Using parallel transmission groups between VTAM and a channel-attached NCP

A transmission group is one or more physical links connecting two subareas. These physical links are viewed as a single logical connection between the two subareas. VTAM®, with NCP Version 5 Release 3 or higher for the 3745 and 3720, allows you to have multiple transmission groups between VTAM and a channel-attached NCP subarea node. You can add up to 16 transmission groups using up to 16 channel links between the same VTAM and NCP subarea nodes. However, because only single-link transmission groups are supported, several channel links between VTAM and NCP cannot be defined as a multilink transmission group.

Procedure

Take the following steps to define a parallel transmission group:

Code transmission group numbers (TGN1, TGN3, TGN255) on the PU definition statement for each channel-link station. In the following LINE and PU definition statements for the NCP, each channel link is defined as a separate transmission group (TGN1, TGN3, TGN255) by coding the TGN operand on the PU definition statement. However, the transmission groups TG7 and TG8 between the two NCPs can consist of multiple SDLC links.

N111CA   GROUP LNCTL=CA             CHANNEL ADAPTER DEFINITIONS
L5CA05   LINE ADDRESS=00,           CHANNEL ADAPTER 5 RELATIVE ADDRESS
              ⋮
              CA=TYPE6,             CHANNEL ADAPTER TYPE
              ISTATUS=ACTIVE        VTAM STATUS
*
P5CA05   PU   PUTYPE=5,             VTAM HOST CONNECTION — PUTYPE5
              TGN=1                 CHANNEL-LINK TRANSMISSION GROUP NUMBER
*
L6CA06   LINE ADDRESS=01,           CHANNEL ADAPTER 6 RELATIVE ADDRESS
              ⋮
              CA=TYPE6,             CHANNEL ADAPTER TYPE
              ISTATUS=ACTIVE        VTAM STATUS
*
P6CA06   PU   PUTYPE=5,             VTAM HOST CONNECTION — PUTYPE5
              TGN=3                 CHANNEL-LINK TRANSMISSION GROUP NUMBER
*
L7CA07   LINE ADDRESS=02,           CHANNEL ADAPTER 7 RELATIVE ADDRESS
              ⋮
              CA=TYPE6,             CHANNEL ADAPTER TYPE
              ISTATUS=ACTIVE        VTAM STATUS
*
P7CA07   PU   PUTYPE=5,             VTAM HOST CONNECTION — PUTYPE5
              TGN=255               CHANNEL-LINK TRANSMISSION GROUP NUMBER

Code a transmission group number on the PCCU definition statement in the NCP major node or on the PU definition statement in a VTAM channel-attachment major node. If a TG number is specified on the VTAM definition statement (PCCU or PU), it must match the corresponding TG number specification on the PU definition statement in the NCP for that channel-link station. Otherwise, code TGN=ANY, which allows the TG number to be assigned by the adjacent NCP, but the NCP subarea node must then specify a TG number. The transmission group number used is the TGN value that is coded on the PU definition statement in the NCP and the TG number that VTAM receives in the exchange identifier (XID) exchange during activation of the channel link.
Transmission group number on the PCCU definition statement

One of the channel attachments must be used to load and activate the NCP from the VTAM host. The transmission group number that is used for that channel attachment can be specified on the PCCU definition statement.

Transmission group number on the PU definition statement

The channel links and link stations between VTAM and a channel-attached NCP must be defined in a channel-attachment major node.
Code PATH definition statements to define different routes using different channel links between the same VTAM and NCP subarea nodes. Although you can define up to 16 explicit routes between VTAM and a channel-attached adjacent NCP, the routing capability between these two subareas can be limited by the number of virtual routes that can be defined. The maximum is eight virtual routes using three different transmission priorities for a total of 24 potential routes. For information about how this affects network routing, see Network routing for subarea nodes.

Results

Following are two examples of how parallel transmission groups are defined:

Figure 1 illustrates a possible configuration of parallel transmission groups.
Figure 1. Parallel transmission groups between VTAM and NCP

In Figure 1, the channel-link station TGN1 can be used to load and activate the channel-attached NCP. In this case, the PCCU definition statement is used to specify the TG number (default is 1). The other two channel-link stations, TGN3 and TGN255, can be defined using a channel-attachment major node for each. In these two cases, the PU definition statement for each of the channel-link stations specifies the transmission group number. Because VTAM does not support the capability of defining multiple channel links as a single transmission group, each channel link in the NCP must be defined as a separate transmission group.
The following is a PCCU definition statement:
```
 
PCCU116  PCCU  AUTODMP=NO,         AUTOMATIC DUMP OPTION
               CUADDR=016,         3745 IPL ADDRESS
               ⋮
               TGN=ANY,            USE TGN OF NCP FROM XID
               ⋮
               SUBAREA=116         OWNING SSCP SUBAREA
```
In this example, the transmission group specification is TGN=ANY. Therefore, when contacted over the channel link identified by the CUADDR operand, VTAM uses the TG number that is specified on the PU definition statement for the channel-link station in the NCP. Because there can be multiple channels between VTAM and an NCP, each channel attachment must use a different TG number so that the PATH statements between the subarea nodes can identify different network routes. Other channel attachments to the same NCP can be defined using the channel-attachment major node specification. In this case, the TG number specification is not on the PCCU definition statement but on the PU definition statement in the channel-attachment major node that represents that channel-link station.