In a subarea network, every T5 and T4 node is assigned a subarea number. The subarea number has to be unique in the SNA network.

The SNA network is assigned a network identifier referred to as a NETID. All the resources in the same subarea network carry the same NETID name. In the same NETID subarea network, you can have more than one z/OS system that implements the SNA protocol.

Every z/OS system with VTAM that implements SNA is referred to as a domain, which is an area of control. Within a subarea network, a domain is that portion of the network managed by the SSCP in a T5 subarea node. When a subarea network has only one T5 node, that node must manage all of the network resources.

A subarea network that contains only one T5 node is a single-domain subarea network. When there are multiple T5 nodes in the network, each T5 node may control a portion of the network resources. A subarea network that contains more than one T5 node is a multiple domain subarea network.

The SSCP can also set up and take down sessions with other domains through the cross-domain resource manager (CDRM). Figure 1 illustrates a cross-domain network. Before applications in one domain can have cross-domain sessions with resources in another domain, a CDRM session must be established between the SSCPs of the two domains.

For a session between SSCPs to exist, VTAM must know about all cross-domain resource managers with which it can communicate. You must define to VTAM its own cross-domain resource manager and all other cross-domain resource managers in the network.

The cross-domain resource manager that represents the SSCP in your domain is called the host cross-domain resource manager. The cross-domain resource managers that represent the SSCPs in other domains are called external cross-domain resource managers.

Figure 1. SNA subarea cross-domain network