faster Handover by ressociation than by selecting network

[Arne Keller]

Hallo,

I'm made some measurements of the connection gap which occur when a 
handover from one to another accesspoint is made.

First I used a configure file like this (both AP on the same channel):

network={
    ssid="AP1"
    scan_ssid=1
    key_mgmt=WPA-PSK
    proto=WPA2
    psk="secret"
}


network={
    ssid="AP2"
    scan_ssid=1
    key_mgmt=WPA-PSK
    proto=WPA2
    psk="secret"
}

To force the Handover I set up a script which major task was to run the 
following line:

"wpa_cli -i ath0 select_network 0" and "wpa_cli -i ath0 select_network 1"

to roam from one to the other AP.

Beside the time the association takes I figured out a gap of about 300ms 
before starting the 4-way handshake where even no pings where send 
through the wlan NIC.
Where does this time-gap comes from?


To avoid the occurrence of this GAP I tried another set-up with the 
following configuration file:

network={
    ssid="AP1"
    scan_ssid=1
    key_mgmt=WPA-PSK
    proto=WPA2
    psk="secret"
}

I also gave the same SSID to both AP.

To simulate the handover I changed my script in the following way:

wpa_cli -i ath0 set_network 0 bssid MAC:OF:AP1
sleep 1
wpa_cli -i ath0 reassociate

and

wpa_cli -i ath0 set_network 0 bssid MAC:OF:AP2
sleep 1
wpa_cli -i ath0 reassociate


Doing the Handover this way is much faster. The 300ms gap is gone.
But there are a some disadvantages like that this way doesn`t work every 
time and that both AP must have the same ssid.


So my question what is the reason for the 300ms gap and can I avoid it?
I already have made some experiences with the supplicant code so maybe 
someone can tell me the right location for tweaking this.



regards

Arne