The business user of data communications most often applies the technical material in BDC4 to the planning and design of a data communications system, or to the operation and management of such a system. The latter issues are discussed in Part 6 of BDC4. In this article we deal with planning and design of data communication systems. We look first in Section I at the larger issues of how the organizational strategy, culture and policies affect planning and designing data communication systems. In Section II, we look at systematic methods for planning and design. Section III is an overview of design algorithms and tools. Appendix A gives some of the more straightforward of the quantitative design techniques. Finally, Appendix B is a case study of on-line book sales.