In 1990 the East Bay Municipal Utility District was using computer assisted design (CAD) on its mainframe, but other applications on personal computers without any link to the mainframe. This led to severe design fragmentation, nontransferable data, work duplication, and split hardware peripheral pools. A plan needed to be devised to integrate personal computers with the existing CAD environment. A very powerful PC network incorporating PCs, Macintoshes, and the mainframe workstations became the primary focus. The development of such a network required the use of the OSI (Open Systems Interconnection) Model to facilitate cross platform connectivity. The OSI model can be simplified to demonstrate the decision making process involved in developing a PC network and the selection of various options. The Personal Computer Interconnectivity model is discussed in this paper from the bottom up, including the following layers: physical, data link, network software. The decision making process is also analyzed. Special attention is paid to duplicating the mainframe environment, Macintosh support, and allowing and promoting network growth. Problems have included corruption of print jobs sent by PCs, slow floor-to- floor network links, and sporadic connection performance due to cabling problems.