What must all wireless devices in a WLAN have in common to communicate successfully?

For wireless devices in a Wireless Local Area Network (WLAN) to communicate effectively, they must share the same SSID (Service Set Identifier). The SSID acts as the network name that identifies a particular WLAN. When devices connect to a wireless network, they use the SSID to join the correct network and establish communication with each other and with the access points.

If devices have different SSIDs, they will not recognize each other as part of the same network, which prevents them from communicating, even if other technical requirements like IP addresses or encryption methods are met. The SSID essentially serves as a way to group devices on a particular network, ensuring they are all operating within the same connection parameters and can interact seamlessly.

While firewall settings, IP addresses, and encryption methods are all important for the security and proper configuration of a network, the crucial commonality needed for devices to join and communicate on a WLAN is the SSID.