It can be difficult to determine if noise is corrupting a wireless (802.11a/b/g/n) signal because the primary symptom for a user is simply slow performance. Because slow performance can caused at any step along the path between a person's computer and a distant Internet site, it can be tough to know what is happening.
Noise that is interfering with an 802.11 wireless signal isn't always constant, but in my experience with 802.11 and with cable TV wired networks using lower frequencies, it doesn't take very much noise to significantly reduce performance.
Why the big drop in performance? TCP. Viewing web pages, downloading files or email, and most other activities on the Internet depend on TCP. TCP was designed to be used on low-noise, wired networks. Every bit of data sent over TCP is numbered sequentially for the conversation between the user's computer and the remote computer or server. If a new bit of data (a "packet") arrives with a number that is too high, then TCP knows that a packet was missed. TCP asks the sender to retry, and tells the sender to slow down.
See the problem? TCP assumes that a packet will be lost only due to high traffic, not because of noise that might corrupt a packet and cause it to be discarded. When a packet is lost because of traffic, slowing down is the best possible response for all parties. When a packet is lost because of a little bit of noise, everything slows down even though the packet really should be resent immediately and the transmission speed should remain constant. Lots of smart people are working on this, although no real standard has emerged. Here is a good academic paper describing this TCP peformance problem and a proposed solution.
Detecting noise with Wireshark
First, download a copy of Wireshark. Start it up for the first time. You need to record, or capture, some packets to look for a particular type of TCP packet.
To capture packets, click on Capture, Interfaces, then click Options on the far right next to your network card, then press Start.
Open up your web browser and go to www.yahoo.com or www.msn.com (or any other site that's been slow for you and that has a lot of stuff on the page). After the page has loaded, click Stop on the capture.
Click on the label of the Info column to sort by the column contents. Then scroll down and look for this type of information about TCP duplicate ACK's:
If you have more than one line like these above, then your wireless network almost surely has interference. Because most of us use inexpensive wireless gear, we have to share the frequencies with other devices belonging to our neighbors.
What can I do about it?
The easiest thing to do is try a different channel. On 802.11b/g wireless access points and routers, they show you 11 possible channels. Don't believe them. Only channels 1, 6, and 11 don't interfere with each other. So if you're using channel 2 and your neighbor is using channel 3, expect problems.
The general rule for trying to reduce interference is to use a channel number farthest away numerically from any other wireless devices in the area. Usually your laptop or other wireless device can tell you what networks it sees around and what channels they are using. Just pick the most remote channel. If you can't see any other wireless networks besides your own, still change the channel to either 1, 6, or 11 and stop using your old channel-- again, try to be as far away from your old channel as possible because you know there is noise on that channel.
I hope this helps!
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