FAQ: Wi-Fi Diagnostics

When trouble-shooting a poorly performing Wi-Fi network it is often the case that RF interference from another wireless transmitter is the culprit. In order to better understand the problem and come up with a solution it helps if you can make the distinction between interference and congestion. There are 4 possible scenarios -- 3 types of interference and one type of congestion:

[1] One type of interference occurs when a non-802.11 transmitter causes your Wi-Fi device to back-off. Included in the 802.11 standard is a sensing mechanism. Before attempting to transmit, a Wi-Fi device will check if the coast is clear – if not, then it will delay a random period before checking again.

[2] Another type of interference occurs when an 802.11 device on an adjacent, overlapping channel, again causes your Wi-Fi device to back-off. This is referred to as Adjacent Channel Interference (or ACI) and is similar to [1]. By the way, ACI occurs in the 2.4 GHz ISM band since adjacent channels overlap, but not in the 5.x GHz ISM band (since channels do not overlap).

[3] The third type of interference occurs when a non-802.11 transmitter causes packets to be corrupted at the receiver – which may be referred to as the hidden node problem. For example, suppose you have an interferer that is far away from an AP – so, the AP thinks the coast is clear and continues to transmit. But the interferer may be close enough to the receiving client device and cause packets to be corrupted.

[4] Congestion occurs when two or more Wi-Fi devices are using the same channel and have to take turns transmitting because the channel's bandwidth is full. This is called Co-Channel Congestion (CCC). To be clear – there is nothing wrong with multiple Wi-Fi devices using the same channel. However, when the traffic exceeds that channel’s bandwidth, then the Wi-Fi devices have to arbitrate use of the channel – that is, take turns -- and this causes performance to suffer.

Of these four scenarios, [3] is probably the worst. This is because it forces the 802.11 protocol and the transmitting device to re-transmit packets. Re-transmitting packets is a very inefficient use of the transmitter's time, resources and CPU.

An AP periodically sends beacon frames at 100 mSec intervals -- these are short advertisements for their network. Since the duration of each beacon is on the order of microseconds and since they occur at intervals, then these pulses of RF energy are difficult to detect. Furthermore, even if an RF spectrum analyzer could detect and display them, the pulse is so short your eyes would have a hard time seeing them on a display.

On the other hand, a Wi-Fi adapter, with its 802.11 chipset, can decode beacons and, hence, can "see" them. However, even though your network discovery tool can detect beacons doesn't mean they pose a problem. Since beacons are so brief and only transmitted at intervals, then this leaves plenty of channel bandwidth for data transmissions. It's the transmission of data packets where most of the work is done.

Keep in mind that whereas APs are constantly beaconing, most of the time they are not transmitting data packets. Only when an AP is actively transmitting data packets can a spectrum analyzer detect its RF energy or WifiMETRIX detect Adjacent Channel Interference (ACI) or Co-Channel Congestion (CCC).

In the North American 2.4 GHz ISM band there are 11 overlapping channels. Each channel is 22 MHz wide, with only 5 MHz unique to each channel. That is, the other 17 MHz of each channel overlaps with adjacent channels.

When APs are configured to use the same channel then they can share its bandwidth. Since they are on the same channel they can communicate with one another and arbitrate the use of the channel -- i.e. take turns.

When two APs are configured to use different, but overlapping channels, then they can not communicate and share bandwidth. Instead, they detect one another as interference or noise. When an AP detects interference it delays transmissions, waiting until their channel becomes clear. As a result, performance of the Wi-Fi network will suffer due to these delays.

So, even though it may appear that intervening channels 2, 3, 4, 5, 7, 8, 9, 10 are free to use, they really are not. If you were to use them, then they would interfere with APs using channels 1, 6, 11 -- and, vice-versa.

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.

The answer is usually 'No'. Network discovery (i.e. Wi-Fi scanner, site survey) applications use your device's Wi-Fi radio that only sees 802.11 packets – that is, it does not measure raw, RF energy. In fact, a site survey tool could be located next to a microwave oven that was emitting tons of RF energy and it wouldn’t see it (since the microwave oven doesn't transmit 802.11 packets). A Wi-Fi site survey tool is designed to measure Wi-Fi coverage by measuring the signal strength of intermittent beacons emitted by access points in your local vicinity. The beacon strength of an access point is a reflection of how close that 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 your Wi-Fi network. And WifiMETRIX goes a step further by computing the best channel in an environment polluted by other RF transmitting devices.

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 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 are rarely concerned about the physical layer because once a computer has been connected to a network and has been working for awhile then it's unusual for a cable to come unattached or to break, the same can not be said 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.

Two choices: (a) if you know the source of the RF interference originates from a non-essential 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 RF interference, choose a channel that will give better performance.

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.