I get asked this question rather too often, so I'm posting my short answer here. The answer is rather more complex than it ought to be, and depends on a huge number of factors.

The most important is the receive sensitivity of your equipment. Many manufacturers fail to publish this data, but those that do will generally rate their radios by dBm at various data rates. As an example, let us take the venerable ORiNOCO Gold 802.11b "Classic" card. Its receive sensitivity is:

• -94 dBm at 1 Mbps
  
• -91 dBm at 2 Mbps
  
• -87 dBm at 5.5 Mbps
  
• -82 dBm at 11 Mbps
  

In theory this means, in order to operate at 11 Mbps, this card must be consistently receiving a minimum signal level of -82 dBm. Any less and it is likely to drop to one of the lower rates; if you get as low as -94 dBm then the connection may drop altogether. As I mentioned before, many manufacturers do not quote their receive sensitiviy for their adapters; if you have one of these, I suggest picking a conservative figure such as -76dBm at 11 Mbps, which is the number for the Belkin F5D6020.
The signal level you receive in an unobstructed environment depends on the transmitter power, the gain of the two antennas involved, and the distance between them, as well as any loss between the antenna and the radio at each end.

In practice, radio waves behave unpredictably in a number of ways. First, the signal will fade out due to multipath effects (radio waves that bounce off objects and increase or decrease the signal that you receive). The further the receiver is from the transmitter, and the more objects between them, the higher this effect will be. Walls, people, electronic equipment, rain/snow/ice/fog are all quite effective at decreasing your signal level. In a typical home or small office environment without too many obstructions, a 10dB variation in signal level is quite normal. So, if you are looking at a NetStumbler scan and the signal is consistently around -65 dBm, it could drop to -75 dBm when somebody comes over to talk to you.

Summary so far:
(Received signal) = (transmit power) - (loss between transmitter and antenna) + (transmit antenna gain) - (path loss) - (multipath and obstruction loss) + (receive antenna gain) - (loss between antenna and receiver)
In order to operate, (received signal) must be greater than (receiver sensitivity).

Another factor is noise. This is "background" radio-frequency junk that your receiver can "hear" but needs to reject. Sources of noise include other wireless networks, cordless phones, microwave ovens, radio hams, medical equipment, Like other radio phenomena, noise may be highly variable. Many wireless network adapters do not report noise, so if you're using NetStumbler with them then you can't even tell how much noise you have in your environment. A typical urban location these days might have an average noise level around -95 dBm. When you switch on the microwave oven or take a call on your 2.4GHz phone, this value will increase. I've seen a 2.4GHz phone produce -50 dBm of noise, which is enough to saturate some Wi-Fi radios and thus kill their connection completely.

Let's take these concepts and combine them. In order to operate, the actual signal level at your receiver needs to be higher than the noise level. The actual signal level varies depending on signal fade, so if you measured -75 dBm one day, it might drop to -85 dBm occasionally. On most radios this is sufficient to make it drop to a lower data rate, and on some it will cause the connection to drop altogether. Likewise your background noise might be around -98 dBm, but then your neighbor takes a call on her cordless phone and it jumps to -78 dBm. With multipath effects, this is sufficient to make your connection drop randomly.

My conclusion, therefore, is:
Q: What signal level should I consider usable for a good wireless link?
A: Depends on your equipment and your environment.

Q: How can I read and write NS1 files?
A: The file format is available here. Be sure to check this document after a new version is released, since it is likely to change without notice.

Q: Can I see the NetStumbler source code? Is it open source? Etc...
A: NetStumbler is not open source. Indeed it contains a lot of code that was developed under Non-Disclosure Agreements, and the source cannot be released to the general public - if I do so then several teams of lawyers will show up on my doorstep.

Q: I have a Windows CE device and a card with an Intersil Prism2/2.5/3 chipset. The card manufacturer does not provide drivers for Windows CE. How do I get it working?
A: You can download the generic Prism drivers from Senao's web site (HPC2000, PocketPC here; CE.NET 3.0 here, CE.NET 4.0 here). Install the driver and then insert the card. When you see the "Unrecognized Card" dialog box, enter PRISMNDS as the driver name.
Additional tip: If you have a device with only a Type 1 CF slot, most Prism devices in the PC card format can be used in it with a CF-to-PCCard adapter. You can also use Type 2 CF cards in a type 1 slot this way, if you have a PCCard-to-CF adapter too.

