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RF Spectrum Analysis versus Network Discovery

Two categories of tools are commonly used in troubleshooting WiFi problems — 802.11 network discovery tools (also known as WiFi scanners) and RF spectrum analyzers.

An 802.11 network discovery tool will report the Service Set Identifier (SSID) for each access point (AP) it detects, along with the channel used by the AP. Approximately every 100 mSec an AP transmits a small beacon packet and a discovery tool (running on your laptop and using its 802.11 wireless adapter) detects the beacon and adds the packet information (including the AP's SSID) to its list of known access points. In addition, the discovery utility may report signal strength (in dBm units) of the beacon as detected by the client adapter. The beacon's signal strength is a reflection of how close the AP is to your current location. Though this is useful information, it does not tell you anything about non-802.11 devices or even how busy the access points are.  That is, your laptop could be sitting next to a microwave oven and the discovery tool would be clueless as to its existence. The discovery tool only knows about beacon packets transmitted by 802.11 devices and can not see non-802.11 transmissions.

In contrast, an RF spectrum analyzer is the instrument of choice for detecting and identifying sources of RF interference. Spectrum analyzers are a basic tool used for observing radio frequency (RF) signals. Since they detect all RF transmissions then they provide a much better picture of the RF environment, which then helps you identify and, perhaps, locate devices that could be interfering with your WiFi network. Typically an RF spectrum analyzer will employ a 2-dimensional display where the vertical axis (Y-axis) represents the strength of a signal and the horizontal axis (X-axis) represents the frequency of a signal.

AP Beacon Strength Is Not A Measure Of Performance

The signal strength reported by a network discovery tool is the signal strength of a beacon as measured by the 802.11 wireless adapter installed on your laptop or desktop machine. Each access point (AP) sends out a short pulse or beacon of information approximately every 100 mSec. It's equivalent to an 'I'm over here!' shout. It does not expect a response from the 802.11 client adapters that may hear it — it's just a one-way shout. The signal strength that the network discovery tool reports is the signal strength of a beacon, and the signal strength of a beacon is a reflection of how close that AP is located to you.  It is not a reflection of the performance or throughput you can expect by associating with that AP — rather, it is an indication of the AP's physical location relative to you. If the AP with the strongest beacon has 24 client adapters associated with it that are actively transmitting and receiving information, and if you connect with that AP then you will be client number 25 and your network connection will seem slow. On the other hand, if you instead choose to associate with an AP whose beacon strength is weaker but which is not associated with other client adapters, then you will likely experience better performance. Furthermore, the AP with the strongest beacon signal may be using a channel that is subject to RF interference — again, degrading its performance. When it comes to networking (both wired and wireless) what we care most about is performance. And the key to performance is 'throughput' (i.e. bytes-per-second). Though a beacon's signal strength can affect it's performance, what's more important is the number of client stations that are competing for the same AP and whether the channel currently used by the AP is subject to RF interference from other wireless devices in the vicinity.

Choosing The Best 802.11 Channel

The frequencies used by the WiFi(i.e. 802.11) standard are unlicensed and shared by a variety of wireless devices — both 802.11 and non-802.11. The public, unlicensed 2.4x and 5.x GHz ISM (Industrial, Scientific, Medical) bands used by WiFi devices are divided into a number of channels. Each channel is a well-defined range of frequencies. WiFi access points provide a set of available channels to choose from — for example, in North America you can choose from channels [1,11] when setting-up a wireless LAN (WLAN) in the 2.4x GHz band. In most environments certain WiFi channels make better choices for a WLAN since they are subject to less RF interference. RF interference is generally caused by transmissions from other wireless devices in the area that compete for the same region of the RF spectrum. Setting the WiFi channel appropriately provides one way to avoid sources of RF interference. But since you can't see RF interference, then how would you know which channel to choose? This is where RF spectrum analyzers come in. Because they allow you to "see" all RF transmissions occurring within the frequency range of interest, then it becomes more clear which frequencies (and channels) should be avoided. The best channels to use for an 802.11 network are ones where there are no peaks of RF interference, as detected by a spectrum analyzer.

Taking this a step further, here's an interesting fact about the 802.11 standard that few people are aware of. Suppose you've been asked to install a new access point, and for reasons beyond your control you can choose from channels 1 or 2. Furthermore, you know the business next door is using channel 1. So, which channel should you use — channel 1 or channel 2? The obvious answer that everyone chooses is channel 2 — since your neighbor is using channel 1 — but you'd be wrong. In 802.11 b/g there is tremendous overlap between adjacent channels — so two 802.11 access points transmitting on channels 1 and 2 will interfere with one another and there will be times when they bring one another to a complete standstill. On the other hand, when two access points are configured to use the same channel they can "see" one another and arbitrate or share the use of the channel. It's not an optimal situation, but because they are using the same channel then they can communicate with one another and coordinate their transmissions. In this case, 50% throughput is better than no throughput. The point here is that though RF interference is probably the most important factor to take into consideration when choosing the best WiFi channel, 802.11 is sufficiently robust and complicated there will be times when the obvious choice will not be the best choice from a performance standpoint.