Segmenting Of Request/Response Units - IBM 3708 Description

Segmenting of Request/Response Units
RUs sent to the
3708
can be larger than acceptable for optimum transfer of data by
the link connecting the terminal to the network. Therefore, a Basic Information
Unit (BIU) consisting of an RH and RU may be divided into smaller elements,
called segments, that are transmitted over the link. The
3708
supports outbound
segmenting on the LU-LU session.
The segment elements are defined as follows. The First in Segment (FIS) element is
equated to Begin-BIU, not End-BIU. The Last in Segment (LIS) element equates to
End-BIU, not Begin BIU. The Middle in Segment (MIS) equates to not Begin-BIU,
not End-BIU. An Only in Segment (OIS) contains the entire BIU.
Sequencing of segments is in the correct order if the sequence consists of:
1. FIS, LIS
2. FIS, MIS, ... , LIS
3. OIS
Errors caused by incorrect sequencing of the segment elements cause the
3708
to
respond with a negative sense code of
X' 8007' .
The PIU delivered to the
3708
must not exceed
265
bytes. A PIU with an OIS or
FIS element can contain a maximum of
256
bytes of RU data plus
6
bytes of
transmission header (TH) and 3 bytes of request/response header (RH). A PIU with
an MIS or LIS element can contain a maximum of 259 bytes of RU data plus 6
bytes of TH. If the PIU exceeds 265 bytes, the 3708 rejects the segment with a
SDLC frame reject.
Although outbound segmenting is supported by the
3708,
it is not recommended
because of slower screen updates. When segmenting is used, the
3708
must receive
all the segments before starting any processing of the data stream. With chaining,
data stream processing is done as each chain is received. With segmenting, none of
the data is processed until all of the segments have been received. The
3708
does not
start transmitting data to the downstream device until segments have been received
and processed. Large segments require the use of more
3708
buffers. The
3708
may
then be forced into slowdown mode and performance may suffer.
It is recommended that the host applications use chaining instead of segmenting.
Chaining can be specified on the BIND command by defining bytes
10
and
11
as
X' 8585 '. If chaining cannot be used, segments should be as small as possible.
3-48
IBM 3708
Description