Fm Data - IBM 3708 Description

FM Data
This command is used to transfer data in the LU-LU or SSCP-LU session. It may
only be sent in LU-LU session when data traffic is allowed (SDT has been issued
and received a positive response).
When communicating with a 3708 SLU, the following FM data protocols are used:
Bracket
Bracket protocol is used to delimit a series of related inbound
and outbound FM data request units (RUs), for example, all the
RUs required to complete a transaction. .
Chaining Chaining logically connects one or more RUs from a single LU,
for example, all RUs required to complete a display image.
Change Direction Change direction informs the receiving LU that the sending LU
has completed transmission and expects the next transmission to
be from the receiving LU; for example, the PLU has transmitted
a complete form image and expects the next transmission to be
from the display operator when the blank fields in the form
image are filled in.
Bracket Protocol:
The 3708 provides a bracket protocol to delimit a series of
related inbound and outbound requests. A bracket may consist of one input and
one output, many sets of inputs and outputs, or a series of requests flowing in a
single direction. The Begin Bracket (BB) and End Bracket (EB) indicators are used
to delimit a bracket. References are made to bracket states (BETB and INB). These
states are described under "Bracket States" on page 3-46.
A bracket is initiated when the Begin Bracket indicator is accepted by the primary or
secondary LU. The bracket is usually ended when the End Bracket indicator is
received by the secondary LU. The specific conditions that end a bracket are
defined by SNA bracket termination rule 1. Two commands, BID and Ready to
Receive (RTR), are implemented to further define the initiation of a bracketed
session. These commands are described under "SNA Commands" on page 3-15.
The following protocols apply for 3708 bracket processing:
• For sessions with type
2
SLUs, the SLU may begin a bracket any time the
session is between brackets. The PL U may request permission to begin a
bracket using BID. If the SLU returns a positive response, the PLU may begin
a bracket. If the SLU returns a negative response, the PLU must wait for the
next BB from the SLU.
• For type 1 and 3 sessions, the PLU may begin a bracket any time the session is
between brackets (the only time the SLU begins a bracket is when the operator
presses the PA key). The PLU may start a bracket by sending a transmission
that contains BB or by sending BID, waiting for a positive response, and then
sending a transmission that contains BB.
The PLU may attempt to initiate a bracket by sending a transmission with BB. If a
contention situation exists (the SLU begins a bracket before receiving BB from the
PLU), the SLU returns a negative response to the PLU's transmission and then
discards all portions of the chain from the PLU. The SLU assumes that its
transmission is accepted by the PLU.
3-34
IBM 3708
Description