Nuts About Nets

Wireless Diagnostics & Troubleshooting

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Troubleshooting Wi-Fi Problems

Performance and stability of wireless / WiFi/ 802.11 networks:

Solving wireless problems can be tricky, but it's not rocket science. There are a few basic concepts to understand, but your success most likely will depend on your own observations and detective skills.  In the end, the majority of problems can be traced to RF interference, misdirected antennas and "dead spots". Four things to keep in mind as you tackle a WiFi-related problem:

1. It is most likely caused by interference, the existence of "dead spots", or poor reception that can be corrected by better locating/aligning an antenna.

2. The beacon strength of an access point is not a measure of the performance of your wireless network. Nor do you want to necessary associate with the access point with the strongest beacon signal.

3. In 802.11 b/g you have a choice of 14 channels (actually, 11 channels in North America) – where each channel is a range of frequencies.  There is tremendous overlap among adjacent channels (take a quick look at the image at the bottom of this page). The reason why it's important to make note of the overlap is that a wireless device that transmits in the frequency range of channel 1 not only interferes with channel 1 but probably also is wrecking havoc on channels 2, 3, etc…

4. When interference is the cause of poor performance, then the most common solution is to reconfigure your access point to use a different channel.  In certain situations you may need to track down the source of interference, but more often than not it will be coming from a source or location you have no control over. So, the simplest "fix" is to identify a channel that isn't subject to interference and reconfigure your wireless network to use that channel.

To get a jump-start in troubleshooting interference-related problems that degrade the performance of your WiFi network, then check-out some of ourlow-cost wireless diagnostic tools…

Troubleshooting Tips:

Can network discovery / WiFi scanning / site survey tools (e.g. NetStumbler, NetSurveyor) be used to solve interference problems?
No. These applications use an 802.11 wireless device that employs a chipset and protocol that only sees 802.11 packets – that is, it does not measure general RF transmissions. In fact, the site survey tool could be sitting next to a microwave oven that was emitting tons of RF energy and it wouldn’t see it (since the format of those transmissions do not conform to the 802.11 standard).  A WiFi site survey tool is designed to measure WiFi coverage by measuring the signal strength of the beacon emitted by an access point. The beacon strength of an access point is a reflection of how close the access point is to you – and does not provide any clue as to the presence or absence of other interfering wireless devices. An RF spectrum analyzer is the tool of choice when it comes to detecting / measuring general RF transmissions that could be interfering with a WiFi network.

When your network discovery wizard reports a list of available wireless networks (i.e. access points) to associate with you should always choose the one with the strongest signal strength — 'True', 'False' or 'Maybe'?
False.  The keyword in the above statement is 'always'.  If 'always' is left out then the answer is 'Maybe'.  All else being equal and without any diagnostic tools at your disposal then, yes, choose the wireless network with the strongest signal strength.  But here is why that may not be your best choice.  The signal strength reported by the network discovery wizard is the signal strength of a beacon.  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 wizard 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 then you will be customer 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.  The point we'd like to emphasize is that the key to performance is 'throughput', and though signal strength can affect that so will the number of client adapters that are competing for the same AP and the presence or absence of RF interference.  To learn more check-out ourNetStress tool.

Can a WiFi packet sniffer be used to solve interference problems?
No – for the same reason as above.  An 802.11 packet sniffer can only see 802.11 packets and can not measure general RF transmissions. If RF interference is a problem then, as a side effect, the packet sniffer might report an increase in packet retransmissions and lower data rates, but it can not be used to solve interference problems.

Besides competing 802.11 networks, what are some examples of other sources of interference?
There are a tremendous number of 802.11 devices in use and they can interfere with your wireless network – which is referred to as co-channel and adjacent channel interference. However, 802.11 is a very robust protocol that was designed to accommodate multiple wireless devices. Also, since 802.11 devices follow the same protocol they can work cooperatively – that is, two access points on the same channel can arbitrate use of the shared channel.

