This allows the wireless network engineer to determine if the plan define in the above steps meets the customers requirements. We measure wifi signal strength, data rates and wireless interference.
Predictive Summary.
We've demonstrated a predictive wifi survey, why we survey and how quick it is to produce. Hopefully we have shown why such tasks are performed before any wireless deployment can be commissioned.
As we can see above, when creating a predictive wifi survey there are many variables which are impossible to accurately measure.
RF attenuation through walls and objects does have some default presets, but not every wall nor object has the same properties. Density of material and thickness of material will soak up RF.
Secondly we have interference from other sources. The wireless emulation software cannot remotely identify other interference caused by external factors and other radios within the environment.
Continue reading as we discuss how we can gather more accurate wireless metrics with a passive wifi survey.
What is a passive wifi survey?
Like we've discussed above. A passive wifi survey can be used in a pre wifi deployment to identify AP placement, AP selection and channel planning. But the main difference is accuracy and time consumption.
Passive wifi surveys are performed on site at a customer site or building. Using spectrum analysis tools and real access points on a stick (APoaS) we can precisely measure real how live environmental factors can affect the wireless.
Below are some of more accurate things we can measure vs a predictive survey:
Channel planning
Channel planning can also be done in a predictive survey. But, as we've seen above. We have no idea on what other channels are broadcasting in that environment and if it will cause us problems. With an onsite passive survey using our spectru analysis tool. We can scan the airways to see if theres any channels what maybe off limits in our deployment. These interference channels may come from an neighbouring access point or even a microwave oven. At Networks Revived we aim to above any interfering channels higher than -80dbm.
RF Attenuation
Wireless RF attenuation is the amount of signal lost through an object. In our case this maybe an internal, external walls or other objects. Again this is something that is accounted for in predictive survey. But every object and wall is different. A passive wifi survey best practise is to measure signal either side of the wall/object. The difference between the two measurements will give you your attenuation value.
AP Selection and Placement
This something thats needs to be done at some point during a site visit but forms part of the passive survey. Buildings have unique features and quirks. This is particular true for older structures. An on site survey can determine if placing an access point in the ideal location is even possible. Factors such as plumbing, electrical and general inaccessibility is not accounted for on a drawing may prevent you mounting the AP in the best place. Due to this, mounting the access point in a different location, for example from ceiling to wall. This will then may the wireless network designer to rethink their AP and antenna of choice.
Passive wifi surveys are not only used for pre wireless deployment. The best passive surveys include the post deployment checks. This is something Networks Revived alway undertake after new wireless network is commissioned.
It is important to valid your work and ensure your wireless network solution has met its expectation and customers requirements. Below are some of the post wireless deployment checks and wifi survey best practises.
Dead Spots
In our post survey we ensure theres sufficient wireless coverage in the required areas of the site. With our spectrum analyser and client devices we can measure that excellent RSSI and low Signal-to-noise ratio has been achieved.
Co-channel interference
The wireless environment may have changed between the pre and post deployment. This can happen if a new neighbour has moved into the building next door and are now broadcasting conflicting channels. This is why a post deployment check is import incase we need to make any fine tuning.
Other performance impact factors
We need to generally test the performance of the wireless network. Software bugs in a new firmware releases may cause issues that weren't foreseen pre and during the wireless deployment(trust me we've seen this).
Hopefully we've now given you some knowledge on whats a passive wifi survey and how it is different to a predictive survey. By following these wifi survey best practises you should have a good basic foundation on deploying successfully.
Theres a lot more factors what going wireless deployment which could fill an entire book. Networks Revived engineers have that experience already for your business wireless deployment.