Wireless Network

Why a Site Survey is crucial before implementing a Wi-Fi network

Planning for a Wireless network is not as simple as Wired network. Unlike wired networks, over-provisioning (of access points) is not a very good idea with wireless networks because of the increased possibility of co-channel interference with many access points placed nearby. That’s why wi-fi site surveys are crucial, and in this article let us understand why.

What are the objectives of Wi-Fi Site Surveys?

In short,

  • Discover RF Coverage Areas/ Coverage Holes
  • Identify sources of RF Interference (internal – microwave owens, cordless phones, satellite dishes, etc; external – neighboring access points, etc)
  • Determine the Quantity & Optimum locations for placement of Wireless devices

Why is a Site Survey required for Wi-Fi Networks?

A wireless network, as you know, is comprised of the Wireless Access Points, Wireless Controller, Wireless clients & the passive and active network components that connect them to the LAN backbone. Unlike wired networks where one can say that so many switch ports are required for so many users, its difficult to predict the number of access points (based on the number of users) for wireless networks.

While an access point could connect to hundreds of computers theoretically, there are a lot of factors like bandwidth per user, distance from the access point, type of wireless technology used (802.11a/b/g/n), co-channel/ external interference, roaming, etc that can influence the actual number of users/ quality. So, a wireless site survey is the best way to determine the number of access points and their positions for optimum wi-fi coverage in the area.

How is a Wi-Fi site survey done?

Some vendors take the soft copy of a floor plan, use their site survey software to determine the number and position of the access points; some of them combine the above process with a manual survey to see the possible sources of obstruction on the site. Some vendors do only the manual survey and guess the number of access points required, based on their prior experience!

The best way to do a site survey is to go to the site, place an access point (battery powered, so that it can be moved) at  places where the new access points are supposed to be placed and check for the received power levels using laptops (loaded with a site survey software) at various distances / positions around the AP. Some site survey softwares automatically record the power levels as they move around, while some enter them manually. If multiple frequencies (2.4Ghz/5Ghz) are going to be deployed, the survey could to be done for each frequency  by using multiple client adapters for all the channels to check the actual coverage area/ intensity for each.

In addition to this, the surveyors should get clear information on – maximum users in each area, minimum required bandwidth per user, number of floors, barriers for RF signals (like metal racks, elevators, walls, steel beams, ducts, concrete, asbestos, etc), the number of voice over wi-fi handsets (maximum concurrent calls/ coverage/ roaming), type of applications used on wireless (Mail/ Internet/ streaming media), etc.

What happens after the On-Site Site Survey?

Within a few days of completing the on-site wi-fi site survey, vendors present a detailed report to the customers. Some vendors charge for this report. Among other things, the report includes the number (and model) of access points required for a given site, floor plans with the access points position marked on them, coverage pattern clearly indicated for each access point (through the usage of various colours – each for a particular signal strength value), bill of materials for the wi-fi project along with the cost estimation, etc.

Do check if the vendor has taken the following factors in to consideration, while preparing the site survey report:

  • RF interference from neighboring access points (hotspots, AP’s from neighboring office, etc)
  • RF interference from non wi-fi devices like microwave ovens, cordless phones, satellite dishes, etc
  • Access Point coverage pattern on multiple floors (Its possible to reduce the total number of access points if  the AP’s from top and below floors are considered)
  • For sites with primarily outdoor coverage, its possible to integrate web based applications like Google earth to prepare (and present) more accurate results
  • Total number of users currently/ expansion planned for the near future/ user density in each floor
  • Channel Interference from other access points/ overlapping of all AP’s where voice roaming is required
  • Passive/ Active components to connect to the LAN backbone – cables, cable routes, POE switches, racks, etc
  • Basic coverage at some places (like lawns) / Powerful coverage at places with higher user density (like conference rooms)
  • Desired (minimum) rate of bandwidth per user
  • Length/ width / number of floors/ wi-fi obstacles/ radio types (802.11a/b/g/n)/ outdoor coverage/ wi-fi leakage outside the building, etc

Limitations/ Dis-advantages of Site Survey for Wireless Networks:

No matter how accurately the wireless site survey is done, its not possible to accurately determine the usage patterns/ expansions/ external interferences in the near future that might affect the wireless coverage. As mentioned earlier, over provisioning is not a good option with wireless networks – even though wireless controller takes care of the channel interference, there are only limited number of channels (especially in) 2.4 Ghz band & 5 Ghz band.

It is also very difficult to replicate the whole set-up for wireless network, during the site survey.  Even if that is done, the results obtained when a large number of concurrent users are simultaneously accessing the wireless network would be quite different from the site survey results. One more disadvantage is the fact that most of the site survey softwares cannot accommodate/ suggest directional coverage (using directional antennas) for special cases and it might have to be done manually.

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Related Article: Wireless (Wi-Fi) Networks – Basics