link quality versus signal strength
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Thread: link quality versus signal strength

  1. #1
    Join Date
    Jan 2004
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    San Diego
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    link quality versus signal strength

    Can anyone explain what is the difference between link quality and signal strength?

    Thanks.

  2. #2
    Join Date
    Jan 2003
    Location
    Florida
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    312
    Signal strength is a simple measure of the amplitude of the signal that is received. The closer you are to the access point, the higher this will be.

    Link quality measures the number of packet errors that occur. The lower the number of packet errors, the higher this will be.

    If you are very close to an access point you will get high signal strength, but you might get low link quality due to a microwave oven or cordless phone in the area causing interference.

    Conversely, you might be very far from the AP and get low signal strength, but high link quality due to the absence of any RF interference, multipath, etc.

  3. #3
    Join Date
    Jun 2007
    Posts
    3
    I found this thread through a Google search for "signal strength vs signal quality wifi". My school's filter will not allow me to create a new post, so we'll see if it lets me reply.

    I wanted to ask if cell phones would affect link quality or signal strength. I am in a college dorm, and our access points don't seem to do very well. Cisco installed the system, so I wouldn't think they are not good access points, but the signal is not very good in a lot of places. If cell phones have an effect, it would make sense, since so many students have their cell phones on all the time.

    Also, while I am posting, I might as well ask which aspect of wifi affects the reliability, speed, etc. of a wireless signal. In other words, what affect does link quality have and what affect does signal strength have?

  4. #4
    Join Date
    Nov 2003
    Posts
    5,553
    1. Signal strength is defined in 802.11 as the Received Signal Strength
    Indicator (RSSI). RSSI, is intended to be used as a ‘relative value’ within the
    chipset. This is a 1-byte value so that it could have values ranging from
    0 to 255, but vendors prefer to use arbitrary scales from 0 to RSSI_Max
    where the latter is vendor-specific. It is not associated with any particular power scale
    (e.g. mW) and is not required to be of any particular accuracy or
    precision.

    The RSSI value is used internally by the microcode in the
    adapter and this is why vendors are not forced to use a compatible
    standard. As an example of its use, if the RSSI value is below some
    threshold, the NIC knows that the channel is idle. Therefore, the signal
    strength numbers reported by an 802.11 card will probably not be
    consistent between two vendors, and should not be assumed to be particularly accurate or precise

    2. Signal quality is the PN code correlation strength, which is a measure of the correlation between the incoming DSSS signal and an ideal DSSS signal.

    3. Signal to Noise Ratio: According to the standard communications systems terminology,
    SNR is defined as the ratio of received signal power to the power of the
    additive Gaussian noise that appears at the output of the receiver.

    4. Link quality analysis: In LQA, signal quality is determined by measuring, assessing, and analyzing link parameters, such as bit error ratio (BER), and the levels of the ratio of signal-plus-noise-plus-distortion to noise-plus-distortion (SINAD). Measurements are stored at—and exchanged between—stations, for use in making decisions about link establishment.

    Link quality is somewhat nebulous and not an official parameter, but the term is used all of the time to denote the outcome of doing a link quality analysis.

    So link quality will affect BER, which will force resends, leading to lower data throughput. Signal strength has a direct relationship with data throughput. 802.11a/g has a real-time signal strength measuring algorithm that will step up or down the data throughput rates through a series of twelve steps depending on the signal strength. 802.11n has an 88 step process making it more granular.
    CWNA, CWSP, K0PBX

  5. #5
    Join Date
    Jun 2007
    Posts
    3

    Question

    ok, what about cell phones? would having lots of cell phones in a building affect the wifi signal?

  6. #6
    Join Date
    Nov 2003
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    5,553
    Not really, they are on frequencies that are typically far enough removed from the frequencies used for 802.11
    CWNA, CWSP, K0PBX

  7. #7
    Join Date
    Oct 2009
    Location
    Ohio
    Posts
    1
    I Read this:

    If you are very close to an access point you will get high signal strength, but you might get low link quality due to a microwave oven or cordless phone in the area causing interference.

    Conversely, you might be very far from the AP and get low signal strength, but high link quality due to the absence of any RF interference, multipath, etc.

    I’m assuming then:

    Signal quality is 100% based on interference? It’s digital. It’s on or off. If it’s on and strength is 100% but quality is 18%, then interference is the problem?

    I have a 2.4 CPE on my aunts house linking to a SkyPilot 5.8/2.4 node a block away. Several months ago I was linking 100% Sig. Str./100% Sig. Qlty. Now I'm fighting to get 18% on sig. qual. and it's running like crap. What changed besides the weather?

  8. #8
    Join Date
    Dec 2009
    Posts
    1
    A reduction in strength could be the cause, but if your signal strength hasn't changed, then interference is the culprit. Additional Wifi users are one source of interference. Only so many channels to go around so as more people fire-up APs interference is inevitable.
    The other common source is cell phones, but not the phone part. M/Q was correct in his post, the cellphone frequencies are sufficiently far enough away to not cause interference. However he did not mention the number of phones with Bluetooth capability. Bluetooth in proximity to an AP or a client will most assuredly cause interference.
    Cordless phones are another potential source of significant interference as many share the 2.4GHz spectrum.

  9. #9
    Join Date
    Jun 2012
    Posts
    2
    Quote Originally Posted by M/Q View Post
    Signal strength has a direct relationship with data throughput. 802.11a/g has a real-time signal strength measuring algorithm that will step up or down the data throughput rates through a series of twelve steps depending on the signal strength.
    Hi, I have a question. I am using 802.11a wireless radios. These radios are spec'd to transmit at 20dBm. they have settings to set the transmit rate. These radios are showing lower than expected signal strengths and I am not sure how to determine whether they are transmitting properly and I am having pathloss issues or whether the radio is scaling back the transmit power because of data rate.

  10. #10
    Join Date
    Aug 2009
    Location
    Henning, MN
    Posts
    192
    OK
    What brand, antenna and any further info you can provide
    Please
    Mitch

    support@abetterwireless.com

    Wireless is supposed to be fun

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