Is it possible to wifi roaming without wifi security aka open network???
Sure. Just make sure the SSID is the same on all APs and that neighboring APs do not use the same channels.
Goes without saying, though, that no password/encryption on your SSID means no security, so make sure you make appropriate security settings for the rest of your network and devices.
is this roaming? or diconnect and connect ap. i want to no disconnection while switching ap's.
Roaming is a client side process. It involves a drop-connect cycle, but if your APs are well tuned for your space, this can be completely (or nearly) unnoticeable.
The only things required for roaming to work are 2 or more APs with the same SSID and security settings (in your case: none). It is also important to set the channels appropriately, as I mentioned earlier, and setting power levels (reducing the power) is also key to a good roaming experience.
i have 3 ap's. i use same SSID but i cant phone call via voip. so i need seamless roaming like 802.11r but 802.11r require wifi security. i dont want to any wifi security.
actually i done with 802.11r (wpa-psk). but wifi must be open network no wifi security.
Standard roaming is a client side process, so if you're not using things like 802.11r to assist the devices, you must ensure that your wifi is well tuned for your environment, and then it is up to the clients to do the roaming properly.
Take a look at this video with respect to the wifi optimization. Although it focuses on Unifi, the concepts apply to all wifi systems (provided the controls are exposed to the user).