Systems Network Architecture Considerations (SNA) › General Considerations › SNA Terminology

SNA Terminology

The following SNA terms are used in this appendix. Special CA IDMS/DC considerations are included along with their definitions:

• A logical unit (LU) is a port through which you access the SNA network, a single network addressable unit (NAU). For example, an LU can be an end-user terminal, a program such as CICS or CA IDMS/DC, or a device such as a display writer.

  Note: Unless otherwise specified, the discussions in this appendix apply to all LU types. Special LU6.2 considerations will be noted.

• A session is a logical connection between two logical units that enables the exchange of messages. Two logical units that share a single physical connection can have one or more sessions between them. Each session is represented in the CA IDMS/DC environment by a single physical terminal element (PTE)/logical terminal element (LTE) pair.
• A conversation is equivalent to one complete transaction between logical units. A conversation is delineated by a begin bracket and an end bracket. In the CA IDMS/DC environment, a conversation is requested by a #TREQ ALLOC statement, or is started by the remote LU, and is terminated by the LAST option on a #TREQ WRITE statement. Data is exchanged by two logical units in a conversation by using various forms of the #TREQ READ and WRITE statements.

Multiple LU-LU Sessions

The following figure illustrates how the SNA driver, functioning as an LU, takes part in multiple sessions. There are four sessions established between the SNA driver and CICS, and one session established between the SNA driver and a display writer. Each session can support only one conversation at a time. This configuration can support up to five simultaneous conversations: four between CA IDMS/DC and CICS, and one between CA IDMS/DC and the display writer.