SNAX/APC Application Programming Manual

Using the Application Program Interface
SNAX/APC Application Programming Manual—138786
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Basic and Mapped Conversations
general datastream (GDS) structures, which define the data transmitted on an LU 6.2
conversation.
However, any TP, including application TPs (ATPs), may use basic conversations. Any
data transmitted or received by a TP on a basic conversation will be in GDS format, and
the TP must provide data in that format. Any SNAX/APC TP that uses a basic
conversation to send and receive data must precede the data with the appropriate GDS
header and must manage logical record boundaries.
The GDS ID for application data is %H12FF. This consists of a header, followed by a
two-byte length field that tells the LU the length of the logical record. Thus, if a
SNAX/APC TP uses a basic conversation to send application data, it has to provide the
data to SNAX/APC in this format. When receiving data on a basic conversation, the TP
may receive either a logical record at a time or a full buffer of data at a time. Depending
on which option is taken, logical record boundaries and WHAT-RECEIVED indicators
on receive verbs need to be interpreted correctly to successfully handle the data received.
Mapped Verbs
The mapped verb set makes it easier for applications to send and receive data. Data is
transmitted and received in units of data records, meaningful only to the application.
SNAX/APC, performing the functions of the LU, then converts the data to GDS format
and issues basic verbs on behalf of the ATP, to send the data as one or more logical
records. Similarly, the ATP receives data records and has only to determine whether the
data record is complete, or if it needs to perform another receive operation to retrieve the
rest of the record. The ATP is also protected from certain error indications that are not
considered the domain of an ATP.
The first verb issued in a conversation, after TP-READY, is ALLOCATE or MC-
ALLOCATE, depending on whether the conversation is basic or mapped.
If ALLOCATE is the first verb issued, then only verbs from the basic set may be
subsequently issued on the conversation. If MC-ALLOCATE is the first verb issued,
then only verbs from the mapped set may be subsequently issued on the conversation.
Type-independent and verbs specific to Tandem may be issued regardless of the set of
conversational verbs being used.
Mixed Mode Conversations
SNAX/APC supports both basic and mapped verb sets, so there should be no reason for
a TP to use mixed-mode conversations, and they are strongly discouraged. Their
continued support by SNAX/APC is for backward compatibility only.
If you choose the basic verb set, and are responsible for allocating the conversation (the
TR-LOCAL-ATTACH-INDICATOR is set to “Y” in the TP-READY verb), you may
optionally request a mapped conversation by means of the AL-PARTNER-TP-TYPE
field of the ALLOCATE verb.
If you choose to use the mapped verb set, and are responsible for allocating the
conversation (the TR-LOCAL-ATTACH-INDICATOR is set to “Y” in the TP-READY
verb), SNAX/APC will send an ATTACH (FMH-5) requesting a mapped conversation.
The partner may or may not be using mapped verbs.