Why you shouldn’t skip a wireless site survey.
By Mike Zeigler
The catch line from a 1980’s Fram commercial sums it all up. Time, money and effort spent up front can save you from spending a lot more time, money and effort after the install. A site survey is basically examining a site for RF interference, existing wireless networks, physical obstructions and network infrastructure, as well as access point placement and performance. There are 2 type of surveys that can be done. Manual and predictive. The manual survey is the more traditional way of surveying. It involves manually placing access points at desired locations and walking the floor measuring signal levels as well as actual throughput and packet loss. This is repeated until the entire are is covered. It can be time consuming, but you’ll know what you are up against physically as well what to expect after the install. Of course there will still be some tweaking, but it should eliminate most surprises. This type of survey should be done using the same type of equipment that will be used for the install. Different clients have different characteristics and surveying with equipment other than what is going be used could result in an invalid survey. A predictive survey uses specialized software that allows you to upload the floor plan of the site you are surveying, including the site specific details such as the type of walls, and other building information. It takes this data and provides recommendations as to the placement on access points and access point settings. The advantage of this method is that it allows you to virtually move access points and adjust power and antenna settings, seeing the results immediatly. This saves time and expense of a manual survey, but the predictive survey is only as accurate as the data put into it. One approach is to start with the predicative survey and verify it with a manual survey. Other pitfalls a good site survey can reveal.
- The network infrastructure.One overlooked item is how far from a network switch the access point is located. The cabling distance is 328ft (100 meters). Power for the access point also needs to be considered. Are the existing switches POE capable? If not power injectors will needed.Another thing to look at is the switch’s capabilities. If it’s running 10/100 and you’re looking at utilizing an “N” radio, it is possible to over subscribe the switch port with enough clients.
- What is the network to accomplish?A wireless data network has different requirements that a wireless voice network.In a wireless data network, dropped packets are acceptable because the data can be re sent and depending the application, slower speeds may be acceptable and as a result requires less access points.Voice is a different story. In a voice network, bandwidth is king and voice tolerates very little packet loss. It can require smaller cell sizes, and more access points. It also means making sure that the network infrastructure supports QOS.A wireless network that supports data without an issue, may not be suitable for voice in its existing state.
- Interference and existing networks.Ideally an RF spectrum scan and a scan on existing wireless networks should be done during the survey.Since the 2.4 Ghz and 5 Ghz are unlicensed frequencies, it means that anyone can use these for a variety of things.Some of the most common sources of interference in 2.4 Ghz band include microwave ovens, wireless phones, first generation Bluetooth devices, wireless video, and other proprietary wireless devices.Because of the half duplex nature of wireless networking, neighboring networks can cause throughput issues.This is because a machine can only transmit data when it detects that the airways are free of transmissions.The more networks using a particular channel, the longer the client has to wait, regardless of which access point it is associated with. Even though 802.11b/g/n has 11 channels in the 2.4 Ghz band, only 3 channels are non-overlapping (interfering). This may not be an issue if you are in the country, but if you are in a city or multitenant building, the competition for an available transmit slot can be great. All you really need to do is look at the number of available wi-fi connections on your laptop or tablet to see what you are competing with. If you have a multi floor location you’ll also need to be aware of what is known as the “3 dimensional trap” This is because signal will bleed to the floors above and below. The good thing about this is it can save on needing additional access points, the issue is channel allocation can be a challenge if you are deploying a large amount of access points Other considerations in the 2.4 Ghz band is that a 802.11b client in the network can drag down throughput due to a built in protection mechanism and 802.11n channel bonding which can spread its channel usage across the band.
- Physical obstacles.The physical layout and building material can effect RF in a surprising way.I once attended a vendor wireless demo several years ago. During a lunch break, the vendor setup and tested his demo while the hotel ballroom was empty and it worked like a charm. After lunch with the ballroom filled to capacity, he proudly launched the demo and it failed miserably. The problem? The room was filled with human beings consisting of 70% water and water absorbs RF energy!Other building materials such as drywall and wood absorb signal strength and can effectively lower the signal reaching the client by 50% or more.Brick and metal reflect signals which can cause multipath issues. This not so much a problem using “N” but can create dead spots in 802.11b/g Chain link fenced areas can cause scattering of the signal which can minimize the available signal. Site surveys should always be performed with the site when it is at its normal state. It doesn’t do you any good to survey an empty warehouse when the RF environment will be totally different when it is at capacity.
While this only scratches the surface, I hope this illustrates that finding and confronting issues that can be found during a site survey can save time, effort and money after the wireless network has been installed.