SNAX/APC Configuration and Management Manual

Introduction
SNAX/APC Configuration and Management Manual—138787
1-2
LU 6.2 and SNAX/APC Concepts
The transaction services layer includes specifications for special programs considered to
be part of the LU. The end user of the LU, usually an application program, is positioned
above transaction services, outside of the LU. Presentation services is designed to
manage the TPs local to the LU, including their interaction with the network through use
of the APPC verbs. Together, these layers are considered to be the upper layer of the 6.2
model within SNA.
Data flow and transmission control are used to send and receive data over the SNA
session. These functions are duplicated for each half-session that the LU is managing—
thus the Figure 1-1
shows multiple instances of these layers. Together, these layers are
known (in IBM terminology) as the half-session protocol machine. They are called half-
session because the LU at each end manages half the session, and protocol machine
because the primary function is to translate the TP verbs and data to a form suitable for
transmission on the lower levels of SNA (the path control network shown in this figure).
When a program begins to communicate with SNA, it must first establish
communication with an LU within the SNA network. SNA communication through an
LU-LU session is established when the program indicates to its LU that it wants to
communicate with another program in the SNA network. The other program must also
be connected to its own LU.
Figure 1-1. Layers of SNA For LU 6.2
001CDT .CDD
Path Control
Network
Path Control
Data Link
Control
Physical
Control
Half-Session
Protocol
Machines
Data Flow
Control
Transmission
Control
Data Flow
Control
Transmission
Control
Data Flow
Control
Transmission
Control
The Upper Layer
Transaction
Services
Presentation
Services