Hi Julian,
1. How does the AP compute the signal strength at the client location to calculate the SNR? Or does the client advertises its signal to the AP?
---- It is actually not the signal strength at the client location. It is the signal strength seen by the IAP at the AP location when the client transmits wireless frames.
2. How does the AP know the number of unassociated clients? Do the clients advertise their presence always? Do the clients advertise their presence even when they do not want to join the WLAN?
---- In addition to client service, IAP also monitors the air for the presense of other APs and clients. Use the command "show ap monitor ap-list" to see the list of SSIDs monitored by the IAP. And the command "show ap monitor sta-list" will show the client list monitored by the IAP. Unassociated client is actually something unassociated to the IAP, but these clients are associated to some other SSIDs. IAP knows these clients by monitoring the air.