gtps2m14ACF/SNA Data Communications Reference

X.25 ALC Interface for Switched or Permanent Virtual Circuits (XALCI)

XALCI support allows ALC-type terminals attached to a packet switched data network (PSDN) to communicate with existing TPF applications. XALCI is similar to TPF NEF/ALCI and AX.25 support of the AX001 and AX002 protocols, but differs in the following ways:

User-Replaceable PSV Routine

The sample PSV routine bases its processing on the NPSI/FTPI command type. The valid command types are:

The sample PSV routine classifies the input based on the FTPI header. The general processing is:

Tables

TPF's XALCI support code uses the WGTA as its control structure. Based on the LEID in the data stream, the WGTA slot is used to store the MCH_ID, VC_ID, and the correlation number of the input message for recall and insertion into the corresponding output message. The WGTA is expanded by 4 bytes to hold this additional information.

Network Definition

TPF Considerations:

The only network definition requirement for XALCI support is the need to:

Defining the NPSI LU

Define to the TPF system the NPSI or NPSI FTPI LU that connects the TPF system to the network and non-SNA terminals using ALCI support. The format and parameters used to define the NPSI or NPSI FTPI LU are shown in Figure 23.

Figure 23. RSC Statement Definition Statement for an FTPI LU


  lu_name   RSC   LUTYPE=XALCI
                  ,PSV=XALCI
                  [,PACING=0]
                  [,AWARE=NO]
                  [,LEID=FFFFFF]

The description of the XALCI values are:

lu_name
Specifies the name of the NPSI LU.

LUTYPE=XALCI
Specifies if the LU uses the GATE/FTPI interface of NPSI and uses XALCI data streams to perform the FTPI header management.

PACING=0
Specifies the pacing value to use for message traffic sent by TPF.
Note:
A value of zero (0) must be specified and means that pacing is not used.

AWARE=NO
Specifies whether the session awareness exit should be started whenever a session with the LU has been established or terminated. NO must be specified.

LEID=FFFFFF
Specifies from 1 to 3 bytes of data to be passed to the PSV routine when an input message is received from the LU. This parameter is optional and is neither required or used by the sample PSV routine supplied with TPF.

An example of the coding of the RSC statement for XALCI support is shown in Figure 24.

Figure 24. RSC Coding Example Statement for an XALCI LU


  CPLU1     RSC   LUTYPE=XALCI,
                  PACING=0,
                  AWARE=NO,
                  PSV=XALCI

For more information about defining SNA resources to the TPF system, see Defining SNA Resources to the TPF System.

Defining the XALCI PSV Routine

TPF provides XALCI support by bridging its support of the NPSI FTPI option with the support for ALCI. The bridge between FTPI and ALCI support is implemented as a user-replaceable PSV routine. To include the PSV routine in the TPF system, you must:

The source code for the PSV table, COBU, includes the default PSV routine for XALCI. Figure 25 shows the coded to be added to segment COBU.

Figure 25. XALCI PSV Entry Shipped with TPF


        IPSVE     NAME=XALCI,
                  PGM=CNEA,
                  RESERVE=NO,
                  DFC=NO

The macro to define a PSV table is shown in Figure 26.

Figure 26. Create PSV Table Macro


        IPSVT    BEGIN|END

The macro to define a PSV entry is shown in Figure 27.

Figure 27. Create PSV Entry Macro


        IPSVE     NAME=XALCI
                 ,PGM=CNEA
                 [,RESERVE=NO]
                 [,DFC=NO]

NAME=XALCI,
Specifies the 1 to 6 character PSV name. The character set is restricted to the letters A through Z and the numbers 0 through 9. The name XALCI was chosen for this example, but any valid name is acceptable.

PGM=CNEA,
Specifies the user-replaceable program that bridges FTPI messages to the ALCI interface. The program can be file or main storage resident but cannot be corefast.

This program segment must reside in the BSS, but may also be resident in other subsystems in an MDBF environment.

RESERVE=NO,
Specifies whether the entry is reserved for future use. If reserved, a SERRC is generated in place of the ENTNC. The default is NO.

DFC=NO
Specifies whether the user PSV provides the SNA-architected DFC layer. The default is NO.

Figure 28 shows a sample of PSV definition for XALCI support.

Figure 28. Sample XALCI PSV Definition Statements


        IPSVT    BEGIN
        IPSVE     NAME=XALCI,PGM=CNEA
        IPSVT    END

For more information about the IPSVT and IPSVE macros, see TPF General Macros.

X.25 Gateway Considerations

TPF's XALCI support is based on a coordinated effort between TPF base code and a remote intelligent workstation (IWS) or gateway. The TPF code performs the blocking and deblocking of data to and from GATE/FTPI, as well as the FTPI header management and ALCI functions. For additional information, see Airlines Line Control (ALC) Support through SNA. The FTPI header management is performed using WGTA and is based on architected XALCI headers discussed in XALCI Header Formats. The gateway is responsible for:

XALCI Header Formats

Figure 29 shows an example of the various XALCI headers.

Figure 29. XALCI Header Formats