Note that this functionality works only for nodes part of the current wireless cell, you can not monitor Access Points you are not associated with (you can use Scanning for that) and nodes in other cells. In Managed mode, in most case packets are relayed by the Access Point, in this case you will get the signal strength of the Access Point. For those reasons this functionality is mostly useful in Ad-Hoc and Master mode.
The iwspy command provides statistics on the quality of the link between your NIC and another wireless device on the network. It doesn't run all the time; you have to activate iwspy on your interface first. When not activated, iwspy gives a "no statistics to collect" error message.You can try the following commands
sudo iwspy wlan0
wlan0: No statistics to collect
Activation requires you to specify the target IP address and the wireless NIC interface through which it can be found.
sudo iwspy wlan0 192.168.1.1
If you use the iwspy command without the IP address it provides WLAN statistics with a typical/reference value against which it can be compared. In the example that follows the signal is considered fairly strong, with a 64/92 quality value versus a typical 36/92 value, but it could be weak by the historical values on your network. It's good to check this from time to time for fluctuations.
sudo iwspy wlan0
eth0 Statistics collected:
00:09:5B:C9:19:22 : Quality:0 Signal level:0 Noise level:0
Link/Cell/AP : Quality:64/92 Signal level:-51 dBm Noise level:-149 dBm (updated)
Typical/Reference : Quality:36/92 Signal level:-62 dBm Noise level:-98 dBm
To switch off iwspy monitoring, use the following command
sudo iwspy wlan0 off