wireless-networking interview questions

Top wireless-networking frequently asked interview questions

How to find a router at an unknown location in a house?

I want to install a WLAN repeater in my father’s holiday house which he rents out to other people.

My father is a digital neanderthal and doesn’t know where his router is, therefore I cannot configure my repeater to this router.

Are there any tools that could help me find that router in the house? I know that there are tools that tell you which Ethernet cable is in use and where it points to, so I figured maybe there are tools that help me find my router?

By “tools,” I don’t necessarely mean software, I’m also thinking about a hardware tool. I tried just going around with my cellphone and searching in the area with the best connection to the network but didn’t find the router.

Edit: Due to some comments that wanted more information about the router: It's a normal ADSL/VDSL router which sends WiFi signals. Distributed by the market leader ISP in my country. It also can do WPS. Here's a link:

Update: I came to the conclusion that it's absolutely not possible to find the router, even though I didn't try the kids trick yet, as raystafarian says, you should only use previously known children and I don't know that many children ;-). The woman who cleans the house said she never saw anything like a router there and the Wifi Analyzer App told me it's most likely inside the wall. Nevertheless it's a great tool and I will accept it as an answer. We're now installing a new router and if, one day, we're able to find the mysterious router location, I'll let you guys know. Cheers!

Source: (StackOverflow)

Is there a cap on simultaneously active Wi-Fi Access points in a single area?

I'm writing an API for controlling an external device. Part of this API is having the device scan for Wi-Fi access points. The API will be implemented across many types of devices, with varying memory capacity. I want to know whether I can just allocate a buffer for found access points once and then forget about it, or whether I need to handle this via dynamic memory allocation.

To make that decision, I need to know how many different Wi-Fi networks/access points can be available in a given area.

At work, when I do a Wi-Fi scan, I pick up 16 different Wi-Fi networks. Even if most of these Wi-Fi networks are poorly reachable, I still wish to pick them up with my Wi-Fi scan.

Is there a cap on simultaneously active Wi-Fi Access points in a single area? More specifically, is there a cap on simultaneously active Wi-Fi networks in a single area? If so, what happens if you go over it?

What I tried (Research)
I tried googling, but the only thing that seems to come up is a limit on the amount of devices per access point. Various searches ("access point limit", "wifi max access point") didn't give me the result I was looking for.

I then tried with different search terms, trying to find out how Wi-Fi scans work. I found out that they work via sending a packet that basically says hello, and then listening for how many hellos they get back.
This seems to indicate to me that there is no cap; I could, in theory, buy many power strips, plug a lot of Wi-Fi access points in (perhaps all connected to one big router so they're connected to the internet, perhaps not), do a Wi-Fi scan and find many access points, provided they have different SSID's. (I don't plan on doing this; even if I did, there'd be no way to know if I'm being limited by the protocol or by the scanner.)
Is this correct? Is there no cap on Wi-Fi access points? Would said theoretical scenario even work in practice?

Source: (StackOverflow)

How do I diagnose and visualize high ping times to wifi router?

I'm seeing erratic and sometimes very long ping times to my wifi router that's just one hop away. Pinging sometimes gives stretches of 400-800ms latencies.

There are plenty of things to try (firmware, router placement, AP channel, etc.), but I would like to attack this problem a bit more methodically:

  • First, how can I visualize the performance of my network?
  • Then, how can I benchmark the performance of a given configuration, so that I can compare reliably after making adjustments?

Source: (StackOverflow)

How to launch a command on network connection/disconnection?

I have a wifi connection that requires to authenticate using a web form once the wireless link is established. I know how to automate the authentication with a script that uses curl/curlIE.

But how can I ask Windows to call my script every time I connect to a particular network connection?

I would be also interested in receiving the name of the wireless profile or the ESSID on the command-line of my script.

Source: (StackOverflow)

Does a wireless-N (802.11n) network have poor performance when in b/g "mixed" mode?

It seems to be an "old" wives tale that, when using a wireless-N router in "mixed mode" to support legacy 802.11b or 802.11g devices, the performance of 802.11n clients will suffer.

Some places claim that when running in mixed-mode, all (some?) N clients run at G speeds. Others make the same claim, but say it only happens when a G client is connected.

Other places say N clients run faster, but still run about 30% slower than if the router were in N-only mode, even if there are no legacy B/G clients connected.

Still others claim there is no speed drop for N clients when running on a mixed-mode network. They say the only issue is that overall network throughput will be lower, because only one client can be transmitting at any time, so some of that transmission time must be shared with the legacy B/G clients running at lower speeds, reducing the overall throughput from what it would be if there were only N clients connected.

So, which is it? Will running in mixed-mode slow down my network, even if there are no B/G clients? If I'm running N, will having another client connected at B/G slow me down substantially vs. if they were running N?

Source: (StackOverflow)

What does the "Fi" in "Wi-Fi" mean?

I just got into a heated discussion about Wi-Fi. What does the Fi in Wi-Fi mean? I would have thought potentially "frequency interface" since all network adapters are classified as interfaces. However I'm not certain.

Source: (StackOverflow)

What's the maximum actual bit rate of an 802.11g connection?

Using a wired connection, I get a 38 Mbit/s download speed. When I switch to wireless one (Linksys WRT54GL router with Tomato firmware), the speed drops to 23 Mbit/s even if the distance between the router and the computer is 2 or 3 meters.

