Systems Network Architecture (SNA) provides users with services needed in a networking environment: sharing of communication lines, directory services, connection establishment, improved reliability, management services, and an appropriate interface and set of facilities for distributed programming. It provides these services within the scope of a layered architecture that has evolved to the rich, set-functions and products that it supports. The major concepts of SNA are reviewed. The three major functions that a network provides-transporting data, enabling distributed programming, and application-level services-are discussed. >