Checking WiFi signal strength from the command line on OS X

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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

If you want to monitor how the signal strength varies over time, you can use commands like those below, which will show the value every x seconds as specified by sleep x, where x represents the number of seconds to sleep between checks. Hitting control-c will terminate monitoring.

$ while true; do /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | grep agrCtlRSSI; sleep 5; done
     agrCtlRSSI: -67
     agrCtlRSSI: -67
     agrCtlRSSI: -68
     agrCtlRSSI: -66
     agrCtlRSSI: -68
     agrCtlRSSI: -69
     agrCtlRSSI: -67
     agrCtlRSSI: -68
     agrCtlRSSI: -67
     agrCtlRSSI: -67
     agrCtlRSSI: -68
^C

When I checked the signal strength seemed to vary little over the course of about a minute, since I ran the check every 5 seconds. When I increased the spacing between checks to 15 seconds and let the check run for several minutes, I saw the signal stength never rose above -67 with the lowest value being -75.

Note: you can have checks run more frequently by specifying a fractional value for the sleep parameter, e.g., sleep 0.5 to reduce the "sleep" period to half a second.

I then examined the noise level for the wifi signal, which is the agrCtlNoise value. When I checked it over a two minute period, it remained high. Less negative values are preferable.

$ while true; do /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | grep agrCtlNoise; sleep 15; done
    agrCtlNoise: -88
    agrCtlNoise: -88
    agrCtlNoise: -88
    agrCtlNoise: -86
    agrCtlNoise: -84
    agrCtlNoise: -89
    agrCtlNoise: -88
    agrCtlNoise: -86
^C
$

The maximum data rate is the maxRate value, which will remain constant, but the lastTxRate value may vary.

$ while true; do /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I | grep Rate; sleep 15; done
     lastTxRate: 5
        maxRate: 144
     lastTxRate: 5
        maxRate: 144
     lastTxRate: 5
        maxRate: 144
     lastTxRate: 7
        maxRate: 144
     lastTxRate: 5
        maxRate: 144
     lastTxRate: 7
        maxRate: 144
     lastTxRate: 20
        maxRate: 144

The maxRate value is the maximum rate, or data throughput, that the Access Point is capable of; the rate is a theoretical maximum rate, which you won't likely realistically attain. In this case the maxRate value was 144 Mbs, but the values I saw for lastTxRate of 5 to 7 Mbs, with one instance of 20 Mbs, were far below the maxRate. Whenever I visited webpages, I seemed to lose connectivity to sites frequently.

If you wish to scan for available networks you can use airport -s.

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -s
                            SSID BSSID             RSSI CHANNEL HT CC SECURITY (auth/unicast/group)
                     VA Internet ec:44:76:81:e0:00 -77  11      Y  US NONE
                         IMM-VAH 00:24:36:a9:a8:9c -84  161     N  US WPA(PSK/TKIP/WEP104) WPA2(PSK/AES,TKIP/WEP104) 
                         IMM-VAH 00:24:36:a9:a8:9b -78  10      N  US WPA(PSK/TKIP/WEP104) WPA2(PSK/AES,TKIP/WEP104) 
                            TEST ec:44:76:81:e4:4d -77  36      Y  US WPA2(PSK/AES/AES) 
                     VA Internet ec:44:76:81:e4:4f -77  36      Y  US NONE
                           M2ZCN 18:1b:eb:c5:f8:2d -77  6       Y  -- WPA2(PSK/AES,TKIP/TKIP) 
   Verizon MIFI4510L DE57 Secure 00:15:ff:26:de:57 -83  4       N  US WPA2(PSK/AES/AES) 
                     VA Internet ec:44:76:81:e8:70 -85  1       Y  US NONE
                     VA Internet 04:fe:7f:49:b9:80 -83  1       Y  US NONE
                     VA Internet ec:44:76:81:e4:40 -66  11      Y  US NONE

You can scan for a particular SSID with --scan="SSID" where SSID is the particular SSID you are interested in.

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport --scan="VA Internet"
                            SSID BSSID             RSSI CHANNEL HT CC SECURITY (auth/unicast/group)
                     VA Internet ec:44:76:81:e4:4f -76  36      Y  US NONE
                     VA Internet ec:44:76:81:e0:00 -76  11      Y  US NONE
                     VA Internet ec:44:76:81:e4:40 -65  11      Y  US NONE

When I last checked the status of wireless connectivity from the waiting room, I saw a lastTxRate of 13. We then went to the cafeteria and after I ate I checked the wifi status again and saw the lastTxRate had gone up considerably from what it was in the waiting room with the value being 130, though the agrCtlNoise value was still similar to what it had been. The agCtrlRSSI value improved, going from -66 in the waiting room up to -58 in the cafeteria indicating a stronger signal in the cafeteria.

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I
     agrCtlRSSI: -66
     agrExtRSSI: 0
    agrCtlNoise: -90
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 13
        maxRate: 144
lastAssocStatus: 0
    802.11 auth: open
      link auth: none
          BSSID: ec:44:76:81:e4:40
           SSID: VA Internet
            MCS: 8
        channel: 11
$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I
     agrCtlRSSI: -58
     agrExtRSSI: 0
    agrCtlNoise: -92
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 130
        maxRate: 144
lastAssocStatus: 0
    802.11 auth: open
      link auth: none
          BSSID: ec:44:76:81:e8:4f
           SSID: VA Internet
            MCS: 15
        channel: 149
$

I did notice that the MCS value had changed from 8 to 15. For 802.11n wireless connectivity, the Modulation and Coding Scheme (MCS) index values range from 0 to 32 with particular index values associated with particular data rates - see the data rates table. An 802.11 Wi-Fi network has a maximum theoretical bandwidth of 600 Mbit/s. The table shows a data rate of 144.4 Mbs for an index value of 15 for a 20 Mhz channel with a 400 nanosecond guard interval.

Some time after leaving the hospital when I performed a wireless status check at another location with a wireless router that supports IEEE 802.11b/g/n, I saw a much better agCtlRSSI value of -34, indicative of the laptop receiving a much stronger wireless signal, with a maxRate of 72 Megabits per second.

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I
     agrCtlRSSI: -34
     agrExtRSSI: 0
    agrCtlNoise: -91
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 73
        maxRate: 72
lastAssocStatus: 0
    802.11 auth: open
      link auth: wpa2-psk
          BSSID: 94:44:52:5a:54:52
           SSID: Maze
            MCS: 7
        channel: 6

An 802.11n network can achieve 72 Mbs (on a single 20 MHz channel with one antenna and 400 ns guard interval)

 

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Created: Tuesday April 28, 2015