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Understanding a Wi-Fi connection.

with 6 comments

Just some more details on how drastically different wireless networks differ from the traditional wired network is understanding the client connection. Surely we all understand how the wired connection works, we plug in a cable two of the four pairs carry data then speed and duplex setting are auto-negotatiated. However when you look at a wireless client you see an antenna, signal strength, data rate, RSSI, power level, and SNR values definitely a little more to think about.

I’ll start with RSSI, which is the Received Signal Strength Indicator this value is typically shown as a negative dBm value (dB and watt values are a topic for another post). RSSI is the measurement of power in an RF signal, the more power in an RF signal the better the connection quality is. So the closer this value is to 0 the stronger the signal is. So a value of -61 is stronger then a value of -74. Now different vendors do have different scales some vendors will have a max value of -100 while others go higher or lower, of course signals that weak should be avoided (and probably won’t work anyway). So it’s best to get some documentation from the vendor of your client WLAN cards to see the RSSI value range. The value of the RSSI will also play a role in the connection speed, and once again vendor documentation will provide the RSSI value to link speed ratio (and do keep in mind many other factors play a role in the connection speed as well).

SNR is the Signal to Noise ratio, this is how much stronger the wireless signal is compared to the noise floor surrounding the WLAN client. This is shown in a positive dB value. Too much RF noise around the WLAN client will cause collisions resulting in frames being retransmitted thus lowering the throughput of the connection. Try connecting a cordless phone that works in the 2.4 GHz range right next to a b/g access point, the phone can generate enough RF noise to cancel out the wireless signal completely. It’s typically best practice to have the SNR value 20 to 25 dB’s away from the RSSI value. So to go back to our previous example if our RSSI is -61 we would want our SNR value to be around -86, or if our RSSI is -74 we would want the SNR to be -99.

The data rate can be one of many values depending on which wireless standard you are connecting with. Be aware though that wireless is a shared medium so it’s half duplex it can not transmit and receive at the same time. So your actual throughput will be about half of what your client is connecting at. A WLAN device showing a connection of 54 Mbps will really have throughput of maybe 30 Mbps. Throughput can be tested using nice little utility called iperf which is available on both Windows and Linux platforms for free.

The power level is measured in mW and depicts how much power a WLAN device is using to maintain the connection. Its typically best practice to design your WLAN infrastructure so your devices operate at half their max output power. This way if an AP goes down neighboring AP’s can double their output power and maintain the availability of the WLAN.

So the overall signal strength/quality registered by client will be a mixture of all those variables.

Below is a screen shot from the Cisco Aironet Site Survey Utility

Here you will see the RSSI at -50 dBm and noise level of -96 dBm, resulting in an SNR value of 46 dB. This utility will also provide you with the BSSID (MAC Address) of the AP you are connecting to along with the RF Channel, 64 in this case utilizing 802.11a.

Written by Stephen J. Occhiogrosso

January 24, 2011 at 9:38 PM

6 Responses

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  1. Steve,

    With respect to SNR and RSSI, does 5 GHz band perform better to 2.4 GHz ? I am pretty sure that the answer is yes. But, I am unsure about the exact reason behind it. For example, if we consider a 802.11n network in deployed in 5 GHz, with certain intentional interference, it performs better than the same 802.11n network deployed in 2.4 GHz. What could be the reason behind it? ( Here the network under consideration is not surrounded by any other network nearby).



    September 21, 2011 at 4:29 PM

  2. RSSI, SNR levels in relation to performance will vary by vendor hardware.

    The reason you suspect this is because there is far less sources of interference in the 5 GHz compared to the 2.4 GHz range. So depending on the surrounding RF interference performance will be degraded in one band while the other band may not be experiencing any interference.

    802.11n has a few more variables compared to it’s predecessors, 802.11n in the 5 GHz range with channel bonding is where you reach Wi-Fi speeds of 300+ MB. 802.11n in the 2.4 GHz is not completely capable of this, So 802.11n will perform better in 5 GHz compared to 2.4 GHz but that is primarily do to channel bonding. (Which increases the amount of sub-carriers)

    Spectrum Analysis tools are what you will want to refer to for identifying sources of interference and also to verify a stable RF signal is being transmitted and received. This tool becomes mandatory with troubleshooting RF WLAN issues, which is what it sounds like you are running into. You should also being seeing a good amount of re-transmissions and CRC errors on your clients/APs.


    September 22, 2011 at 10:17 PM

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