BISYNC (BSQ) Definition/Meaning:
Abbreviation for binary synchronous communications (protocol). A line
protocol created by IBM for synchronized communication between mainframe
computers and remote job-entry terminals. BISYNC is a character-oriented
protocol: it uses special control characters to mark the beginning and end of a
message, to acknowledge previous messages, to request retransmission of missing
or damaged messages, etc. The BISYNC protocol may be used with the 6-bit Transcode, 7-bit ASCII, or 8-bit EBCDIC character codes, and multidrop or
point-to-point communication lines.
A BISYNC block (message) generally starts
with two or more synchronization characters. The next field is an optional
start-of-header code and header. The presence and contents of the header depends
on the devices that are communicating. The header is followed by the
start-of-text code and text (the data of the message). Finally an end-of-text
code is followed by the block check characters, which terminate the message.
Several variations of the general format may be employed. Special codes are
inserted to acknowledge previous messages, or to request retransmission. The
particular character's used to represent the BISYNC functions depend upon the
character set being used. The protocol has a transparent mode, which allows the
presence of arbitrary data in the text field. Special provision is made for the
automatic insertion and deletion of idling characters, which are used in
synchronous communications when a sender has more data to send but is temporarily unable to
send it on time. There are various other options.
The protocol is inherently half duplex: a message is sent, a reply is sent, the
next message is sent, etc. Thus BISYNC communication usually uses half duplex
communication lines and modems. Full duplex communication lines and modems may
be used but most of the additional capacity is wasted.
BISYNC is being replaced in computer communications by newer data link control
protocols, such as SDLC and HDLC. BISYNC's
retransmission and acknowledgment scheme does not work efficiently over
connections with long delay times. This is particularly important in the US and
other areas where the telephone system is converting to satellite transmission systems for voice and data traffic.