gtps2m0wACF/SNA Data Communications Reference

X.25 NPSI Support

TPF supports connections to packet switched data networks (PSDNs) through the use of the NCP Packet Switching Interface (NPSI).

SNA devices that attach to TPF through NPSI are not defined as NPSI device types; instead, they are defined using other supported TPF RSC device types.

Defining a network containing X.25 resources using NPSI is described in the NPSI publications for the single-domain case. This task requires understanding of VTAM and NCP as well as NPSI and X.25, and consists of many considerations not relevant to TPF X.25 support or even a TPF application using the X.25 network. Such topics, mostly concerning X.25 link and physical level protocols, or X.25 vendor options, are not covered here because they have no influence on the TPF X.25 support, or on the definition of NPSI LUs to TPF.

TPF provides connectivity support for the following NPSI facilities defined by Logical Link Control (LLC) type:

LLC=0 :
Both Switched Virtual Circuits (SVCs) and Permanent Virtual Circuits (PVCs)

LLC=3 :
Both Intermediate Network Node (INN) and Boundary Network Node (BNN) function forms

LLC=4 :
General Access to X.25 Transport Extension (GATE) option with integrated Fast Connect support

LLC=5 :
Packet Assembler/Disassembler (PAD) Options:

TPF uses the ROUTC interface for TPF applications to communicate with logical end points reached through a PSDN. This is achieved by extending TPF SNA support to the LU_T1 interface with the NPSI product in a local or remote NCP node. Additionally, user-written system modules, which collectively form the basis of a user Communication and Transmission Control Program (CTCP), provide the user with the capabilities offered by NPSI GATE support. Process selection vector (PSV) routines can be used for this purpose. See Process Selection Vector (PSV) for additional information.

The NPSI link, PU, and LU resources are owned by VTAM. TPF needs network awareness of only the NPSI LU resources. Therefore, all LU sessions with NPSI LUs, or in the case of LLC=3 with the LU residing across the PSDN, have cross-domain/network session characteristics.

Non-SNA devices are supported as follows:

Non-SNA devices appear to TPF as LU_T1 LU_LU sessions and use the following session characteristics:

SNA devices are supported using NPSI LLC=3.

The following functions are not supported:

NPSI Message Length Considerations

An "X.25 message" is a sequence of one or more X.25 data packets ending with a packet with the "more data" bit in its packet header turned off (indicating no more data). During a RECEIVE, for example, NPSI combines related data packets in one X.25 message and places it in 1 inbound PIU for application processing. During a SEND, an outbound PIU contains 1 X.25 message that NPSI transforms into 1 or more X.25 data packets for network transmission.

TPF applications, therefore, send or receive X.25 messages only. Furthermore, TPFs message length support allows up to a 4K block maximum. Because TPF message blocks contain headers and control bytes, user applications must adhere to the following data length constraints:

NPSI LU-LU Session Characteristics

The NPSI session protocols that are used by TPF X.25 support are:

These options are given in the BIND image detailed in TPF ACF/SNA Network Generation.

Because different LLC types use different message formats, the TPF application must be aware of the LLC type used on each NPSI session. This information is not maintained by TPF X.25 support and must be supplied and tracked by the-user in user defined tables. (See the user exits information in TPF System Installation Support Reference.)

The SNA commands shown in the Table 1 are used for the TPF NPSI support:

Table 1. SNA Commands Supported for NPSI

Session Control Data Flow Control
BIND
CLEAR
SDT
UNBIND
CHASE
LU_STAT
RSHUT
RSHUTD
SHUTC
SHUTD
SIG (LLC=5 Integrated PAD)