Before programs on two different systems can converse in an APPC network, connections on various levels must exist. Basically the connections are on the physical level, the logical level, and the program level. The physical level of connection is the hardware that links like and unlike systems together. After the physical connection is made, a logical level of connection is possible. Logical connections are the definitions that permit VTAM® to establish a session. The session allows transaction programs on different systems to communicate on the program level. The program level connection is the transaction program identification and the use of callable services, which establish a conversation.

A hardware connection can support multiple sessions. A single session supports one conversation at a time. Figure 1 shows the relationship of the three levels of connection.

Figure 1. Levels of Connections

When using RACF®, a security administrator can define security for an APPC network on both the logical level and the program level. Physical level security is accomplished through facilities planning.

For information about security, see Setting up Network Security.