Using Basic or Mapped Conversations

Another design decision is whether to use a basic or mapped conversation. This decision affects the format and, potentially, the performance of data transmission.

Data travels between APPC transaction programs in buffers provided by the LUs involved. APPC defines a logical record format for the data as a sequence of length and data fields, with the 2-byte length fields (LLs) indicating the amount of data to follow before the next length field. The typical data pattern is “LL, data, LL, data.” The transaction programs can either format their data in this pattern themselves or leave the formatting to APPC, depending on the needs of the application.

Transaction programs that format their own data for transmission use what are called basic conversations. Those that let APPC do the formatting use mapped conversations.

Basic conversations allow transaction programs to define logical records to their own specifications. The transaction programs can specify the exact lengths of their records using the “LL,data,LL,data” format and avoid the overhead of mapping done by APPC. Basic conversations give potential performance benefits and greater control.
Mapped conversations are easier to code; they allow applications to send data in any format the partner programs expect. APPC itself maps the data into and out of the “LL,data,LL,data” logical record format for each transmission. Mapped conversations are recommended for most applications.

Both CPI Communications and LU 6.2 callable services support basic and mapped conversations. To specify a mapped or basic conversation, you can use the CMSCT (Set_Conversation_Type) service, or the Conversation_type parameter of the LU 6.2 Allocate service.