How to Minimize Wireless Interferen
01-17-2010 06:02:22 PM
This guide will give you a brief introduction on how to minimize wireless interference in your home network. In the home environment, many things can interfere with consumer wireless network devices. These devices, referred to by their IEEE standard number (802.11b, 802.11g, and 802.11n) operate in the 2.4 GHz band, which is a highly crowded spectrum. By use of a simple freeware tool, you can optimize your wireless network to avoid interference from other wireless networks operating near you.
The following terms are used in this guide:
802.11b - This is an older wireless standard that can operate at data rates up to 11 Mbps. Access points using only 802.11b are not very common anymore.
802.11g - The most common wireless standard, and the one that is used in the U-Verse RG. It can operate at data rates up to 54 Mbps by the standard, although some manufacturers have used proprietary methods to achieve higher data rates, such as up to 108 Mbps. These data rates can usually only be achieved if the same manufacturer's equipment is used at both the access point and on the client computer.
802.11n - The newest wireless standard, can operate at data rates up to 300 Mbps. 802.11n is subject to more interference than 802.11b or g because it uses a larger slice of the 2.4 GHz spectrum.
Channel - The 2.4 GHz spectrum is divided into channels, numbered 1 through 11. Most wireless access points, including the U-Verse RG, can be set to use a particular channel. Selecting which channel to use is the most important thing to avoid interference. You would ideally like to select a channel that is not in use by neighbors, and far away from other channels that are in use.
Received Signal Strength (RSS or RSSI) - This is the strength of the wireless signal in the air at the receiver. This ranges from -100 dBm (the noise floor) up to 0 dBm (theoretical maximum). Signal strengths below about -80 dBm are generally too weak for a laptop to connect to. Typical signal strength inside the house, if your wireless access point is working properly, is -60 dBm up to -40 dBm.
SSID - This is the name of the wireless network, which is set at the access point. The U-Verse RG has a default SSID of "2WIRExxx", where xxx is a 3-digit number.
Frequency Spectrum - This is a range of elecromagnetic frequencies. The ones we are interested in are the ones used by 802.11, which ranges from 2.412 GHz (center frequency for channel 1) up to 2.462 GHz (center frequency for channel 11).
Once you have downloaded InSSIDer, install it and run it on a laptop with a wireless network card. At the top of the screen will be a pull-down menu listing all wireless network cards in the computer. Select your wireless network card (most laptops only have one). Also select the "2.4 GHz" button to the right of the pull-down menu. After those selections are complete, click the "Start Scanning" button on the left. After a few minutes of scanning, you will end up with a screen that looks like this:
The top part of the window shows all the different wireless networks that have been seen by your laptop. For each wireless network, listed is the vendor (the manufacturer of the access point), a check box which controls whether this wireless network is graphed in the bottom graphs or not, a signal strength meter (this also has a lock icon on it if the wireless network is protected by some type of encryption), the MAC address of the access point, the SSID (name of the wireless network), the channel that the wireless network is using, the received signal strength indicator (RSSI) in dBm, the encryption/security protocol the network is using, whether the network is an access point or an ad-hoc (computer-to-computer) network, the highest supported network speed in Mbps, and the time that the network was first and last seen. The location column only functions if you have a GPS peripheral installed in your laptop.
On the lower left is a graph that shows the received signal strength (RSSI) over time. The colors correspond to the color of the check box in the list at the top.
On the lower right is the most important graph, it is a graph of all the wireless networks seen and the frequency/channel that they are operating on. As you can see here, my home network (SSID = danzone) is operating on channel 9.
802.11 wireless does not use just one certain channel, unfortunately. The channel that the wireless network is operating on is where the frequency is centered, but the actual frequencies used extend away from that channel in both directions, and actually overlap 2 channels on each side. Thus, my access point, while it is centered on channel 9, is using frequencies from channel 7 up to channel 11, and is subject to interference anywhere in that range. The width of each set of frequencies used is 20 MHz (this is important to know later when we talk about 802.11n). The set of frequencies used can also be called a lobe, named from the shape of the frequency curve.
Wireless Access Point Location
If you have a large house, especially a 2-story, you may want to take the laptop at this point to all the rooms in the house where wireless is likely to be used, and watch the received signal strength graph (lower left) over time. Make sure that you have an adequate wireless signal (greater than -60 dBm) in all rooms where wireless will be used. If you have a room where the received signal strength drops below -60 dBm, you may want to look into relocating the wireless access point, or adding an additional wireless access point in your home.
Identifying the Best Channel to Use
Next, you may want to clean up the channel graph by removing wireless networks that are too weak to interfere. Go through the list and uncheck any wireless network where the received signal strength (RSSI) is less than -85 dBm. These networks are generally too weak to worry about, and removing them from the channel graph will let you see the stronger networks.
From the graph on the lower right, you will be able to get a feel for what will be the best channel for you to use. Take a look at the other wireless networks in your area, and see if there is a spot in the channel map that looks the least busy. For my location shown above, that would be channel 8 or 9. There are 2 networks operating on channel 6 (SARA and Buffalo), and one network operating on channel 11 (user-PC-wireless). SARA and Buffalo have frequency components extending up to channel 8, and user-PC-wireless has frequency components extending down to channel 9. However, the frequency components at the tail ends of those networks are small, leaving a lot of room at channel 8 or 9 for my network to operate. The lower part of the spectrum (channels 1 through 6) is very crowded in my area, so I probably shouldn't try to use those channels. They may work, but there is more potential for interference.
