characters. A data record to be transmitted is segmented into the
optimum number of character strings (to take full advantage of the
identical character compression) by the transmitting program. A special
SCB is used to indicate the grouping of character strings that compose
the original physical record. The receiving program can then
reconstruct the original record for processing.
Control I Characters I Usage I DLE STX BCB FCS FCS
RCB
SRCB
SCB
DATA
SCB
DATA
SCB
RCB
SRCB
SCB
DATA SCB RCB
DLE
ETB
Figure 48 .• BSC Leader (SOH if no transparency feature)
BSC Start-of-Text
Block Control Byte
Function Control Sequence
Function Control Sequence
Record Control Byte for record 1
Sub-Record Control Byte for record 1
String Control Byte for record 1
Character String
String Control Byte for record 1
Character String
Terminating SCB for record 1
RCB for record 2
SRCB for record 2
SCB for record 2
Character String
Terminating SCB for record 2
Transmission Block terminator BSC Leader (SIN if no transparency feature)
BSC Ending Sequence
A Typical MULTI-LEAVING Transmission Block
In order to allow multiple physical records of various types to be
grouped together in a single transmission block (see Figure 48), an
additional eight-bit control field precedes the group of character
strings representing the original physical record. This the
Record Control Byte identifies the general type and function of the physical record (input stream, print stream, data set, etc.). A
particular RCB type has been designated to allow the passage of control
information betwe.en the various systems. Also, to provide for
simultaneous transmission of similar functions (that is, multiple input
streams, etc.), a stream identification code is included in the RCB. A
second eight-bit control field, the Sub-Record Control Byte (SRCB), is
also included immediately following the RCB. This field is used to
supply additional information concerning the record to the program. For example, in the transmission of data to be printed, the
SRCB can be used for carriage control information.
For actual MULTI-LEAVING transmission, a variable number of records
may be combined into a variable block size, as indicated previously
(that is, RCB,SRCB,SCB1,SCB2, ••• ,SCBn, RCB,SRCB,SCB1, ••• The MULTI-LEAVING design provides for two (or more) computers to exchange
transmission blocks, containing multiple data streams as described
above, in an interleaved fashion. To allow optimum use of this
capability, however, a system must have the capability to control the
flow of a particular data stream while continuing normal transmission cf
all others. This requirement becomes obvious if one considers the case
of the simultaneous transmission of two data streams to a system for
immediate transcription to physical I/O devices of different speeds
(such as two print streams). 350 IBM'VM/370 System Programmer's Guide

To provide for the metering of the flow of individual data streams, a
Function Control Sequence (FCS) is added to each transmission block.
The pes is a sequence of bits, some of which represent a particular transmission stream. The receiver of several data streams can
temporarily stop the transmission of a particular stream by setting the
corresponding FCS bit off in the next transmission to the sender of that
stream. The stream can subsequently be resumed by setting the bit on. However, since only single data streams are supported, RSCS does not
support this metering capability. If bit one of the FCS (wait-a-bit) is
on, or if bits 4, 9, or 15 (print, console, punch stream identifiers)
are off, transmission will be suspended. Thus, the bit pattern of X'SSC1' represents the m1n1mum acceptable FCS configuration for
transmission to be continued.
Finally, for error detection and correction purposes, a Block Control
Byte (BCB) is added as the first character of each block transmitted.
The BCB, in addition to control inforaation, contains a hexadecimal
block sequence count. This count is maintained and verified by both the
sending and receiving systems to exercise a positive control over lost
or duplicated transmission blocks.
In addition to the normal binary synchronous text control characters (STX, ETB, etc.), MULTI-LEAVING uses two of the ESC control characters, ACKO and NAK. ACKO is used as a "filler" by all systems to maintain
coamunications when data is not available for transmission. NAK is used
as the only negative response and indicates that the previous
transmission was not successfully received. MULTI-LEAVING Control Specification
This section describes the bit-by-bit definitions of the various MULTI-LEAVING control fields and includes notes concerning their Appendix B: MULTI-LEAVING 351