Monday, September 10, 2012
Onsite Pre-Installation Site Survey vs. Predictive Pre-Installation Site Survey
Onsite Pre-Installation Site Survey vs. Predictive Pre-Installation Site Survey
In contrast to wired LANs, a Wireless LAN deployment isn’t that straight forward. Wired LANs can have a range of Ethernet switches, routers, wiring outlets, servers, etc. When all of these devices are configured for a certain design they can be deployed in any type of environment, building or country. When properly deployed, the overall performance (throughput, capacity…) will be the same in all scenarios.
Wireless LANs are a whole different ball game. You will also need wiring (not that funny), Ethernet switches, routers, servers, etc. but the big difference is the access layer which is the "air" for Wireless LANs as oppose to an RJ45 wired outlet when dealing with wired LANs. The overall performance of a wireless client depends on where it is physically located in relation to the access point that it is connected to.
Every building has its own specific characteristics in regard to the propagation of RF signals. It is for this reason that you cannot copy one specific Wireless LAN design to another building in contrast to the wired LAN.
With this, we can say that the design of the access layer is a very crucial part of a successful Wireless LAN deployment. The design of this access layer is done through a Wireless LAN site survey. The goal of a Wireless LAN site survey is to determine the amount and correct positions of access points & antennas so that the Wireless LAN requirements (coverage, throughput, capacity…) are met throughout a predetermined coverage area.
There are two ways to do this, the "onsite pre-installation site survey" and the "predictive pre-installation site survey".
The Onsite Pre-Installation Site Survey
During this site survey the Wireless LAN engineer will go onsite with the documented Wireless LAN requirements and ground plans. After a kick off meeting the survey will which includes:
- · A visual inspection of the coverage areas
- · Spectrum analysis
- · Tune/calibrate the measuring device(s) with the weakest wireless client device that will deployed (upstream and downstream)
- · Start measuring the requested WLAN application requirements
- · Onsite documentation of the results
When the onsite measurements are complete, the results of the entire analysis will be documented in the WLAN Site Survey Report.
The Predictive Pre-Installation Site Survey
This site survey can basically be done anytime & anywhere and there is no need to go onsite. The Wireless LAN engineer will import all of the ground plans into the planner software, and will enter an estimated attenuation value for each wall, elevator, stairwell or any other attenuating object. Next, the software will be configured with the Wireless LAN requirements of the applications. Finally, the planner software will disperse the access points’ layout on the imported ground plan. A Wireless LAN Site Survey report will be generated automatically or can be written manually.
So, which is the most preferred way? Let's compare both methods and discuss the pros & cons.
This comparison is based on a Wireless LAN site survey project for a warehouse/factory with offices on multiple floors, and our experience with both types of site surveys throughout Europe.
Onsite Access
Predictive Site Survey pros
One of the advantages of a predictive site survey is that you don't have to go onsite. You can simulate the entire site survey on imported ground plans in a planner software tool. This could come on handy when the facility is not yet constructed or is still under construction.
Predictive Site Survey cons
The accuracy of the site survey highly depends on the details and accuracy of the received ground plans as well as the accuracy of the predicted attenuation of the objects that have been drawn on the plan. Do the ground plans reflect the latest construction works and are all the interior objects/furniture displayed? Getting accurate ground plans from the customer could be challenging with old buildings.
Drawing in the walls and other objects also requires a thorough knowledge of the attenuation of these materials. Let's take an office with glass walls as an example. RF signals pass easily through normal glass but coated glass is a real RF killer.
Measurements are calculated for downstream RSSI only.
Onsite Site Survey pros
Being onsite get's you more involved in the project. Contact with the customer and a walkthrough of the facility gives you a better perspective on the characteristics of the facility and the aesthetic options for mounting the access points.
Measurements can be done with the weakest wireless device (iPad, iPhone) and can be tested downstream and upstream.
Onsite Site Survey cons
Well, you need to be onsite which could mean travel and hotel expenses, and if the facility is not yet constructed or under construction you can't do the survey.
Time consuming & budget
Predictive Site Survey pros
A predictive site survey will take less time to do, and will therefore be advantageous for urgent & low budget projects. Less time also means less budget!
Predictive Site Survey cons
On the other hand, an inaccurate predictive site survey could be a budget killer (if any is left over). If the Wireless LAN performance does not meet the requested Wireless LAN requirements after deployment, additional access points & cabling could be needed. The same goes if too many access points are installed and need to be disabled/removed. Either one of these scenarios will have a ripple effect on the overall access point layout.
Onsite Site Survey pros
An onsite site survey will be for 99% accurate so that there will be no surprises at end of the road.
Onsite Site Survey cons
An onsite site survey will take more time and budget.
Visual inspection of the coverage area
Predictive Site Survey pros
N/A
Predictive Site Survey cons
The coverage area is only visible on the ground maps.
Onsite Site Survey pros
It is an added value to inspect what the RF barriers are, and what to take in account regarding the building aesthetics and cabling for access point and/or antenna installations.
Onsite Site Survey cons
To be onsite.
Access points #
Predictive Site Survey pros
Fast and cheap way to have an estimation of the number of access points and antennas needed.