In most environments the presence of non-802.11 devices dwarfs the number of 802.11 devices. These non-802.11 devices could include microwave ovens, cordless phones, Bluetooth devices, wireless video cameras, wireless security devices, outdoor microwave links, wireless game controllers, Zigbee devices, fluorescent lights, motion detectors, automation sensors, WiMAX, and so on. Even bad electrical connections can cause broad RF spectrum emissions. Non-802.11 types of interference can significantly impact the performance of a wireless network by causing a loss of data throughput. In addition they can cause secondary effects such as rate back-off, in which retransmissions caused by interference trick 802.11 devices into thinking that they should use lower data rates for backward compatibility with older/slower 802.11 devices.

If your wireless network “appears” to be working, could interference still be a problem?
Yes.  Though 802.11 is a robust protocol that was designed to be resilient to interference from other wireless devices, its performance can be degraded by other wireless devices that transmit in the same bandwidth. When an 802.11 device detects RF interference then it will delay its own transmissions. As a result, though 802.11 packets generally reach their destination, the performance and throughput of your wireless are negatively impacted.  Whether they are degraded to the extent that users notice the problem depends on how heavily the network is used and whether or not it is important to users that the wireless network perform optimally.

Is it sufficient to perform a single RF sweep during installation to identify all sources of interference?
No. It is important to understand that, by nature, sources of interference come and go. New wireless devices are being installed all the time. Some devices run continuously – while others are used only at certain times of the day. There is nothing to be alarmed about, just take note that a single RF sweep on Tuesday morning doesn’t assure you that your environment is free of RF interference.

If there is a problem with the wireless network, is testing for RF interference one of the first or last things to check for?
One of the first…   In any communication network (wired or wireless) you need to have confidence that the physical layer is solid and intact. For example, in the case of a wired network, if the Ethernet cable is disconnected then why worry about troubleshooting the higher level protocols?  When we connect a computer to a wired, Ethernet network and the machine cannot communicate with other devices on the network then the first thing we check is that the cable is properly attached and the link lights on network adapter indicate the physical layer is functional.  The same is true of a wireless network, except there are no cables or link lights.  Whereas with a wired network we don’t worry so much about the physical layer because once a computer has been connected to a network and has been working for awhile then its rare for a cable to come unattached or to break, the same is not true for a wireless network. In the case of a wireless network, the quality of the physical connection can frequently change as new wireless devices or obstructions are introduced into the local environment that create interference or dead spots.

If RF interference is a problem, then what’s the solution?
Two choices: (a) if you know the source of the interference and it is an unnecessary device then remove it, or (b) reconfigure your wireless network to use a different channel that is not subject to RF interference. In our experience, (b) is the more likely solution since in a busy environment not only is it difficult to track down the source of the interference it may not be directly under your control (e.g. from an adjacent business or a neighbor).  So, assuming not all 802.11 channels are subject to interference, choose a channel that will give better performance.

A 2.4 GHz spectrum analyzer is the tool of choice for detecting and identifying sources of RF interference. 

Spectrum analyzers are a basic tool used for observing radio frequency (RF) signals – they give you a better picture of the RF environment to help identify and find devices interfering with your Wireless LAN (WLAN). Typically they’ll employ a 2-dimensional display where the vertical axis (Y-axis) represents the magnitude of a signal and the horizontal axis (X-axis) represents the frequency of a signal. Dedicated hardware spectrum analyzers can run into the tens of thousands of dollars (they employ many arrays of analog-to-digital converters). Recently, PC-based analyzers have appeared on the market. Check-out some of ourlow-cost wireless diagnostic tools that help you detect the presence of interfering devices and then choose the best channel for your wireless network.

Nuts About Nets develops innovative diagnostic tools for installing, optimizing and troubleshooting 802.11 (WiFi) wireless networks.  Our tools include RF spectrum analyzers, 802.11 channel analyzers, 2.4x and 5.x GHz channel / signal generators, and 802.11 packet injectors.  Our low-cost products offer an unbeatable value — they are easy to use, sensitive, accurate and a fraction of the cost of proprietary hardware test equipment.

Let’s get started on your wireless project…