2. USAGE CONSIDERATIONS › 2.2 General Analysis Considerations › 2.2.2 SNA Overview
2.2.2 SNA Overview
IBM's Systems Network Architecture (SNA) is a collection of
standards which define in detail the requirements for
communications within an IBM network. SNA is not a product
in itself, although many of the standards which comprise SNA
are implemented in hardware and software products marketed by
IBM and other vendors. Not all of the features and
facilities defined by SNA are currently implemented in
products; also, additions to the architecture are made
frequently (usually with the implementation of a new
product). Thus any description of SNA is limited by the
necessity to describe a "snapshot" of an evolving entity in
terms of the products that are currently implemented. Such a
description is presented, without further apology, herein.
In the most general terms, SNA is a hierarchy consisting of
seven layers. Each layer performs a specific function
consisting of one or more services for the next higher layer.
In turn, each layer requests services from the next lowest
layer and communicates with peer layers throughout the
network. Because the layers are functionally self-contained,
most changes to a layer are restricted to that layer. Only
changes to the interface between layers can affect another
layer and then only the next higher layer is affected.
Figure 2-3 shows a diagram of the seven SNA layers.
/\
/ \
/ \
| |
+-----------------------------------------------------------+
| TRANSACTION SERVICES |
|Implements service transaction programs. |
|Provides configuration, session, and management services. |
+-----------------------------------------------------------+
| |
+-----------------------------------------------------------+
| PRESENTATION SERVICES |
|Defines protocols for program-to-program communication. |
|Controls communications between transaction programs. |
+-----------------------------------------------------------+
| |
+-----------------------------------------------------------+
| DATA FLOW CONTROL SERVICES |
|Assigns sequence numbers, correlates requests and responses|
|builds chains and brackets, coordinates send/receive modes.|
+-----------------------------------------------------------+
| |
+-----------------------------------------------------------+
| TRANSMISSION CONTROL SERVICES |
|Verifies received sequence numbers, enciphers/deciphers, |
|manages session-level pacing, provides boundary support. |
+-----------------------------------------------------------+
| |
+-----------------------------------------------------------+
| PATH CONTROL SERVICES |
|Selects paths, routes data, segments/reassembles data, |
|controls virtual and explicit routes (including VR pacing).|
+-----------------------------------------------------------+
| |
+-----------------------------------------------------------+
| DATA LINK CONTROL SERVICES |
|Transmits message units across links, manages link-level |
|flow, performs link-level error recovery. |
+-----------------------------------------------------------+
| |
+-----------------------------------------------------------+
| PHYSICAL CONTROL SERVICES |
|Defines the electrical and transmission characteristics |
|required to establish, maintain, and terminate links. |
+-----------------------------------------------------------+
Figure 2-3. The Seven SNA Layers
The functions of the seven architectural layers of SNA are
implemented by combinations of hardware and software
components. Hardware components include the following:
o Host Processors (mainframes)
o Communications Controllers (37XX and equivalents)
o Cluster Controllers (3274, etc.)
o Workstations (3278, etc.)
o Printers (3287, etc.)
Software components include the following:
o Telecommunication Access Methods (ACF/VTAM, etc.)
o Application Subsystems (TSO, IMS, CICS, etc.)
o Network Control Programs (ACF/NCP, etc.)
These components are defined and discussed in greater detail
in the following sections:
1 - SNA Terminology
2 - SNA Network Design Fundamentals
3 - SNA Session Activation and Termination
