Application programs send normal and expedited information over conversations with the SEND, SENDRCV, PREPRCV, DEALLOC, DEALLOCQ and SENDEXPD varieties of the APPCCMD macroinstruction, according to the full-duplex or half-duplex capabilities of a session. Application programs exchange both data and conversation-related control information, such as confirmation requests and responses.

The VTAM® program imposes restrictions on when an application program can send data or control information over a conversation, but it also handles many of the tasks involved in managing the flow of data handled by the application program for non-LU 6.2 sessions.

Both half-duplex and full-duplex conversations allocated to sessions negotiated to full-duplex capability have the ability to send and receive data in an expedited manner, bypassing normal, paced protocols. Expedited data is sent using APPCCMD CONTROL=SENDEXPD, QUALIFY=DATA macroinstruction.

There are some important concepts the application programmer must understand in order to send and receive information effectively and efficiently. The application programmer must fully understand the differences between half-duplex-capable sessions and full-duplex-capable sessions. The application programmer also needs to understand the capabilities of half-duplex conversations and full-duplex conversations. To review these concepts, see Full-duplex and half-duplex protocols.

This chapter contains information about VTAM's normal send services and interface. Better send performance is available to applications that use the interface for high performance data transfer (HPDT). For information about HPDT, see Sending and receiving data using high performance data transfer. Before reading that chapter, however, the reader should be familiar with the information presented in this chapter.