ABSTRACT
System Network Architecture (SNA) has evolved towards greater flexibility and can be considered as a decentralized network. The objectives of SNA include: Distributed Processing Orientation, Unification, Reliability, Ease of modification and Versatility. SNA is built around a seven layer concept. The seven SNA Layers are called Physical Control, Data Link Control, Path Control, Transmission Control, Data Flow Control, Function Management, and Application. The protocol used by SNA at the Data Link Layer is called Synchronous Data Link Control (SDLC). It was created before High Level Data Link Control (HDLC) and greatly influenced HDLC's specifications. Digital Network Architecture (DNA's) Physical Link Layer is the equivalent of the Open System Interconnection Physical Layer. Digital Data Communications Message Protocol (DDCMP) allows the transmission of variable length messages over a physical link, and provides mechanisms for error detection and error recovery. DDCMP also provides for full or half-duplex, point-to-point or multipoint communications. There are three types of messages in DDCMP: data, control, and maintenance.
