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Tue, Apr 28, 2015 10:51 pm

Checking wifi signal strength from the command line on OS X

I took a friend to a VA hospital today. I took my MacBook Pro laptop with me and while I was there I used the wireless guest service at the facility. The performance of the wireless service was awful today as it was on a previous visit. To determine if the problem might be attributable to signal strength, I used the airport command, which is available on OS X systems. The utility is located in /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/. Help information on the command is available using airport -h.

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -h
Supported arguments:
 -c[<arg>] --channel=[<arg>]    Set arbitrary channel on the card
 -z        --disassociate       Disassociate from any network
 -I        --getinfo            Print current wireless status, e.g. signal info, BSSID, port type etc.
 -s[<arg>] --scan=[<arg>]       Perform a wireless broadcast scan.
				   Will perform a directed scan if the optional <arg> is provided
 -x        --xml                Print info as XML
 -P        --psk                Create PSK from specified pass phrase and SSID.
				   The following additional arguments must be specified with this command:
                                  --password=<arg>  Specify a WPA password
                                  --ssid=<arg>      Specify SSID when creating a PSK
 -h        --help               Show this help

To view the status for a wireless connection, use airport -I or airport --getinfo.

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I
     agrCtlRSSI: -67
     agrExtRSSI: 0
    agrCtlNoise: -86
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 7
        maxRate: 144
lastAssocStatus: 0
    802.11 auth: open
      link auth: none
          BSSID: ec:44:76:81:e4:40
           SSID: VA Internet
            MCS: 0
        channel: 11

The signal strength for a connection is the value for agrCtlRSSI. "RSSI" stands for received signal strength indication. The higher the number, the stronger the wireless signal. The maximum value that may be reported will depend on the wireless devices being used. Cisco Systems wireless cards have a maximum value of 100 while Wi-Fi chipsets from Atheros will return a value from 0 to 128. For Apple OS X systems, the value will range from a high of 0 down to minus 100 (-100). The closer the number is to zero, the stronger the signal while the closer the number is to negative 100, the weaker the signal strength.

Since I only wanted to check the signal strength initially, I looked just for that value. The signal strength seemed to be ok for the waiting room I was in, though not terrific.

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | grep agrCtlRSSI
     agrCtlRSSI: -74
$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | grep agrCtlRSSI
     agrCtlRSSI: -68

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