The Functional Layers of SNA

An SNA network consists of a set of Network Addressable Units (NAUs) connected by a common path control network. The logical connection between NAUs is called a session. Each NAU is organized into functional layers. Each layer serves the next highest layer in its own node and relates to its peer layer on another node. Direct communication with another node occurs only at the lowest layer of a network. The functional layers of SNA implemented by the SNA IMF and SNA link products, beginning with the lowest level, are as follows:

Physical Control, which sends and receives bits between nodes. It defines the mechanical and electrical interfaces and the bit-level data flow to the network.
Data Link Control, which schedules and sends data across a link (physical connection) between two nodes and monitors errors that occur on the link.
Path Control, which provides paths between end users (terminal operators, programs, or devices) and routes data between these end users.
Transmission Control, which synchronizes and paces session-level data traffic, checks the sequence numbers of requests, and codes and decodes end user data.
Data Flow Control, which monitors and controls the flow of data between two logically connected Network Addressable Units.
Presentation Services, which formats data to be displayed or printed.
Application, which provides services that directly support end users such as resource sharing, file transfers, remote file access, and data management on LU-LU sessions.

You can find introductory material about SNA and data communications in the Communicating With IBM primer and the Getting Started with SNA Node Management manual.

SNA IMF, along with the SNA link product, implements the seven functional layers of SNA.

The Functional Layers of SNA

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