Devices in peer-oriented SNA networks participate in Advanced Peer-to-Peer Networking (APPN). Each device, known as a physical unit type 2.1 (PU 2.1), handles all network routing functions as well as normal computing activities and applications.
PU 2.1 devices such as AS/400s and SNA Server computers are associated with LU 6.2. Devices or programs in an APPN network appear as LU 6.2 entities and communicate over sessions that use APPC.
Figure 6.1 Peer-oriented SNA network model.
APPC enables applications on different APPN systems to communicate directly with each other across a network. In APPN networks, SNA Server provides support for APPC and emulates a low-entry networking (LEN) node.
In the AS/400 environment, APPC is used for a variety of applications, including:
Programs that use APPC to communicate are called transaction programs (TPs). Transaction programs use APPC LU (LU 6.2) names to access other systems and other transaction programs (Figure 6.2). With SNA Server, a transaction program — such as a 5250 terminal emulator — may also use an APPC LU alias to access another TP. In this case, the LU alias maps to an LU name that is actually used to access the other system’s TP.
Figure 6.2 Transaction programs in APPC.
APPC uses pairs of LUs to facilitate simultaneous, bidirectional communication between transaction programs. To achieve this, a local LU and a remote LU are defined on each device in the APPN network.
The perception of local and remote LUs is dependent on the system that you are configuring. When configuring SNA Server, the local APPC LU corresponds to the SNA Server computer and the remote LU corresponds to the AS/400. Local LUs on one system communicate with remote LUs on another system. If you view the configuration from the AS/400 perspective, the SNA Server computer is the remote system and the AS/400 is the local system.
Figure 6.3 APPC conversation components.
When a client-server network TP, such as a 5250 terminal emulator, requests a conversation with a TP on the AS/400 (remote system), the server (local system) acts on behalf of the client request and negotiates an LU 6.2 - LU 6.2 session to the AS/400. The data sent or received from the AS/400 TP is handled by the server and sent to the client TP over the selected client-server protocol
(Figure 6.3).