I need my device in some conditions to create an alarm which will be readable for any BLE device in range. Is it possible? What is the right way of doing so? Thank you.
Can you explain a bit more on how your Alarm would work ? Would your device have the "Alarm" Service that other device will query or will your device just broadcast the "Alarm", broadcasting may make more sense, but it would be good if you can explain you needs a bit more.
The Alarm can be broadcast and the receivers can re-broadcast the alarm so that it can be propogated. The Alarm broadcast may require some authentication or signing of the packets so that only the right system can set off the alarm. The receivers that are re-broadcasting may restrict their period of re-broadcasting based on their power source.
You can consider the trickle algorithm ( See tools.ietf.org/.../rfc6206 ) for your broadcasting. Hope this gets you started.