Predictive Site Survey cons
Accuracy is the big drawback here. Normally an additional percentage of access points are calculated in the Bill Of Materials. Having extra access points to fill in the gaps afterwards is not the most professional approach; not to mention the additional cabling that will be needed.
Onsite Site Survey pros
Accurate number of the "measured" access points, if done professionally there will be no need for additional access points or cabling.
Onsite Site Survey cons
To be onsite.
Positioning of the access points
Predictive Site Survey pros
No need to be onsite, the planner tool will automatically disperse the access points on the ground maps.
Predictive Site Survey cons
Accuracy. This is especially true in warehouse (storage racks) and factory (robots, objects) environments.
In 802.11n deployments it's vary hard to predict the effect of multi-path, which is beneficial in 802.11n deployments.
Onsite Site Survey pros
Accuracy. Taking the measurements on a measured location will highly increase the accuracy of coverage, overlap and capacity. Multi-path effect can be measured more precisely.
Being onsite you can take in account aesthetic limitations if applicable.
Onsite Site Survey cons
To be onsite.
Mounting accessories
Predictive Site Survey pros
I can't think of any here.
Predictive Site Survey cons
It is especially difficult to predict which type of access point mounting accessory would be suitable for each access point location in non-office environments.
Onsite Site Survey pros
While onsite and measuring correct access point locations, you can perfectly determine the mounting accessory needed for the access point. Let's say that the access point needs to be lowered down 6m to provide service between storage racks but needs to be high enough so that the forklifts will not hit the access points when they load the pallet to the highest rack. This will require a specific mounting accessory.
Onsite Site Survey cons
To be onsite.
Spectrum analysis
Predictive Site Survey pros
N/A. unless you could say that the deployed access points will have spectrum analysis onboard.
Predictive Site Survey cons
It is not possible to perform a spectrum analysis that will detect non-WiFi interference with a predictive site survey.
Onsite Site Survey pros
A spectrum analysis can be performed during the onsite pre-installation survey.
The spectrum analysis will scan for non-WiFi related sources of interference. These types of interference could/will disrupt Wireless LAN transmissions because they do not respect WiFi protocols. This is why they are defined as non WiFi related sources of interference. Even if the Wireless LAN access points that will be deployed have spectrum analysis onboard it's highly recommended to do a spectrum analysis in the pre-install phase, due to the fact that you want to deal with this interference as soon as possible. Detection of non-WiFi interference in a post-installation phase will cause deployment delays.
Onsite Site Survey cons
To be onsite.
Overall Conclusion
Taking everything in consideration there are two big differences between both site surveys, the first one is that for an onsite pre-installation site survey, you need to be onsite and have access to the coverage area (which speaks for itself), while you don't need to be onsite for a predictive site survey. The onsite site survey will initially be more time consuming and more expensive (especially if travel expenses and hotel costs are added to the bill).
The second big difference is the accuracy of the survey, which is way more efficient with the onsite site survey. Putting these two together it will be a trade off between the 2 types of site surveys.
You can save money on the site survey by doing a predictive site survey, but loose more money and time if after the deployment the Wireless LAN does not meet all the requirements and results in bad performance, new cabling, repositioning of access points, adding access points. Keep in mind that one bad positioned access point could have a ripple effect on the whole access point layout.
Or, you can go for the onsite pre-installation site survey, which will initially cost more and will take more time, but will provide accurate results with no surprises at the end of the road.
Recommendations
Unless you're Wireless LAN project consists out of small office floors or separate meeting rooms that require non-latency sensitive Wireless LAN applications we highly recommend onsite pre-install site surveys.
Although cost and time will be higher you are sure you've got what you bought.
Regarding the overall cost of an onsite pre-install site survey, there are many manuals and best practice guides out there which speak of the integration of the site survey in overall project phases meaning several visits to the customer.
To decrease the overall cost of the onsite pre-install site survey we recommend to do a "clean cut" site survey, which means that you cut as much overhead as possible during the onsite pre-installation site survey (rogue detection, heat maps, etc.). Most of these measurements can be done by the Wireless LAN infrastructure before deployment and be dealt with during the verification site survey.
Take note that with either type of site survey a verification site survey before deployment is "highly" recommended. That’s right folks it ain't done with the site survey only. A fine-tuning of the RF, data rates, TX powers, etc. to the requested Wireless LAN applications is crucial for a fully optimized Wireless LAN deployment.
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8 comments:
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This is my first time i visit here. I found so many entertaining stuff in your blog, especially its discussion. From the tons of comments on your articles, I guess I am not the only one having all the enjoyment here! Keep up the good work.
I don't fully agree with you because if you're an experienced WLAN engineer you can manually finetune the results of the prediciton software and you get very good results
So what your saying is that it is ok to have your wireless over-provisioned... Not to mention having APs in strange places like the boiler room?
I'm not saying that it is OK over-provision, I would recommend to use the building infrastructure (attenuation) to mitigate the co-channel contention as mush as possible, in some cases access points could end up in strange places in benefit of the RF design.
cheers
If you are an experienced WLAN engineer, you will get more out of the predictive WLAN software, but one thing that predictive software is not able to do, is predict refections. In an office environment not a big problem, but in a warehouse and production environment could be different results. Measurements onsite will always give you the best results.
Cheers Joeri
i liked the guide...thanks
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