This product is either discontinued or we cannot get that from suppliers.
Cisco AIR-CT5520-50-K9 Cbn Only - 5520 Wireless Controller, Rack-mountable - 1U, 10 Gigabit Ethernet, Throughput 20 Gbps, Virtual interfaces (VLANs) 4096, Rack mount kit, Power AC 120/230 V (50/60 Hz).
The Cisco 5520 Wireless Controller Supporting 50 Access Point with Rack Kit AIR-CT5520-50-K9 is part of the Cisco 5520 Series Wireless LAN Controller that is a highly scalable, service-rich, resilient, and flexible platform.