When most individuals are asked to define a wireless site survey, the usual response is that a site survey is for determining RF coverage.
While that definition is absolutely correct, the site survey encompasses so much more, including looking for potential sources of interference as well as the proper placement, installation, and configuration of 802.11 hardware.
In the following we will discuss the often overlooked, yet necessary, spectrum analysis requirement of the site survey and the often misunderstood coverage analysis requirement.
During the coverage analysis process, a determination will be made for the proper placement of access points, the transmission power of the access point radio card, and the proper use of antennas.
Although not mandatory, performance and application testing might also be an optional requirement of an 802.11 wireless survey.
Depending on the purpose of the wireless network, different tools can be used, which is why the site survey interview and planning process is so important. We will also cover the variety of tools that may be used as part of your site survey arsenal.
Mandatory Spectrum Analysis
Before conducting the coverage analysis survey, locating sources of potential interference is a must. Unfortunately, many site surveys completely ignore spectrum analysis because of the high cost generally associated with purchasing the necessary spectrum analyzer hardware.
Spectrum analyzers are frequency domain measurement devices that can measure the amplitude and frequency space of electromagnetic signals. Spectrum analyzer hardware can cost upward of $40,000 (in U.S. dollars), thereby making them cost prohibitive for many smaller and medium size businesses.
The good news is that several companies have solutions, both hardware and software based, that are designed specifically for 802.11 site survey spectrum analysis and are drastically less expensive.
Figure below depicts a 2.4 GHz hardware spectrum analyzer.
In order to conduct a proper 802.11 spectrum analysis survey, the spectrum analyzer will need to capable of scanning both the 2.4 GHz ISM band and the 5 GHz UNII bands. A company named Cognio sells a software-based solution that works with a special PCMCIA card.
This software-based spectrum analyzer was designed specifically for 802.11 site surveys and can correctly identify specific energy pulses such as a microwave oven or cordless phone.
So why is spectrum analysis even necessary? If the background noise level exceeds –85 dBm in either the 2.4 GHz ISM band or 5 GHz UNII bands, the performance of the wireless network can be severely degraded.
A noisy environment can cause the data in 802.11 transmissions to become corrupted. If the data is corrupted, the cyclic redundancy check (CRC) will fail and the receiving 802.11 radio will not send an ACK frame to the transmitting 802.11 radio.
If an ACK frame is not received by the original transmitting radio, the unicast frame is not acknowledged and will have to be retransmitted. If an interfering device such as a microwave oven results in retransmissions above 10 percent, the performance or throughput of the wireless LAN will suffer significantly.
Wi-Fi data networks can handle a retransmission rate of up to 10 percent, but a Voice over Wi-Fi (VoWiFi) network needs to limit packet loss to a rate of 2 percent or less.
Interfering devices may also prevent an 802.11 radio from transmitting. If another RF source is transmitting with strong amplitude, an 802.11 radio can sense the energy during the clear channel assessment (CCA) and defer transmission.
If the source of the interference is a constant signal, an 802.11 radio will continuously defer transmissions until the medium is clear. In other words, a strong source of RF interference could actually prevent your 802.11 client stations and access point radios from transmitting at all.
It is a recommended practice to conduct spectrum analysis of all frequency ranges, especially in the 2.4 GHz ISM band. The 2.4 to 2.4835 GHz ISM band is an extremely crowded frequency space.
The following are potential sources of interference in the 2.4 GHz ISM band:
- Microwave ovens
- 2.4 GHz cordless phones, DSSS and FHSS
- Fluorescent bulbs
- 2.4 GHz video cameras
- Elevator motors
- Cauterizing devices
- Plasma cutters
- Bluetooth radios
- Nearby 802.11, 802.11b, or 802.11g WLANs
- Wireless Internet service providers (WISPs)
One of the first things you should determine during the site survey interview is the location of the microwave ovens. Microwave ovens typically operate at 800 to 1,000 watts. Although microwave ovens are shielded, they can become leaky over time.
Commercialgrade microwave ovens will be shielded much better than a discount microwave oven that you can buy at many retail outlets.
A received signal of –40 dBm is about 1/10,000 of 1 milliwatt and is considered a very strong signal for 802.11 communications. If a 1,000 watt microwave oven is even .0000001 percent leaky, the oven will interfere with the 802.11 radio.
Figure below shows a spectrum view of a microwave oven.
Note that this microwave operates dead center in the 2.4 GHz ISM band. Some microwave ovens can congest the entire frequency band. Because of the extreme crowding of the 2.4 GHz ISM band, many enterprise deployments are switching to 802.11a equipment that transmits in the 5 GHz UNII bands.
Switching to an 802.11a WLAN is often a very wise choice in the enterprise because the 5 GHz UNII bands are currently not very crowded and there are more choices on channel reuse patterns.
Not nearly as many interfering devices exist, and there are just not many neighboring 802.11a networks that can potentially cause interference. Although there is much less interference currently present at 5 GHz as compared to 2.4 GHz, this will change over time.
Just as everyone moved from 900 MHz to 2.4 GHz to avoid interference, the “band jumping” effect will also catch up with 5 GHz. Current potential sources of interference in 5 GHz UNII bands include the following:
- 5 GHz cordless phones
- Radar
- Perimeter sensors
- Digital satellite
- Nearby 802.11a WLANs
- Outdoor wireless 5 GHz bridges
Although many devices can cause problems in both frequencies, one of the most common causes of interference will be other wireless LANs. Strong signals from other nearby WLANs can be a huge problem, especially in a multitenant building environment.
You may need to cooperate with the neighboring businesses to ensure that their access points are not powered too high and that they are on channels that will not interfere with your access points.
Once again, due to the proliferation of 2.4 GHz WLAN equipment, many businesses are now choosing to switch to 802.11a solutions. After locating the sources of interference, the best and simplest solution is to eliminate them entirely.
If a microwave oven is causing problems, consider purchasing a more expensive commercial-grade oven that is less likely to be a nuisance. Other devices, like 2.4 GHz cordless phones, should be removed and a policy should be strictly enforced that bans them.
5.8 GHz cordless phones operate in the 5.8 GHz ISM band, which overlaps with the upper UNII band (5.725 GHz to 5.825 GHz). Indoor use of 5.8 GHz phones will cause interference with 802.11a radios transmitting in the upper UNII band.
If interfering devices cannot be eradicated in the 2.4 GHz bands, consider moving to the less crowded 5 GHz UNII bands. A Voice over Wi-Fi (VoWiFi) network needs to limit packet loss to a rate of 2 percent or less, meaning that a very thorough spectrum analysis of the 2.4 GHz ISM band is a necessity.
Currently, Wi-Fi VoIP phones operate using High-Rate DSSS (HR-DSSS) technology and the radios therefore transmit in the very crowded 2.4 GHz ISM band.
In the very near future, 802.11a VoWiFi phones will exist that transmit in the less crowded 5 GHz UNII bands. If your WLAN is being used for either data or voice or for both, a proper and thorough spectrum analysis is mandatory in an enterprise environment.
It is important to make sure you know what your client devices are capable of before determining the spectrum to use. If all your client devices are restricted to using 2.4 GHz, then that may be your only option and you will need to be able to plan and engineer around the environment.
