| Wi-Fi wireless networks have grown in popularity | | | | are. That is, your laptop could be sitting next to a |
| in recent years and are found ubiquitously in | | | | microwave oven, sending all kinds of RF energy |
| business, industry and the home. Wi-Fi networks | | | | into the room, and the discovery tool would be |
| operate in the 2.4 GHz Industry, Scientific, Medical | | | | clueless to its existence. The discovery tool only |
| (ISM) band - a public band that is unlicensed by | | | | knows about 802.11 devices - that is, devices that |
| the FCC. Many types of wireless devices | | | | transmit RF energy according to the 802.11 |
| compete for air space in the 2.4 GHz ISM band - | | | | protocol - and can not see non-802.11 |
| and this introduces interference that negatively | | | | transmissions. Here's an analogy - you're in a large |
| impacts the performance of a wireless network. | | | | auditorium and you hear someone across the |
| A 2.4 GHz spectrum analyzer is the tool of choice | | | | room occasionally yell out "Hello, I'm Joe!". Joe is |
| for detecting and identifying sources of | | | | an access point and he occasionally sends a |
| interference and providing information that allows | | | | beacon to let whoever is out there know that |
| optimal configuration of a Wi-Fi network. | | | | he's there. It might be that Joe is really busy and |
| Overview | | | | talking with many people or he might be by |
| With wireless systems it is very difficult to predict | | | | himself. Or there might be a lot of noise in the |
| the propagation of radio waves and detect the | | | | auditorium that would make it difficult to carry on |
| presence of interfering signals without the use of | | | | a conversation with Joe from a distance. The |
| test equipment. Radio waves don't travel the | | | | discovery tool can't help you with that - it only |
| same distance in all directions - instead walls, | | | | hears Joe's occasional beacon "Hello, I'm Joe!". |
| doors, elevator shafts, people, and other | | | | 2.4 GHz Spectrum Analyzer. This is the |
| obstacles offer varying degrees of attenuation, | | | | instrument of choice for detecting and identifying |
| which cause the Radio Frequency (RF) radiation | | | | sources of RF interference. Spectrum analyzers |
| pattern to be irregular and unpredictable. In order | | | | are a basic tool used for observing radio |
| to achieve optimal reliability and throughput for | | | | frequency (RF) signals - they give you a better |
| your Wi-Fi ( 802.11) wireless network it is | | | | picture of the RF environment to help identify and |
| necessary to detect and identify sources of | | | | find devices interfering with your Wireless LAN |
| interference that impact negatively on its | | | | (WLAN). Typically they'll employ a 2-dimensional |
| performance. | | | | display where the vertical axis (Y-axis) represents |
| There are a multitude of electronic devices that | | | | the magnitude of a signal and the horizontal axis |
| transmit RF energy into the airspace. Wi-Fi (802.11 | | | | (X-axis) represents the frequency of a signal. |
| b/g) operates in the 2.4 GHz Industrial, Scientific, | | | | Dedicated hardware spectrum analyzers can run |
| and Medical (ISM) band. This particular range of | | | | into the tens of thousands of dollars (they employ |
| frequencies (2.401 GHz through 2.473 GHz) is | | | | many arrays of analog-to-digital converters). |
| public and its use does not require licensing by the | | | | Recently, PC-based analyzers have appeared on |
| FCC. As a result, this band tends to get rather | | | | the market. Yet they are also fairly expensive - in |
| crowded - not only with 802.11 devices but also | | | | the neighborhood of $4000. AirSleuth is an |
| microwaves, bluetooth devices, cordless phones, | | | | affordable 2.4 GHz spectrum analyzer available |
| baby monitors, audio/video senders, wireless | | | | from Nuts About Nets, LLC that displays RF data |
| cameras, etc. | | | | in a variety of diagnostic views that help you |
| Professional installers of wireless computer | | | | detect the presence of interfering devices and |
| networks attempt to optimize their client's Wi-Fi | | | | then choose the best channel for your wireless |
