Hi .. basically the ASA 5520 can provide full packet inspection by adding a module in to it. This module is the AIPSSM. Once this module has been inserted in the correct slot, then you need to establish connectivity between the ASA and the module. once you have done that, then you can proceed with configuration by using the ASA graphic interface ASDM. The below link will give you a better idea.
http://cisco.com/en/US/products/ps6120/products_configuration_guide_chapter09186a008063705f.html
Once configured the results is :
" The ASA 5500 series adaptive security appliance supports the AIP SSM, which runs advanced
IPS software that provides further security inspection. The adaptive security appliance diverts packets
to the AIP SSM just before the packet exits the egress interface (or before VPN encryption occurs, if
configured) and after other firewall policies are applied. "
I hope it helps .. please rate it if it does !!!
