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wireless-networking interview questions

Top wireless-networking frequently asked interview questions

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)

Which routers do you prefer for DD-WRT or OpenWRT? [closed]

I'm aware of the giant Supported Devices list. I'm wondering which ones you all prefer (e.g., support for Mega install, cheap, reliable hardware, USB port, etc.)

I've used an ASUS WL-500G Premium for 1.5 years, but I've been eyeballing the WL-520gU recently.


Source: (StackOverflow)

How can I get the same SSID for multiple access points?

I need to upgrade my existing wireless infrastructure and this time I want 2 access points to cover my house, since I get blind spots no matter what with a single AP. I have physical cabling to my central network available for both access points.

I would really like these two to interoperate seamlessly as a single SSID. How do I do this? What are the features that the new access points I'm buying would need to support?


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)

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 192.168.1.1 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)

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)

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?

Related:

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)

What is a "Magic Packet" for waking a computer?

My wireless adapter (Intel Dual Band Wireless-N 7260) has two settings in Device Manager which I cannot explain.

Wireless adapter properties

Wake on Magic Packet
Wake on Pattern Match

After a bit of research, I found this Microsoft Technet article which defines the feature as follows:

Defines if a network adapter is enabled to wake a computer on the magic packet.

This rather cryptic description is a bit low on details. Can anyone help?

I would prefer that my laptop not be woken up remotely under any circumstances. I've disabled Allow this device to wake the computer on the Power Management tab, but these settings appear to be separate. My assumption is that I can set these two settings to Disabled without negative consequences. Is that right?


Source: (StackOverflow)

Make Windows 7 ignore WiFi when ethernet is available

When I dock my Windows 7 laptop, I want it to prefer the wired ethernet connection over WiFi.

This is a pretty straightforward thing to do on my Mac - I just reorder my network preferences, and it "does the right thing." I just can't figure out how to achieve the same thing on my Win7 laptop.

So, when I'm docked, it connects to WiFi, and then fails to connect to servers on the local wired network. How do I fix this?


Source: (StackOverflow)

Is it really possible for most enthusiasts to crack people's Wi-Fi networks?

Can most enthusiastic users (even if they are not professionals) use well-known techniques to break through the average home router's security?

Some basic security options are:

  • strong network password with various encryption methods
  • custom router access password
  • WPS
  • no SSID broadcast
  • MAC address filtering

Are some of these compromised and what to do to make the home network more secure?


Source: (StackOverflow)

Why does my microwave kill the Wi-Fi?

Every time I start the microwave in the kitchen, our home Wi-Fi stops working and all devices lose connection with our router! The kitchen and the Wi-Fi router are in opposite ends of the apartment but devices are being used a little here and there. We've been annoyed by the instability of the Wi-Fi for some time and it wasn't until recently we realized it was correlated to microwave usage.

After some testing with having the microwave on and off we could narrow down the problem to only occurring when the router is in b/g/n mode and uses a set channel. If I change to b/g mode or set channel to auto then there is no problem any more...but still!

The router is a Zyxel P-661HNU ("802.11n Wireless ADSL2+ 4-port Security Gateway" with latest firmware) and the microwave is made by Neff with an effect of 1000W (if this information might be useful to anyone). There is an "internet connection" light on the router and it doesn't go out when the interruption occurs so I think this is only an internal Wi-Fi issue.

Now to my questions:

  • What parts of the Wi-Fi can possibly be affected by the microwave usage? Frequency? Disturbances in the electrical system?
  • How can setting Auto on channels make a difference? I thought the different channels were just some kind of separation system within the same frequency spectrum?
  • Could this be a sign that the microwave is malfunctioning and slowly roasting us all at home? Is there any need to be worried?

Since we were able to find router settings that cooperate well with our microwave's demand for attention, this question is mainly out of curiosity. But as most people out there...I just can't help the fact that I need to know how it's possible :-)


Source: (StackOverflow)

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: https://www.swisscom.ch/en/residential/help/device/internet-router/centro-grande.html

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)