Hierarchical Domains and Subareas

Hierarchical SNA networks organize nodes into domains and subareas. In a hierarchical SNA network, an SNA domain represents a set of network resources that are managed by a SSCP implemented within the host system VTAM. Each domain has only one SSCP.

Some large SNA networks contain hundreds of domains. When end users in different domains need to communicate through an LU to LU session, the SSCPs in each domain must first establish communications through an SSCP to SSCP session.

SNA domains typically include several subareas. A subarea is composed of one subarea node (a type 5 host node or a type 4 node [an FEP]) and the resources it controls, including type 2 nodes, as illustrated in Figure A.5.

Enlarge figure

Figure A.5 Single Hierarchical SNA Domain with Three Subareas

Subarea nodes can communicate with the peripheral nodes in its subarea and can also establish one or more links with other subarea nodes. Links between subarea nodes are called transmission groups. The capability to establish transmission groups allows subarea nodes to create routing tables of other subareas, which are used for SNA network addressing and routing sessions.

Maintaining multiple links in a transmission group maximizes network availability and performance. If one link should fail, SNA reroutes the data over one of the other links in the transmission group.

Note

In SNA peer-to-peer networks, domains and routing functions are defined differently than in SNA hierarchical networks. Peer-oriented networks are described in the following section.

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