Is that a maximum effective bit rate I can expect from an 802.11g connection?
Are there any settings I can tweak to to increase the download speed?

Source: (StackOverflow)

Do two computers connected on the same Wi-Fi have the same IP address?

Do two computers connected on the same Wi-Fi have the same IP address (for example, my dad's computer and my computer, at home)?

If so, how does the outside world distinguishes one computer with the other? (for example, when a server wants to send us back some data).

Source: (StackOverflow)

RSSI value of wifi connection - how to interpret?

In Mac OS X you can obtain the RSSI value of your by wifi connection by holding Alt and clicking on the Wifi-indicator icon.

My questions are:

  • What range of values can RSSI take?
  • What RSSI ranges corresponds to "good", "normal" and "bad" connectivity?

Source: (StackOverflow)

Is looking for Wi-Fi access points purely passive?

Say I carry a Wi-Fi enabled phone or laptop through an area where there are WAPs. Assuming that I don't actively try to connect to them or otherwise interact with them, is it possible for the owner of that WAP to know that I was there?

I'm asking this in the context of my earlier question: Looking for MACs on the network

I was talking with a friend about my newfound ability to detect phones (and other devices with MAC addresses) on the network, and he pointed out that it might be useful to detect unknown phones on the network; I could use that data to track down anyone who was in my house and brought a Wi-Fi phone with them.

So, if I set up a logging fake WAP with no security or encryption, can I glean any useful information about the devices that come into the house? Assuming that the thief doesn't actively try to connect...

Source: (StackOverflow)

If there are 13 Wifi channels, can I only use 13 Wifi devices on the same room?

As wikipedia reference, 802.11 standards (which defines Wi-fi networks) tell us that wireless networks works with 13 different channels on OFDM (depending on the release, a, b, g or n). From this I was wondering, if I have more than 13 machines on the same room (one room work for example with 50 notebooks), it would be impossible to connect all of them to internet at the same time? I mean, each device would use one specific channel to communicate with the acess point, limiting the acess point to 13 permanent connections.

How does all this stuff really work?

Source: (StackOverflow)

Is it better to use a crowded 2.4GHz Wi-Fi channel 1, 6, 11 or "unused" 3, 4, 8, or 9?

I understand that 2.4GHz Wi-Fi channels overlap, and that the most popular non-overlapping set of channels in the US is 1, 6, and 11. Generally, my signal strength on channels 1, 6, and 11 are much stronger than my neighbors' on the same channel. However, these channels usually have 4 or 5 APs already using them. In this scenario, is it better to use 3, 4, 8, or 9? Or is it better to use the crowded channels 1, 6, and 11?

As a secondary question, does it even matter that my signal strength is much higher than theirs?


Why use wifi channels other than 1, 6 or 11?

Source: (StackOverflow)

Paranoid Parent: "WiFi safe for baby?" [closed]

I am most likely being an overprotective parent but since the birth of our newborn, my wife and I have been wondering about credible studies dealing with Wi-Fi and health concerns. I love my Wi-Fi, it's the cornerstone to all my gadgets and computer setup through out my house, and it makes my world easier plain and simple, but having a newborn enter that world changes the way I think about everything.

Now before people start writing that Wi-Fi is safe because they use it in hospitals and schools, let me be clear, I'm aware of all that, but the idea of having it 24/7 for years to come around this little person that is our responsibility to look out for makes me want to have a definitive answer to the subject.

I will put on my tin foil hat and await for some well thought out/educated answers.

Source: (StackOverflow)

Does it make sense to keep different SSIDs for 2.4GHz and 5GHz wireless networks?

My router can have different SSIDs for 2.4 and 5 GHz and I'm not sure if it's better to have the same SSID or not.

Initially I put the same network but I got confused when the MacBook Pro displayed me two networks with the same name, an I was not able to distinguish between then.

So the next step was to configure two networks, "home" and "home-slow".

I would like to know what are the pros/cons for these configs.

Note, I do have a Cisco E4200 router, configured:

  • 5GHz - SSID "home" - Mixed - Auto 20 MHz/40MHz - Auto-DFS
  • 2.4GHz - SSID "home-slow" - Mixed - Auto 20 MHz/40MHz - Auto

Note, iPhone4 and HTC Desire HD do not see the 5GHz network, only the 2.4 one, not sure why. MacBook Pro seems to detect both of them.

Source: (StackOverflow)

How does Windows know whether it has internet access or if a Wi-Fi connection requires in-browser authentication?

In Windows 7, the notification area networking icon will show an error indicator if there is no internet access wifi-err, and the error icon goes away once there is a successful connection to the internet networking normal. Sometimes, if the WiFi connection requires an in-browser authentication step, like on many guest networks in hotels or universities, then the following pop-up bubble appears, saying as much: additional log on information may be required, click to open your browser

How does Windows know whether or not it has a successful internet connection?
Presumably it is checking some online Microsoft service to see whether it has a successful connection, gets redirected to some other page, or doesn't get any response at all, but I haven't seen anywhere that this process or the services used are documented. Can anybody explain how this works? I would prefer answers that refer to facts, rather than just guessing, but if you have a really good guess, then go for it.

This question was a Super User Question of the Week.
Read the May 16th, 2011 blog entry for more details or submit your own Question of the Week.

Source: (StackOverflow)