2. USAGE CONSIDERATIONS › 2.2 General Analysis Considerations › 2.2.3 Comparison of SNA and the OSI Model

2.2.3 Comparison of SNA and the OSI Model

SNA is the IBM data communication network architecture.
Other organizations and vendors have developed their own
architectures.  Often, data communications architectures are
compared to the generalized structure developed by the
International Standards Organization (ISO).  The ISO
architecture is know as Open Systems Interconnection or OSI.
Since the OSI model is commonly referred to, its fundamentals
are briefly presented here.

The OSI model consists of seven functional layers organized
similarly to the seven SNA layers.  Figure 2-12 shows the
hierarchical arrangement of the layers in the OSI model.

Confusion sometimes exists due to comparisons between SNA and
OSI because, while the structures are similar, the design
objectives are different.  SNA defines the characteristics
required for exchanges among nodes made up of products which
conform to a single architecture (i.e., IBM and compatible
products).  OSI is designed to standardize protocols, thus
allowing communications between nodes adhering to dissimilar
architectures.


                              /\
                             /  \
                            /    \
                             |  |
+-----------------------------------------------------------+
|                     APPLICATION LAYER                     |
|Provides Common and Specific Application Services.         |
|Contains the user interface.                               |
+-----------------------------------------------------------+
                             |  |
+-----------------------------------------------------------+
|                     PRESENTATION LAYER                    |
|Defines protocols that allow systems to control the        |
|representation of the user data stream.                    |
+-----------------------------------------------------------+
                             |  |
+-----------------------------------------------------------+
|                       SESSION LAYER                       |
|Establishes, maintains, and terminates sessions.           |
|Manages normal and expedited data exchange.                |
+-----------------------------------------------------------+
                             |  |
+-----------------------------------------------------------+
|                      TRANSPORT LAYER                      |
|                                                           |
|Provides end-to-end control between two user nodes.        |
+-----------------------------------------------------------+
                             |  |
+-----------------------------------------------------------+
|                       NETWORK LAYER                       |
|Provides control between two adjacent nodes such as:       |
|addressing, flow control, and error detection/recovery.    |
+-----------------------------------------------------------+
                             |  |
+-----------------------------------------------------------+
|                      DATA LINK LAYER                      |
|                                                           |
|Provides for transmission over a single data link.         |
+-----------------------------------------------------------+
                             |  |
+-----------------------------------------------------------+
|                       PHYSICAL LAYER                      |
|Defines the electrical and signaling characteristics used  |
|to establish, maintain, and terminate physical connections.|
+-----------------------------------------------------------+

  Figure 2-12.  The OSI Model