Q: How can I develop an application like Netstumbler/Ministumbler?
A: See the following web sites.

• Look at NDIS 5.1 on MSDN.
• For some clues about ORiNOCO support on Windows CE, look at this source code which doesn't work but is still useful, particularly when read in conjunction with this document.
• For Intersil support, visit their web site.
• Try looking at the Wireless Research API.

Q: I just bought an iPAQ 5455 and the battery seems to be dead! It fully charged it and it powers off immediately when it's not in the cradle.

A1: Make sure the battery is snapped right in. Then go Start > Settings > System tab > Power > Standby tab. Move the "Standby period" tab left and right a few times between 12 and 72. Leave it somewhere in the middle. This fixed it for me; it might fix it for you too.
A2: Try loading the ROM update.

Q: I get good GPS data for a minute or two, then a timeout message. What could be wrong?
A: As a test, try restarting Netstumbler. If the GPS data comes back (and then goes away) again, your machine has an infrared port (or other serial device) that shares the same IRQ as the serial port. Try disabling the infrared port from Device Manager.
A: Is it plugged in? If restarting Netstumbler does not appear to fix the problem, check that your serial cable is still connected. The Garmin eTrex and eMap connectors sometimes pop out.

Q: How do I get the WL110, Truemobile 1150, and other similar cards to work with MiniStumbler?
A: Uninstall any existing driver that you are using with the card. Install the ORiNOCO driver on your device. Insert the card. When it asks what driver to use, enter WLLUC46 in the dialog box.

Q: Did you know that the D-Link DWL-650+ card does not work with NetStumbler?
A1: Yes. I have one and will get it working when I have time.
A2: Updated drivers from D-Link are reported to fix this.

Q: I get the message The NETSTUMBLER.EXE file is linked to missing export MFC42.DLL:6625 when I run NetStumbler on Windows 95. What's wrong?
A: You have an outdated version of MFC42.DLL. Please download this version and copy it to your WINDOWS\SYSTEM directory.

Q: How do I install MiniStumbler on an iPaq?
A: Copy the executable from the "ARM" subdirectory of the zip file onto your device. Create a shortcut or place it directly on the Start Menu.

Q: What CompactFlash cards are supported by MiniStumbler/NetStumbler?
A: None.

Q: Can I sell CDs containing NetStumbler or MiniStumbler?
A: No. Please read the license, which can be found both in the about box and in the readme file. Please contact me for permission to redistribute the software. I usually deny all requests, with one exception (publishers of magazines or books that include a cover CD).

Q: I set my GPS to COM3 and now MiniStumbler locks up. How do I fix this?
A: This is the result of unexpected behavior by the Windows CE serial drivers. It will be fixed in the next version of MiniStumbler. In the meantime, you can set your system back to COM1 as follows:

1. Install a registry editor (such as PHM Registry Editor)


2. Go to HKCU\Software\Bogosoft\MiniStumbler


3. Set the value of GPS Port to 1.

Q: How do I use Netstumbler's new scripting feature?
A: Start by reading the Scripting Guide. Try looking at some of the scripts that other users have posted here.

Q: When will cards such as D-Link, Linksys, Netgear be supported?
A: If you are using Windows XP, they are already supported in version 0.3.30. If you are using Windows 2000, this version may possibly work for you too. If you are on Windows Me, 98 or 95, it will not work for you.

Q: Why do both netstumbler.com and stumbler.net exist, and have different content?
A: stumbler.net is marius's personal soapbox, with easy-to-find status information. netstumbler.com is an online community, providing news and forums for Netstumbler users.

Q. What is Netstumbler? A. NetStumbler is a Windows tool that allows you to discover 802.11b (and 802.11a, if using Windows XP) wireless LANs. It includes GPS integration and a simple, intuitive user interface. Though primarily targeted at owners of wireless LANs, it has been the de facto tool for casual users such as "war drivers" since 2001. NetStumbler 0.3 won the eWeek / PC Magazine i3 award for Innovation In Infrastructure, 2002.