Ref: 99980063
Title: What is SNA?
Date: 6/1/86

Copyright 3Com Corporation, 1991.  All rights reserved.

SNA is a Systens Network Architecture developed by International
Business Machines Corporation to govern communications in IBM
environments.  An SNA network is a collection of IBM computers,
communications controllers, cluster controllers, and devices.
These are all called nodes.  Any component in the environment
that can be communicated with is called a Network Addressable
Unit (NAU).  SNA provides communications between NAU via sessions
that are logical connections.  Each Network Addressable Unit
consists of subcomponents that describe its characteristics.
These are called Logical Units (LU) and Physical Units (PU).  In
every SNA network, at least one NAU must also contain a System
Service Control Point (SSCP) which provides control and
monitoring of network resources.  The LU and PU types are often
used to define levels of compatibility between non-IBM equipment
and the IBM environment.  The different types are defined below.

Logical Unit (LU)

The LU type defines the logical aspects of the node.  There are
currently 7 types of LU defined.

LU Type 1.  Defines the protocols between a host application and
            remote batch terminal such as 3770 device.

LU Type 2.  Defines the protocols between a host application and
            3270 display terminal.

LU Type 3.  Defines the protocols between a host application and
            3287 printer.

LU Type 4.  Defines the protocols between a host application and
            work processor (display writer) or two terminal
            devices.

LU Type 6.  Defines the protocols between host applications; for
            example, CICS and IMS.

LU Type 6.2 Used in new IBM products for process-to-process
            communication.

LU Type 7.  Defines the protocols between a host application and
            5250 display terminal.

Physical Unit (PU)

The PU type defines the more physical aspects of the node.  There
are five types of PU.

PU Type 1.  Display writers (3278 or 3279) or printers (3287).

PU Type 2.  Terminal cluster controllers (327X) or remote batch
            terminals (377X).  Most systems that emulate IBM
            equipment in SNA are PU Type 2.

PU Type 3.  Not currently used.

PU Type 4.  37X5 Communications Controller (local or remote).

PU Type 5.  Host node that contains a System Service Control
            Point (SSCP).

SNA and the OSI Model

SNA is layered into a grouping of related functions that are
logically separate from the functions in other layers.  The
implementation of the functions in one layer can be changed
without affecting functions in other layers.  While SNA predated
the OSI Reference Model, its layers roughly follow the ISO model
as shown in the figure below.

      Mapping Between SNA Layers and the 7-layer OSI Model

               SNA                        OSI
   ---------------------------------------------------------
  |          End User        |     Application Layer        |
  |--------------------------|------------------------------|
  |   Presentation Services  |     Presentation Layer       |
  |--------------------------|------------------------------|
  |   Data Flow Control      |                              |
  |--------------------------|     Session Layer            |
  |   Transmission Control   |                              |
  |--------------------------|------------------------------|
  |                          |     Transport Layer          |
  |   Path Control           |------------------------------|
  |                          |     Network Layer            |
  |--------------------------|------------------------------|
  |   Data Link Control      |     Data Link Layer          |
  |--------------------------|------------------------------|
  |   Physical Control       |     Physical Layer           |
   ---------------------------------------------------------

