The "back end" of the AppleLink system was hosted on General Electric's Information Services (GEIS) (division) Mark III time-sharing mainframes and worldwide communications network. AppleLink translated the user's GUI actions into a transaction protocol that was sent into a server program running on the mainframe. The connection used a proprietary error free sliding window protocol called EF3. Later versions upgraded this protocol to GEIS' "EFX" (Error-Free eXtended) protocol. GEIS charged fairly substantial prices to Apple, both for maintaining the service (about $30 million a year per year), and the end-users' connection fees of about $15 an hour during business hours (which Apple rebilled to the users). For some years prior to the discontinuation of the system, GEIS maintained a number of employees on the campus of Apple's Cupertino, California, headquarters, co-located with the Apple On-Line Services organization. Repeated attempts to negotiate a lower cost failed, and Apple management chafed at paying for a service that had no obviously measurable income.