Connection to Express via APN or Internet (Carrier)
 

I am really confused how the BB device connects to the BES Express server in order to synchronize its mail/contacts etc. Is this done via an APN connection that is configured on our internal network by our carrier or is it a full blown internet connection (so through our FW/Proxy via the internet profile on the BB device)? In attached architecture layout all connections are routed via wireless network. Is this the same as in APN of current carrier?
I cannot find these specific details in the documentation on the BB forums. We currently have to merge another company that is on a different carrier that does not have a private entry on our network. So I am trying to find out all requirements to have a sound implementation of either BES Express or BES Enterprise with or without private APN.
Second I would like to know what the function of the RIM connection is.
Thanks in advance