Specialized VTAM Nodes

As the name implies, "the specialized VTAM nodes" are implemented only in the mainframe. These nodes enable the mainframe to connect directly to both subarea and APPN networks.

Interchange (network) nodes (ICNs)

An interchange node resides on the border of an APPN network and a subarea network. It provides protocol conversion between subarea and APPN networks to enable the integration of the two types of networks. Because an interchange node can convert session requests from one protocol to the other and can provide intermediate routing, it can establish sessions from one type of network to the other.

An interchange node combines the function of a subarea node and a network node. It controls resources and functions as a network node in the APPN network and as an SSCP and a cross-domain resource manager (CDRM) in the subarea network. All of the characteristics described for network nodes and subarea nodes apply to interchange nodes.

An interchange node:

Uses subarea path definitions to determine routes within the subarea network
Uses the topology database to determine routes within APPN networks
Uses both SSCP-SSCP and CP-CP sessions to communicate with other nodes
Has a subarea number and is defined as a network node, NODETYPE=NN
Can own and activate network control programs (NCPs)

The interchange node communicates network control data by using SSCP-SSCP sessions with other subarea nodes and CP-CP sessions with other APPN nodes. To enable it to participate in the subarea network, it is defined with a unique subarea number and requires subarea path definition statements. It can be connected to other APPN nodes, low-entry networking nodes, and subarea nodes.

Many IBM mainframe installations implement interchange nodes because both APPN and subarea components must co-exist in most mainframe networks. Those installations still have subarea networking but are starting to implement APPN.

VTAM determines the node type of the mainframe using two parameters in VTAM's start option. If HOSTSA is set to a subarea number and NODETYPE=NN, an interchange node is implemented by VTAM. If HOSTSA is not specified and CONNNTYPE=NN or EN, VTAM implements an APPN node.

Composite network nodes (CNNs)

Because a Network Control Program (NCP) does not have a control point, NCPs cannot function as APPN nodes by themselves. Instead, NCPs work with their owning VTAM (the one that activated the NCP) to present the appearance of single APPN node to other APPN nodes. This collection of a VTAM network nodes and its owned NCPs is referred to as a composite network node; note the following:

The composite network node can have either APPN functions only, or both APPN and subarea functions.
Existing subarea protocols are used within the composite network node for communication between the T5 node and its T4 nodes.
APPN protocols are used to communicate with other APPN network nodes and end nodes. The T4 node provides boundary function services for attaching other APPN nodes.

The rationale for composite network nodes is to ease the migration from subarea network to APPN. With composite network nodes, an installation can preserve its current hardware while still providing a migration path to APPN networking. Figure 1 shows a composite network node.

Figure 1. Composite network node

A composite network node configuration provides the functional combination of a single T5 (VTAM) node. All the T4 (NCP) nodes that the composite network node owns appear as one logical APPN network node to other low-entry networking and APPN nodes to which it is interconnected.

Figure 1 shows a composite network node connected to two VTAM hosts (HOSTB and HOSTC) acting as an interchange node. The interchange node supports SSCP-SSCP sessions with other VTAM nodes as well as CP-CP sessions with adjacent APPN network nodes and end nodes. This enables the interchange node to use both APPN and subarea data flows to locate LUs. From the APPN node's viewpoint, LUs owned by subarea VTAMs (for example, LUB on HOSTD) appear to reside on APPN end nodes.

Migration data hosts (MDHs)

A migration data host (MDH) combines the function and roles of an APPN end node and a subarea node, and resides on the periphery of a combined APPN and subarea network. A migration data host:

Uses subarea network routing definitions
Does not perform intermediate session routing or interchange node functions in combined APPN/subarea network.
Uses CP-CP and SSCP-SSCP sessions to communicate with other nodes
Is defined as an end node, NODETYPE=EN
Can attach to NCPs over APPN or subarea links, but cannot activate NCPs
Has a subarea number defined on the HOSTSA start option

Like a data host in a subarea network, a migration data host is dedicated to processing application programs and does not control network resources. It also participates as a cross-domain resource manager (CDRM) in the subarea network. The migration data host also functions as an end node in the APPN network. All of the characteristics previously described for end nodes apply to migration data hosts.

To enable the migration data host to participate in the subarea network, it is defined with a unique subarea number and supports subarea path definition statements.

The migration data host communicates network control data by using SSCP-SSCP sessions with other subarea nodes and CP-CP sessions with its network node server. It can be connected to other APPN nodes, low-entry networking nodes, and subarea nodes.