WLAN Controller is a term often used in medium and large Enterprise companies having a large number of users connecting to the WiFi network.
For a very small home WiFi network (where 1 to 5 users are connected) your wireless router would have integrated with one or more wireless radios, antennas, switch / hub, xDSL modem, DHCP server, USB hub, miniature firewall, etc., We can call this device as a THICK AP. This device may not have to coordinate with other network devices connected to the same network.
For a medium to larger enterprises network (typically more than 100+ WiFi users) you may have connected one or more Access Points(AP) to the network at multiple locations of your office. One AP’s wireless radio coverage can support 50 to 100 meter radius. So, about every 100 meters you could have one AP installed. Unlike wired network, where network access parameters are fixed once you plug your Ethernet cable, Wireless network is highly dynamic in nature.
Factors like number of users, type of applications, building wall thickness/materials, ceiling height, flooring, furniture property, glass doors etc can affect the properties of wireless signals. Technically speaking, some good open space / path is required for electromagnetic signals generated by APs to propagate correctly. But practically, providing open space / path for electromagnetic signals is not possible – we have to manage only with existing office infrastructure.
Since multiple APs would be located inside the office floors, ‘someone’ should closely monitor the wireless environment, network usage, interference, channel availability, user movement, AP availability, user density etc. And everything happens in micro/nano seconds. This ‘someone’ will be a WLAN Controller which will sit inside a datacentre. All the controlling mechanism / processor / memory / switch / firewall / QoS / DHCP/ policing / etc., are usually offloaded to the WLAN controller and only radio & some memory information is provided in the APs that are distributed across the floors. We can call such APs as THIN APs.
The WLAN controller will gather (hear) all the information shared by each APs connected to your Ethernet network from multiple locations and use its own logic (varies from one OEM to other depending upon cost / features) to make sure the users are not dropped / disconnected due to dynamic changes in Wireless environment. Whatever changes happen in your network or RF environment, this WLAN controller will take care to stabilize every change or try to adopt to it.
Again these days WLAN Controller has different form factors: In an appliance or in a cloud or a virtual controller etc. When choosing your WLAN controller, you may have to choose the right one as per your needs & budget. Always go with expert advice to choose one or else your wireless network will always get into trouble over a period of time!!!