Example 1. IMS-to-IMS message switch routing

The following figure shows the terminal attached to IMSA uses the ISC message switch to input TRANX to be executed in IMSB.

Message flows from terminal to Basic Edit and Message Switch in IMS A, then to ISCE in Control Region of IMS B, then TRANX in MPP of IMS B, then ISCEDT and Default MOD in IMS A, then back to terminal.

An MPP in IMSB processes TRANX, and the reply is routed back to the terminal in IMSA, which was the source of the message switch. The output terminal reply appears as an output message switch.

Assume the terminal is a 3270 display device. Based on this assumption, basic edit can only be invoked by entering input from a cleared screen. Therefore, in this example, the terminal operator enters:
```
LTISC1 | TRANX | Data...
```
LTISC1 is the IMSA logical terminal name associated with an ISC session between IMSA and IMSB.
Basic edit edits the input data stream from the terminal, and the message is placed on the IMSA message queue with a destination of LTISC1.
On output to LTISC1, ISC support in IMSA:
1. Strips the destination LTERM name (LTISC1) from the data stream
2. Builds the FMH required to send the transaction to IMSB
The data stream that is sent to IMSB looks like:
```
FMH: DPN=SCHEDULER
```
```
FMH: DPN=ISCE,PRN=,RDPN=,RPRN=T | TRANX | Data...
```
1. DPN=ISCE because IMS ISC support supplies this value as a default if no DPN is supplied when output is to be sent to another subsystem.
2. PRN= is not supplied and also not required for this example. TRANX is part of the user data. Only through the use of MFS can PRN= be supplied.
3. RDPN= is not supplied and also not required for this example. Only through the use of MFS can RDPN= be supplied.
4. RPRN=T is automatically inserted by IMSA as a default function of the message switching logic incorporated in IMS ISC.
The TRANX data stream is edited by ISC edit (ISCE) on input to IMSB, because the FMH specified DPN=ISCE. After editing, the data stream is placed on the message queues, and looks like:
```
TRANX | Data...
```
When scheduled, an MPP retrieves TRANX from the message queues and processes the transaction. Output inserted (ISRT) by the MPP to the originating input terminal looks like:
```
Data...
```
This output can be inserted (ISRT) by the MPP to its I/O PCB (on the same parallel session) or to an alternate PCB. The I/O PCB or an alternate response PCB must be used if TRANX is a response-mode transaction. An alternate PCB could be used if TRANX were a nonresponse transaction. Alternate PCB output is sent on the same or a different parallel session, depending on the session to which the output LTERM is assigned.
On output from IMSB to IMSA, the FMH is built and sent with the data:
```
FMH:  DPN=SCHEDULER
```
```
FMH:  DPN=ISCE,PRN=T,RDPN=,RPRN | Data...
```
The output is sent on the same session as that on which the input was received.
1. DPN=ISCE is specified, because ISC support supplies this value as a default if no DPN is available from input, or through MFS when asynchronous output is to be sent to another system.
2. PRN=T is supplied, because the input FMH to IMSB specified RPRN=T. IMS ISC support automatically wraps an input FMH RPRN value to the output FMH PRN field.
3. RDPN= is not required but can be added by MFS DPM. The RPRN is not supplied, because the reply is returned on the same session as the input transaction. However, MFS can also be used to set the RPRN.
On input to IMSA, the reply from IMSB is edited by ISC edit (ISCE), because the FMH specified DPN=ISCE.
Because a PRN is supplied in an input FMH, ISC edit uses the PRN as the IMS destination and appends the PRN value to the input data. After ISC edit, the message is placed on the input message queues and looks like:
```
T | Data...
```
If MFS had been used to process the reply within IMSA, the insertion of the LTERM name can be suppressed. See Example 6.
An IMS default MOD is used to format the data for output to the terminal. The data displayed to terminal T is:
```
T | Data...
```