wireless-networking interview questions

Top wireless-networking frequently asked interview questions

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)

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 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 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 can I protect myself while using public wifi hotspots?

I frequently need to access secure resources (gmail, banking, remote desktop, etc) while on public wifi hotspots. What can I do to ensure that nobody can sniff my passwords or my other browsing activity?

Source: (StackOverflow)

What is the difference between 2.4GHz and 5GHz wireless?

I just got a new "dual band" wireless router. The sales rep didn't really understand the difference between the "2.4GHz" and "5GHz" wireless networks that the router supports.

Can anyone please explain the difference to me?

Source: (StackOverflow)

How to put fear of God (law) into Wi-Fi hacking neighbors [closed]

I live in an apartment and some new guys have apparently moved into one of the apartments. They have been shamelessly hacking into my WiFi.
Mine was initially a WEP encrypted network and out of laziness I just limited and reserved the IPS on my router for the people in my house.
Yesterday I had to free up an IP for a guest in my house but before he could join the network these guys connected in.
I have changed my encryption to WPA2 and hope they dont have the hardware/patience required to hack into it, but there are many wi-fi networks in my apartment most of which are secured using WEP. I don't really want to call the police on them. Is there any way to deter them from misusing other people's wi-fi ?

I have gone through I think someone else has access to my wireless network. What next? but I have already taken the steps mentioned there.

Source: (StackOverflow)

How do I tell Windows to use 802.11 in preference to 3G?

I have a Samsung NC-10 netbook which I take to work every day. Most of the time I use it just on the train/bus, but I also use it at work and home.

It has a built-in 3G card which I want to use when travelling, but I'd prefer to use wifi when I'm at work or home, for obvious reasons. Unfortunately, if the 3G connection is up, Windows appears to use that in preference to wifi.

Starting up and shutting down the 3G modem is a bit of a pain - it's not hard as such, just a bit inconvenient. Ideally I'd like it to always be up, and even have the connection itself up all the time, but without routing traffic through it if there's a wifi connection up. This is what my Android phone does, for example.

Is there somewhere in Windows which lets me express an ordering for network interfaces? I suspect the routing table may be relevant, but it's a bit of a pain to mess around with. I'd really expect there to be a simple GUI way of setting this up - after all, it would equally be useful when dealing with wired vs wifi connections.

I'm currently using Windows XP Home, but Windows 7 answers would also be useful as I'll be migrating soon.

Source: (StackOverflow)

Can I make two wireless routers communicate using the wireless?

I want to make a setup like this:

cable modem <-cable-> wireless router 1 <-wireless-> wireless router 2 in another room <-cables-> PCs in another room

Basically, I want to extend my network access across the house and then have a bunch of network jacks available for my office PCs.

Right now, I have a cable modem going to a wireless router in one room and a PC with a wireless PCI card in it in the office on the other side of the house. I use internet connection sharing with the other PCs in the office. The problem is that ICS is flaky, especially when I switch to VPN on the Windows box to access files at work. I picked up a wireless USB adapter that I thought I could share among the PCs I work on but I'm not very happy with it so I'm going to return it (NDISwrapper support for it is poor).

Is this possible? My wireless experience so far has been pretty straightforward so I have no idea what kind of hardware is available. I've looked at network extenders but those just look like repeaters for signal strength. I want wired network jacks in my office.

Source: (StackOverflow)

How much Wi-Fi signal strength do I lose per foot of antenna cable?

I'm thinking of buying an antenna for a router so I can extend my Wi-Fi. I have seen a few products like TP-Link TL-ANT2405C Indoor Desktop Omni-directional Antenna:

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The length of the wire is 130 cm. Is it OK if I increase the length of the wire or will it affect the performance of the router? What is the maximum cm of length that I can increase?

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)

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'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)