Changing The Channel on the U-Verse RG
To change the channel on the U-Verse RG, go to the following link:
At the top, you will see a pull-down menu for the wireless channel. Select the channel that you identified as being the least busy, and then click Save at the bottom.
Changing the Channel on other Wireless Access Points/Wireless Routers
Other routers have similar settings to let you change the wireless channel. Follow your manufacturer's directions to configure your router or wireless access point.
Automatic Channel Selection
Some routers have the ability to automatically select the wireless channel, and will try to select the one that is the least busy automatically. The U-Verse RG is supposed to do this, but it doesn't always work. Other routers may also do this, and you may have to turn this feature off if you want to select the channel manually.
Once you have changed your channel, your laptop will disconnect and reconnect to your wireless network. Verify with InSSIDer that your network is operating on the channel you selected.
Using your Own Wireless Router or Access Point
If you have your own wireless router or access point, you will want to turn the wireless on the U-Verse RG off to prevent it from interfering with yours. Go to the following link:
In the upper right, click the "Disable" button next to where it says "Wireless" to turn the built-in wireless in the U-Verse RG off.
If you have an 802.11n wireless access point or wireless router, there are additional things you need to know. 802.11n operates as if it was two separate 802.11g access points. One set of frequencies is the "primary" set and operates just like an 802.11g access point. This set of frequencies (the primary lobe) is 20 MHz wide, just like an 802.11g access point. Any clients that are 802.11g-only connect to this primary lobe.
The 802.11n access point then selects another 20 MHz wide lobe (the secondary lobe) exactly 4 channels away from the primary channel. Thus, for example, if the primary lobe is on channel 9, the access point begins operating a secondary lobe on channel 5. This results in a total of 40 MHz of bandwidth being used for the 802.11n access point, overlapping a total of 8 channels (in this example, channels 3-7 for the secondary lobe, and 7-11 for the primary lobe).
This is why 802.11n is generally subject to more interference than 802.11g, because the large slice of the spectrum that it uses can pick up more interference (double the width of an 802.11g access point).
InSSIDer will report the channel that the 802.11n access point is operating on in 2 different ways depending on whether you are scanning it from an 802.11g-only laptop or from an 802.11n laptop. If you are scanning from an 802.11g-only laptop, the channel will be reported as the channel of the primary lobe (channel 9 in the above example), and you will not be able to see the secondary lobe. If you are scanning from an 802.11n laptop, the channel will be reported as the channel where the primary and secondary lobes touch each other, which is channel 7 in the above example. Also note that InSSIDer will not report the correct width of an 802.11n access point -- it always shows a width of 20 MHz, even though the access point frequency spread is double that.
As a further complication, some laptops with 802.11n network cards can be set to operate in 802.11n single-lobe mode. In this mode, the laptop connects using 802.11n protocols, but only to the primary lobe, resulting in a maximum speed of 150 Mbps. If you have a laptop set to do this, InSSIDer will report the 802.11n access point channel as if the laptop was operating in 802.11g mode (channel reported as the primary lobe).
Potential Sources of Interference other than 802.11 Networks
Many consumer devices operate in the 2.4 GHz band. All of these devices can interfere with your wireless network, and they won't show up on InSSIDer's graphs. Some of these devices are:
Microwave Ovens - A very common offender. Microwave ovens can have interference that appears all over the spectrum, but many microwave ovens have interference specifically near channel 11 and channel 7, and somewhat in between as well. If the microwave oven is a problem in your household, try using the lower channels (1-3) for your wireless network.
Cordless Phones - Another common offender. Older cordless phones operated in the 900 MHz band, these won't interfere with wireless. Some new cordless phones offer 5 GHz capability, or 1.9 GHz capability (the 1.9 GHz phones are marketed as "DECT 6.0"). Either of these types of cordless phones won't interfere with 802.11 wireless. 2.4 GHz phones have 2 different types: One type hops frequencies all over the 2.4 GHz spectrum, causing a lot of interference on every channel. A second type stays on one particular frequency, and can be selected to others by the user using a switch or by programming the phone through the keypad. If you have the former, you may be out of luck. If you have the latter, they typically operate in the low channels (channel 1-3), but you will have to look at the specs to see the exact frequencies. To correlate the frequencies with the 802.11 channels see the 802.11g article at Wikipedia.
Bluetooth - Bluetooth operates in the 2.4 GHz frequency band, but is typically very low power. Additionally, there are protocols that wireless and Bluetooth use to peacefully coexist, so it shouldn't be too much of a problem.
Wireless Video Senders - Most of these devices operate in the 2.4 GHz band, and their frequencies can overlap 4-6 channels. You will have to look at the specifications to see what frequencies are in use.
Baby Monitors - These devices operate very similarly to the wireless video senders, but typically don't overlap as many channels.
802.11 NIC Quality
Another item frequently overlooked is the quality of the 802.11 NIC used in the laptop. Most laptops with built-in wireless have a decent wireless chipset, but some do not. In particular, the Intel 2200 NIC is one known to have frequent problems. For 3rd-party 802.11 NICs, it is unfortunate but many people have problems with the 2-Wire NICs provided by AT&T. A name-brand 802.11 NIC such as one from Linksys, D-Link, NetGear, TrendNet, etc. will likely have much better performance.
While the use of InSSIDer won't eliminate all wireless problems, it can be used effectively to identify a channel that has the least likelihood for interference from other wireless networks, can be used to verify that your access point is operating on the channel that is assigned, and can be used to verify signal strength in all parts of your house. WIth that plus the other points in this guide, you should be able to get optimum network performance on all of your wireless computers.