Subarea links are the different types of connections that can be defined between subareas. For example, links include a channel connection between two VTAMs, a channel connection between VTAM® and NCP, or an SDLC, frame-relay, or token-ring connection between two NCPs.

Transmission groups (TGs) allow links with similar characteristics to be logically associated. Several links between the same two subarea nodes might have the same TG number, or the TG number might represent only one link.

Explicit routes (ERs) consist of a list of TGs and subarea nodes in one direction between the two end points of the route. For each ER in one direction, there must be a corresponding ER in the opposite direction that follows the same path.

Virtual routes (VRs) represent the complete route path. Each VR has a specific number and the VR definition associates that VR number with the ER to be used for a session. The VR specifies two one-way ERs to create a round-trip path.

Explicit routes and VRs are defined within path definition statements. Each path definition statement gives the ultimate destination point of the path. The ER gives the transmission group and subarea number for the next hop on the path. The association of the VR number with the ER within the path definition connects the individual hops together into a complete path from one end point to the other.

Each LU-LU session has an associated logon mode table entry. This entry specifies, among other things, the name of a subarea COS that contains a list of pairs of VR numbers and transmission priorities that can be used to establish the session. Using the subarea COS, you can specify the VR that best fits the characteristics needed for that session such as capacity, security, and speed.

For more information about subarea routing, see Network routing for subarea nodes.