You will first need to convert your AP from autonomous mode to LWAPP. This is done by dowloading from Cisco web site an LWAPP image for this AP, and the migration tool (download > wireless > access points > 1100 series).
Then, if you use a 5.0 code release, be aware that you will not be able to use some 5.0 features, such as the global credentials feature... might not be big deal.
Just to add a note to the good info from Jerome (+5 points for this Jerome :)
Sadly, the 1120 with "B" Radio is not supported :(
Support for the 1121 was added in WLC version 18.104.22.168 Have a look at these related docs;
Software release 22.214.171.124 New Features
LWAPP for Cisco Aironet 1100 Series Access Points (802.11g radio only) Allows the Cisco Aironet 1100 Series Access Point to be upgraded from autonomous access point mode to lightweight mode using the autonomous to lightweight mode upgrade tool.
Cisco has released a free tool called the "Autonomous to Lightweight Mode Upgrade Tool" that allows selected Cisco Aironet autonomous access point models to be configured for lightweight mode operation.
The Autonomous to Lightweight Mode Upgrade Tool supports the following models:
Cisco Aironet 1240AG Series access points
Cisco Aironet 1230AG Series access points
Cisco Aironet 1200 Series access points that contain 802.11g (AIR-MP21G-x-K9) and/or second-generation 802.11a radios (AIR-RM21A-x-K9 or AIR-RM22A-x-K9)
Cisco Aironet 1130AG Series access points
**Cisco Aironet 1100 Series Access Points that contain 802.11g radios (AIR-AP1121G-x-K9)
Cisco Aironet 1300 Series Access Points/Bridges (AIR-BR1310G-x-K9 or AIR-BR1310G-x-K9-R). A Cisco Aironet 1300 Series operating in Lightweight Access Point Protocol (LWAPP) mode only operates as an access point. This series does not support LWAPP bridging mode.
The Autonomous to Lightweight Mode Upgrade Tool supports a process to migrate an autonomous access point from autonomous mode to lightweight mode. Unlike a VxWorks to Cisco IOS Software upgrade, this process is a Cisco IOS Software upgrade to the existing Cisco IOS Software image-not an operating system "swapout". In converted access points operating in lightweight mode, Cisco IOS Software continues to run on the access point, while LWAPP is used to communicate with a wireless LAN controller. Since LWAPP supports automatic access point configuration, there is no need to retain or convert the original autonomous Cisco IOS Software access point configuration.
The reason the older APs are not supported for conversion is that they do not support AES encryption. This is required as you will have an x509 certificate installed on the AP and open an encrypted LWAPP tunnel to the controller. No AES no tunnel.
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
This is the start of a display filter cross reference between Wireshark
and OmniPeek. The 1st installment is a table of advanced filters. More
filters will be added as time allows. It is a living doc, so check back
for changes every so often Please feel f...