Abstract: A mediation server assigns customized “plug-ins” to particular network access servers (NAS) that allow a service provider application to use a single interface to communicate with multiple types of NASs. Vendor specific details of NAS transaction processing are hidden in the customized plug-ins implementations in which a service provider application has no knowledge of such transaction processing information. The mediation server extracts a user identifier from a call request, and uses the user identifier to select a mediation handler plug-in. The mediation server then uses the mediation handler plug-in to communicate with a particular NAS. In addition, a RADIUS server uses a NAS identifier to assign a RADIUS handler plug-in to a NAS session, and uses the assigned RADIUS handler plug-in to communicate with the NAS.