SNA Terms and Concepts

Basic SNA Concepts

A logical record consists of a two- or four-byte header starting with a two-byte length field, often represented as “LL,” followed by up to 32,765 bytes of data. Logical records can be grouped together and sent as a block, transmitting more than one logical record with a single call to the SEND function.

•In a mapped conversation, information is passed to the SEND function as a pointer to a single, unformatted block of data; the length of the block is passed as another parameter. The block cannot be received as one or more logical records; the receiving TP must do whatever record-level formatting is required.

NOTE

Only LU type 6.2 supports mapped conversations.

Modes

Each LU-LU session has an associated mode that defines a set of session characteristics. These session characteristics include throughput parameters, session limits (such as the maximum number of sessions between two LUs), message sizes, and routing parameters.

Each mode is identified by a unique mode name. The mode name must be the same on all SNA nodes that use that mode.

Route Selection

To establish an LU-LU session, a route must be calculated between the nodes where the two LUs reside. A route is an ordered sequence of links and nodes that represents a path between the two nodes.

SNA networks support the following methods of route selection:

•For subarea networks, you must predefine all routes between subarea nodes.

•For peer networks that do not support APPN, type 2.1 nodes can support sessions only with adjacent nodes; their sessions cannot be routed through intermediate nodes.

•For APPN networks, SNA can compute routes dynamically at the time of session initiation, using a class of service specified for the mode used by the session (see “Class of Service”).

Chapter 1

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