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(Systems Network Architecture) IBM's mainframe network standards introduced in 1974. Originally a centralized architecture with a host computer controlling many terminals, enhancements, such as APPN and APPC (LU 6.2), adapted SNA to modern peer-to-peer communications and distributed computing environments. SNA has mostly been replaced with TCP/IP as the world has migrated to the IP protocol. Following are some of SNA's basic concepts.

Nodes and Data Links
In SNA, nodes are end points or junctions, and data links are the pathways between them. Data links include high-speed local channels, the SDLC data link protocol, Token Ring and Ethernet.

Nodes are defined as Type 5 (hosts), Type 4 (communications controllers) and Type 2 (peripheral; terminals, PCs and midrange computers). Type 2.0 nodes communicate only with the host, and Type 2.1 nodes communicate with other 2.1 nodes (peer-to-peer) without going to the host.

SSCPs, PUs and LUs
The heart of a mainframe-based SNA network is the SSCP (System Services Control Point) software that resides in the host. It manages all resources in its domain.

Within all nodes of an SNA network, except for Type 2.1, there is PU (Physical Unit) software that manages node resources, such as data links, and controls the transmission of network management information. In Node Type 2.1, Control Point software performs these functions.

In order to communicate user data, a session path is created between two end points, or LUs (Logical Units). When a session takes place, an LU-LU session is established between an LU in the host (CICS, TSO, user application, etc.) and an LU in the terminal controller or PC.

An LU 6.2 session provides peer-to-peer communication and lets either side initiate the session.

VTAM (Virtual Telecommunications Access Method) resides in the host and contains the SSCP, the PU for the host, and establishes the LU sessions within the host.

NCP (Network Control Program) resides in the communications controller (front end processor) and manages the routing and data link protocols.

SNA Layers

SNA is implemented in functional layers starting with the application that triggers the communications down to the bottom layers which transmit packets from station to station. This layering is called a "protocol stack." The SNA stack is compared with the OSI model below. Although SNA had major influence on the OSI model, there are differences in implementation.

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