I'm trying to understand if compression is available using a CISCO VPN client 5.x to a CISCO device (ASA, 871 etc.)
Our site has recently moved from Windows dial-in, where compression was enabled, and we have noticed the CISCO client show 'no compression'.
This URL describes how to configure compression on the ASA.
Compression can be configured as a parameter within the crypto ipsec transport-set in the IOS.
Compression/decompression takes a signficant toll on the resources of the Cisco appliance if it lacks dedicated hardware for those functions. You may want to confine its use to only where it is needed for dial-up clients.