The difference between logical and physical configurations is the most significant distinction between SNA, BSC, and TCP/IP protocols. In BSC NJE, the physical configuration is the logical configuration. In SNA or TCP/IP NJE, the logical and physical configurations can be quite different. Various routing tables in the host and the communication controllers and routers determine the logical configuration.

In Figure 1, Node A can have a direct session with Node C even though no direct line exists between them. Node A and Node C view themselves as being directly connected to each other. ACF/VTAM^* and Multi-Systems Networking Facility (MSNF) support this connection through the communications controllers (cc) and the network control programs (NCP), which are transparent to the nodes. Figure 1 represents a simple SNA NJE configuration.

Figure 1. Simple SNA NJE Configuration