On my MacBook Pro laptop running OS X El Capitan (10.11.6), I've been losing Internet connectivity periodically. Though it appears I still have Wi-Fi connectivity, when I attempt to access websites through a browser I find that I can't access sites at times, though a moment before I had no issues browsing the Web. Firefox will display a "Server not found" message. If I go to a Terminal window and try to ping any IP address, I see "request timeout" messages.

$ ping 8.8.8.8
PING 8.8.8.8 (8.8.8.8): 56 data bytes
Request timeout for icmp_seq 0
Request timeout for icmp_seq 1
^C
--- 8.8.8.8 ping statistics ---
3 packets transmitted, 0 packets received, 100.0% packet loss
$

If I check the status of the WiFi connection using the airport command, I see that it is very noisy, though the signal stength is good, which I would expect, since the laptop is only a few feet from the wireless router.

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I
     agrCtlRSSI: -39
     agrExtRSSI: 0
    agrCtlNoise: -93
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 73
        maxRate: 72
lastAssocStatus: 0
    802.11 auth: open
      link auth: wpa2-psk
          BSSID: 94:44:52:4a:43:40
           SSID: Rain
            MCS: 7
        channel: 11
$

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When I used the airport utility from a Terminal windowto check the noise level and signal strength for the WiFi connection, I saw the following:

$ /System/Library/PrivateFrameworks/Apple80211.framework/Versions/Current/Resources/airport -I
     agrCtlRSSI: -56
     agrExtRSSI: 0
    agrCtlNoise: -87
    agrExtNoise: 0
          state: running
        op mode: station 
     lastTxRate: 73
        maxRate: 72
lastAssocStatus: 0
    802.11 auth: open
      link auth: wpa2-psk
          BSSID: 94:44:52:4a:43:40
           SSID: Rain
            MCS: 7
        channel: 11

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The following parameters will be displayed for the wireless connection currently in use:

• PHY Mode, e.g. 802.11n or one of the other wireless standards, such as a, b, g, etc.
• Basic service set identification (BSSID)
• Channel
• Security, e.g., the wireless security method employed, or "None" if no security mechanism is emmployed, such as for a wireless network that is intended to be available for public access.
• Received signal strength indicator (RSSI), which provides a numeric value indicating the strength of the signal. For Apple OS X systems, the value will range from -100 as the worst value to 0 as the best. So a value of -56 as shown above is about at the midpoint for signal strength.
• Transmit Rate, which indicates the maximum transmission rate you might achieve through the wireless access point or router, though you likely won't be able to actually achieve that maximum rate.
• Modulation and Coding Scheme (MCS) Index

For information on checking signal stength and other wireless parameters from the command line on an OS X system, see Checking WiFi signal strength from the command line on OS X.

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$ /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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