SNA Functional Layers

The functionality of each SNA layer operating in the context of a hierarchical network model is summarized in Table A.5. The transaction services layer and the physical layer are both outside of the original scope of SNA specifications but are included here to show the full range of IBM host networking.

Table A.5 Functional Layers in Hierarchical SNA Networks

SNA Functional Layers


Transaction Services

A conceptual layer to represent host applications that establish and terminate SNA user-to-user sessions.

Functional Management

Formats data streams and converts character codes for presentation (for example, 3270 data streams). Also controls active sessions.

Data Flow Control

Provides protocols for managing data integrity in sessions, synchronizing data exchange, and packaging data units.

Transmission Control

Uses VTAM and NCP to manage active end-to-end sessions. Controls data sequencing and pacing. Optionally supports data encryption and decryption.

Path Control

Routes data between hierarchical SNA nodes using VTAM and NCP.

Data Link Control

Manages data transmissions between nodes and performs error detection and recovery. Manages transmissions over standard WAN (including SDLC), LAN (Token Ring, Ethernet, FDDI, Asynchronous Transfer Mode (ATM)) and channel interfaces.

Physical Control

Transmits bits over industry standard physical/electrical circuits. SNA generally relies on industry standard LAN and WAN specifications at this layer, although specifications for channel attachments are unique to SNA.



APPN uses different methods to accomplish networking functions at some layers.

Because many networking professionals are familiar with the Open Systems Interconnection Reference Model (OSI model), it is sometimes helpful to describe the SNA functional layers within the context of the OSI model.

SNA functional layers define protocols and services that are similar in scope to layers in the OSI model. In fact, the OSI model was developed in response to SNA and was influenced by SNA functions. Originally, SNA did not include specifications for layers that correspond to the physical and applications services layers in the OSI model. However, recent descriptions of SNA include all seven layers, including the physical and transaction services layers (equivalent to the application layer in the OSI model). The transaction services layer plays an important role in IBM host system networking because transaction-based applications often initiate SNA networking sessions.

While the scope of the networking layers described by the SNA and OSI models are similar, the methods SNA uses to perform networking functions within each hierarchical networking layer are quite different from methods described by the OSI model, as is made apparent in Figure A.4.


Figure A.4 SNA Functional Layers Mapped to the OSI Model

Path Control

The three lower SNA functional layers comprise the SNA path control network components, also called the SNA transport network. These layers control the routing and flow of messages through the network and provide the interfaces to physical media that carry transmissions between network devices.

Network Addressable Units

The four upper SNA functional layers provide end-to-end communication protocols between network addressable units (NAUs). NAUs include the following SNA components:

  • Physical units

  • Logical units

  • Control points (such as SSCP in VTAM)

SNA nodes might each contain several NAUs to manage specific networking functions. Each NAU has a unique address so data can be routed to the NAU during an SNA session.