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Thread: How does inSSIDer scan in real-time without disturbing the internet connection ?

  1. #1

    Default How does inSSIDer scan in real-time without disturbing the internet connection ?

    Dear all,

    I have a question about the channel scanning procedure and I wish you can help me.

    I expect that any 802.11 device is able to sense one channel at a time. Therefore, when a device is associated at channel 1 but is scanning channel 9, in this case this device looses all the packets transmitted at channel 1 during the time period of scanning channel 9.

    But,

    I noticed that inSSIDer is able to show all the available networks at all the (2.4 and 5GHz) channels, in real time while the device is connected to the internet without any problem. At the same time, when I ask the network manager to update the list of the available networks, I notice that the WiFi card takes an important time (about 4 seconds) to make the scan, and during this time, the connection to the internet is pending.

    So may question is: what does inSSIDer do to scan the different channels in real-time without disturbing the internet connection, and why the network manager can not do the scan in the same way like inSSIDer?
    Do you have any idea about what is happening.

    Thank you in advance and best regards

  2. #2

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    I'm not associated with the developer, but my understanding of inSSIDer is that it only reports data that is available from the wireless NIC. It does not physically scan any channels. inSSIDer reports both 2.4 GHz channels and 5 GHz channels in near real time.
    Old Mod by the Sea

  3. #3

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    Quote Originally Posted by ydal View Post
    Dear all,

    I have a question about the channel scanning procedure and I wish you can help me.

    I expect that any 802.11 device is able to sense one channel at a time. Therefore, when a device is associated at channel 1 but is scanning channel 9, in this case this device looses all the packets transmitted at channel 1 during the time period of scanning channel 9.

    But,

    I noticed that inSSIDer is able to show all the available networks at all the (2.4 and 5GHz) channels, in real time while the device is connected to the internet without any problem. At the same time, when I ask the network manager to update the list of the available networks, I notice that the WiFi card takes an important time (about 4 seconds) to make the scan, and during this time, the connection to the internet is pending.

    So may question is: what does inSSIDer do to scan the different channels in real-time without disturbing the internet connection, and why the network manager can not do the scan in the same way like inSSIDer?
    Do you have any idea about what is happening.

    Thank you in advance and best regards
    I have no experience with inSSider, but I will say this. Most software that records signal strengths on all channels for you to see, simply reads the management beacon frames sent out by the access points. To do this the client device must go off channel to scan the other channels for beacons. You can prove this with a WiFi protocol analyser. If you can take a capture of your current channel whilst running the software you will see a "Null Data" or "Null QoS data" frame being sent to your connected AP. This is a frame that a client sends to an AP instructing it to buffer data while it either a) goes to sleep, or b) goes off channel to scan for other networks.

    I found this out to my detriment. When running Ekahau (another WiFi surveying tool), I discovered that my wireless (and subsequently my internet) throughput was dramatically decreased. This was because Ekahau was using my laptop to go off channel and scan other channels for management beacon frames. I suspect that InSSider will do something similar, but I stand here to be corrected .

    You are correct that a Wi-Fi client can only use a single channel at any one time and communications on the channel it is using will be temporarily suspended when it goes off channel for scanning.

  4. Default

    Quote Originally Posted by svosper View Post
    Most software that records signal strengths on all channels for you to see, simply reads the management beacon frames sent out by the access points.
    Entire thing sounds right to me!

  5. #5

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    Here is a bit more detail on how software written for Windows obtains a list of available wireless signals using the Native WiFi API.
    Old Mod by the Sea

  6. #6

    Default

    Thank you every body for your answers.

    In fact I asked the question because I was expecting the throughput to fall and the connection to be paused temporally during the channel scanning, but non of this occurs. The issue is that inSSIDer is showing the right list of available devices in real-time (sometimes with a delay of 2 or 3 seconds). So I am confused! This is contrarily to what is supposed to be! I know that inSSIDer reads available information. but the real-time nature which does not disturb the internet connection is the mystery.

    The link of ua549 confirms may expectations which are: 1) by default the scan is achieved once very 60 seconds and 2) the scan time period should not exceed 4 seconds. But I learned the following news: in some cases (when already connected to wireless network) the Wireless LAN Service does not ask for scans at all. But the real-time scanning of inSSIDer is difficult to explain. May be current devices are able to scan more than one channel at a time! After all, a channel is nothing but a portion of the radio band, and current devices are able to operate at 20MHz and 40MHz. So why not being associated at one channel of 20MHz while scanning other channels with the other 20MHz filter capability! Of course this is an example, but nothing forbids the device to scan all the 2.4GHz and 5GHz bands at the same time while only forwarding packets arriving at the appropriate channel to the kernel.

    Thank you again for your help.

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