In order to implement Nortel Converged Office, you must have an OCS Proxy server and a Mediation server in addition to your existing OCS Environment.
OCS Proxy server will be used by Nortel MCM application, it's a normal OCS role without users, and no application filter installed, the only application filter is being installed is Nortel MCM.
MCM application on the proxy server is taken care of all SIP signaling to and from Nortel environment and if you want to implement Remote Call Control, you must have at least this role
Mediation server is for transcoding between RT-Audio and G.711 and the other way, as with all other voice deployments, you must have this role if your are going to use Communicator as your "softphone"