| network by strategically choosing a subset of the | | | | network. |
| 2.4 GHz band for use by their wireless network | | | | Examples |
| devices. The 2.4 GHz range of 2.401 - 2.473 GHz | | | | Click here to see 5 examples of measurements |
| is separated into 11, 22 MHz channels. Note that | | | | taken with the AirSleuth 2.4 GHz spectrum |
| 2.473GHz - 2.401GHz = 72 MHz, yet 11 times 22 | | | | analyzer. In each figure there are 3 items to take |
| MHz is 242 MHz. So, how does 242 MHz fit into | | | | note of:a. The maximum value on the Y-axisb. |
| 72 MHz? Answer - the channels overlap. An | | | | The horizontal white lines that denote the |
| installer can configure the wireless router or | | | | boundaries of the 11, overlapping 802.11 channelsc. |
| access point (AP) to use channel 6, and then all | | | | The location (i.e. frequency) and height (relative |
| data communication will occur over the range of | | | | signal strength) of the major RF peaks |
| frequencies associated with channel 6 (i.e. 2.426 | | | | Figure 1. In this figure the horizontal white lines |
| GHz - 2.448 GHz). But if other wireless devices | | | | show the boundaries of the 11, 22 MHz channels in |
| (802.11 or non-802.11) are also transmitting over | | | | the 2.4 GHz band. Notice how adjacent channels |
| this range of frequencies then your wireless | | | | overlap with one another. |
| network will suffer. So, when installing a wireless | | | | Figure 2. The air space is relatively quiet and there |
| network or troubleshooting a poorly performing | | | | are no wireless devices transmitting in the 2.4 |
| one, it is important to choose a channel that is not | | | | GHz band. Notice the maximum value on the |
| subject to interference from other devices - i.e. | | | | Y-axis is '3.0' Relative Signal Strength units - this is |
| you don't want your wireless network to | | | | small. The reason you see peaks (and the output |
| compete with other devices for the same range | | | | isn't totally flat) is the display employs automatic |
| of frequencies. | | | | scaling, such that the largest peak (regardless of |
| Wi-Fi Diagnostic Tools | | | | how small it really is) will fill-up the screen. We |
| There are two categories or types of tools that | | | | know this is what background noise looks like |
| are available for helping you choose the best Wi-Fi | | | | because the maximum value on the Y-axis is |
| channel for your wireless network - WiFi Scanners | | | | Figure 3. A microwave oven is running. Here you |
| (i.e. 802.11 discovery tools) and 2.4 GHz Spectrum | | | | can see that it emits RF energy across the entire |
| Analyzers: | | | | 2.4 GHz band. |
| Wi-Fi scanner or 802.11 (WLAN) discovery utility. | | | | Figure 4. A 2.4 GHz cordless phone has been |
| The best known in this category is NetStumbler ( | | | | powered on. Notice the height of the peak in the |
| . The discovery tool will report the Service Set | | | | vicinity of channel 1 is around '140' units and the |
| Identifier (SSID) for each access point (AP) it | | | | peak in the vicinity of channel 11 is around '110' |
| detects, along with the channel used by the AP. | | | | units. These are strong enough to knock-out |
| The way this works is that roughly every 100 | | | | wireless networks configured to use either |
| mSec an AP sends an "I'm here" beacon - and | | | | channel 1 or channel 11. In addition, because of the |
| the discovery tool (running on your laptop and | | | | way that channels overlap the first peak would |
| using its 802.11 wireless adapter) picks up that | | | | probably also interfere with a wireless network |
| beacon and adds the SSID to its list. In addition, | | | | using channel 2, and the second peak would also |
| the discovery utility may report the SNR | | | | interfere with wireless networks using channels 9 |
| (signal-to-noise) ratio for each AP, which is | | | | or 10. |
| approximately an indication of how close the AP is | | | | Figure 5. Shows the pattern of activity from a |
| to your current location. Though this is useful | | | | wireless network (configured to use channel 6) as |
| information, it doesn't tell you anything about | | | | it is actively transmitting a large stream of data. |
| non-802.11 devices or even how busy the APs | | | | |