gtps2m4fACF/SNA Data Communications Reference

User CTCP Control Blocks

This section suggests a control block structure that could be used by the CTCP you implement in TPF to define resources accessed via an X.25 network.

Exit points exist which permit you to create, initialize, and maintain these user control blocks. These exit points include:

See the TPF System Installation Support Reference for additional information about these exit points.

In SNA, the LU represents a port into the network. NPSI uses the LU to represent both virtual circuits and to provide a port for controlling an X.25 link. The virtual circuit LU is called a VC_LU. The port for controlling the X.25 link is called an MCH_LU. An MCH_LU may relate to one or many VC_LUs. Both VC_LUs and MCH_LUs are defined in the RVT and have a:

The suggested user control blocks fall into two categories:

  1. The user control blocks defining the network accessed by the X.25 interface protocol and the links and virtual circuits that provide access to the network.
  2. The user control blocks defining the terminal and terminal controller resources that can be reached through the X.25 Network.

The CTCP control blocks pertinent to network access are:

The control block defining terminals is the End Point Control Block (EPCB).

In addition to these control blocks, all of which are assumed to be main storage resident, the XNCL (X.25 Network Contact List), which could be DASD resident, contains the CPCBs (Contact Point Control Blocks) which define the terminal controllers in the switched networks. Figure 105 illustrates the relationship between elements of the X.25 Network and the suggested user control blocks.

Figure 105. TPF X.25 Network Elements and Corresponding User Control Blocks




X.25 Network Control Block

The X.25 Network Control Block (XNCB) provides one place to store information about the X.25 network (for example, TRANSPAC or TELENET), and to anchor and index the Network Contact List (XNCL) and X.25 Link Control Blocks (XLCBs) for the network. Information contained in the XLCB might be:

X.25 Link Control Block (XLCB)

The X.25 Link Control Block corresponds to a NPSI MCH_LU and contains information about an X.25 link:

Virtual Circuit Control Block (VCCB)

The Virtual Circuit Control Block corresponds to a NPSI VC_LU and contains information about a virtual circuit:

The VCCB is a logical extension of the TPF RVT entry that defines the corresponding VC_LU. A VCCB entry is either active or idle. When a VCCB is idle, it is available for call in or call out. When the virtual circuit is in use, it contains information about the remote node.

User Terminal Control Blocks

The remaining control blocks define terminals and terminal controllers that can access TPF applications. The key information needed by these control blocks is a description of the device and its network identification.

The purpose of these terminal tables is two-fold:

  1. The terminal tables contain the information to convert the terminal addressing used by the network to the addressing technique used by TPF and its applications (either LEID or RID addressing).
  2. The terminal tables define the terminal characteristics to enable conversion from the character set and presentation space definition used by the application to that passed between TPF and the terminal.

Terminals and terminal controllers in an X.25 network are identified by a call user ID. The call user ID-to-device definition relationship requires a way to index to a device definition given a call user ID, and a way to discover the call user ID given a device. The control blocks used are the:

End Point Control Block (EPCB)

The EPCB defines the remote resource, usually a terminal. The EPCB contains the following information:

Contact Point Control Block (CPCB)

The CPCB defines a remote terminal controller, its call identifier, and its characteristics. The CPCB contains the following information:

Control Block Residence

Figure 106 illustrates the structure and residency of the suggested CTCP Control Blocks while calls are active.

The control blocks that are referenced for every packet residing in main storage include:

The CPCBs may reside on DASD because they are used only when a CALL_REQUEST command is received or a call out is required. The contact information from the CPCB, <ci>, needed for processing messages is copied to the VCCB when the virtual circuit has been established.

Figure 106. TPF CTCP Control Block Structure