Traditional SNA networks consist of peripheral nodes, called physical units (PUs), containing several types of logical units (LUs). The physical units are typically type PU2.0 (for example, a 3174 control unit) or type PU2.1 (such as a Windows-based workstation, AIX, or iSeries, which runs applications, gateways, or servers). The LUs (in PU2.0 or PU2.1 nodes) can be type LU0, LU1, LU2, or LU3. Types LU0, LU1, LU2 and LU3 are called "dependent LUs" because they require VTAM services to establish LU-LU sessions.

Another LU type is LU6.2. An LU6.2 can act as either an dependent or independent LU. LU6.2 (APPC) acts as a dependent LU when it resides in a type PU2.0 node and as an independent LU in type PU2.1 nodes. As an independent LU, an LU6.2 can initiate sessions on its own (is capable of sending BINDs) without services from VTAM.

Historically, type PU2.0 and PU2.1 nodes have been attached to a subarea boundary function, typically to an NCP. As an alternative, many customers have used Open Systems Adapters, channel-attached routers, or 3172s to attach these devices to VTAM's boundary function, rather than NCPs.

The dependent LU server (DLUS) is implemented only in Type 5 (VTAM) network nodes. The DLUS function enables VTAM to provide SSCP services for dependent LUs located in remote APPN end nodes or network nodes, which act as the dependent LU requester (DLUR).

Two LU 6.2 sessions (one inbound, one outbound) are established between a DLUS and a dependent LU requester (DLUR), and these LU 6.2 sessions are collectively known as the CPSVRMGR pipe. SSCP-PU and SSCP-LU session flows use the CPSVRMGR pipe. An SSCP-PU session is established between a VTAM network node and the PU that owns the dependent LU, and an SSCP-LU session is established between VTAM and the dependent LU.

Session initiation flows for the dependent LU are sent over the SSCP-LU session, and VTAM can use subarea or APPN flows to initiate a session with the PLU. BIND and session data are then routed directly between the PLU and the dependent LU.

Dependent LU requester/server (DLUR/DLUS) illustrates a skeleton configuration of DLUR/DLUS.

Figure 1. DLUR configuration