That special signal is called beacon frame.
Explanation
You have bought a Laptop PC from your local retailer shop. You decide to pass by a coffee joint and turn your Wi-Fi signal on. By default, it should prompt you to log on to the joint’s wireless network.
What I have described above is known as an association. An association involves packet exchange between my laptop and the AP. As long as my laptop is on, it will do a passive scan and look for access points that are within its surroundings. It will listen on available channels within range and look for a special signal called beacon frame that always resonates from an Access Point. This special signal contains information that my laptop needs to associate itself with the AP. Information such as the network name (SSID) and the transmission rate of the network. Once my laptop detects a beacon frame, it will choose to associate itself with the AP or not.
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