APPC conversations can either be mapped or basic. If you are interested in CICS®-to-CICS applications, you need only use mapped conversations. Basic conversations (also referred to as "unmapped") are useful when communicating with systems that do not support mapped conversations. These include some APPC devices.
The two protocols are similar. The main difference lies in the way user data is formatted for transmission. In mapped conversations, you send the data you want your partner to receive; in basic conversations, you have to add a few control bytes to convert the data into an SNA-defined format called a generalized data stream (GDS). You also have to include the keyword GDS in EXEC CICS commands for basic conversations.
Table 4 summarizes the differences between mapped and basic conversations. Note that it only applies to the CICS API. CPI Communications, introduced in the next section, has its own rules.
Mapped | Basic |
---|---|
The conversation partners exchange data that is relevant only to the application. | Both partners must package the user data before sending and unpackage it on receipt. |
All conversations for a transaction share the same EXEC Interface Block for status reporting. | Each conversation has its own area for state information. |
The transaction can handle exceptional conditions or let them default. | The transaction must test for exceptional conditions in a data area set aside for the purpose. |
A RECEIVE command issued in send state causes conversation turnaround. | A RECEIVE command is illegal in send state. |
Transactions can be written in any of the supported languages. | Transactions can be written in assembler language or C only. |