In addition to specifying start options and coding configuration lists, you'll need to identify resources in the network to VTAM.

Depending on your network, you might need to define a combination of the following resources:

1. Application programs
   1. Every SNA application program like CICS, IMS, TSO, in-house VTAM applications, and applications developed by other companies, is defined to VTAM.
   2. When the application initializes, it connects to VTAM and informs VTAM that it is ready to accept requests and service the LUs in the network.
2. Network control programs (NCPs) and peripheral nodes
   1. The NCP is the software residing in the communication controller. The first task the systems programmer performs is the definition of all the lines, the peripheral equipment (T2.0 and T2.1), links that connect to other SNA domains and the explicit routes that traverse the NCP and virtual routes that originate in the NCP.
   2. During VTAM initialization, VTAM connects to the NCP, activates the resources in the NCP, and determines if a load (IPL) of the NCP is required.
3. Physical units (PUs) and logical units (LUs)
   PUs and LUs are defined either in the NCP or in VTAM. PUs are connected to the mainframe by any of the following means:

   1. Directly attached through a channel (either parallel or ESCON)
   2. Leased lines to the NCP
   3. Switched (dialed) lines to the NCP
   4. LAN-connected, either directly to an NCP attached LAN, or directly to the mainframe using an OSA card

   PUs connected through dialed lines and LAN-connected PUs are defined to VTAM as switched major nodes. The definition includes the PU and the LUs associated with that PU. Because a switched connection is casual (comparable to a phone call) and un-authorized people can dial the mainframe or connect to a LAN jack, the switched major node includes some very basic security definitions that can be used to identify the switched major node during the connection phase.

4. If your network is a multiple domain network, additional definitions are required. These definitions include:
   1. Cross-domain resource manager (CDRM). CDRM defines the logical connection to an adjacent mainframe. The CDRM connection is used to send and receive control information between the two mainframes.
   2. Cross-domain resource (CDRSC). CDRSC is an SNA resource that exists in the domain of another host and sometimes sets up or actually maintains a session with a local resource in the domain of the host where the CDRSC is defined.
   3. Adjacent SSCP table (ADJSSCP). The ADJSSCP is used to control the search for SNA resources in a multi-domain SNA network. The ADJSSCP determines the order the search is performed, or in other words, which mainframe is queried first to determine if it owns the resource, and the subsequent hosts if the query was not successful.

Any of these resources can be predefined to VTAM using a static definition statement. For example, applications are predefined using an application major node and application (APPL) statements; switched PUs are predefined using a switched major node for the PUs and LUs. In addition, many VTAM resources can be dynamically defined as VTAM learns of them. A switched PU may be defined when it dials in. Not all resources need be initially defined to subarea nodes. There are several methods by which resources may be dynamically defined to the network.

Although static and pre-defined resources are burdensome, many mainframe installations are reluctant to allow dynamic definition of PUs or LUs. The rationale for preventing dynamic definition is security. When every resource is pre-defined the installation has more control over who acceses the mainframe.

Another method for dynamic, but controlled, definition is called dynamic reconfiguration, which allows a system programmer to add or delete peripheral nodes dynamically. The resources are added or deleted through a VTAM major node configuration statement (stored as a member in SYS1.VTAMLST) or by using VTAM commands entered from the console.

Following are examples of dynamic definitions of SNA resources and how VTAM determines the credentials and attributes for dynamically defined resources.

Dynamic definition of independent LUs allows an independent LU to activate an LU-LU session with an LU in a subarea network without prior definition of the independent LU to the subarea network. The SSCP controlling the independent LU's owning node dynamically stores the name and location of the independent LU as determined from the bind request sent by the independent LU. Subsequently, other LUs in the network can activate LU-LU sessions with the independent LU, because the independent LU is then known to the subarea network.

Dynamic definition of dependent LUs allows dependent LUs to be defined using information that specifies how many dependent LUs can be on switched or non-switched lines.

Dynamic definition of switched resources uses information from the exchange ID (XID) message exchange for switched PUs to allow switched PUs and their associated dependent LUs to be dynamically defined. As with dynamically-defined independent LUs, other LUs in the network can activate LU-LU sessions with the dynamically defined dependent LU, because the dependent LU is then known to the network.

Dynamic PU definition dynamically defines an adjacent link station (or PU). This method, like dynamic definition of switched resources, relies on information obtained from the XID to create dynamically a definition of the resource. After the adjacent link station is defined, it may be used for connectivity to its independent logical units.