6100 BSC Programming Manual

6100 BSC Concepts and Context
Data Link Control Procedures
The orderly exchange of data relies not only on well-defined
message formats, but also on well-defined data link control
procedures. The following procedures are part of the BSC
protocol:
• Circuit assurance. Lets stations on a dialup line identify
themselves to one another.
• Bidding for the line. Lets a station obtain the right to
transmit a series of blocks or messages.
• Data transfer. Lets stations exchange messages and
acknowledgements or conversational replies.
• Relinquishing the line. Lets a station indicate the end
of its transmissions and allow the other station to bid
for the line.
• Disconnect. Severs the link between the stations on a
dialup line.
CIRCUIT ASSURANCE. Circuit assurance mode applies only to
dialup lines, and only to those whose configuration calls for
id exchange. You can prescribe id exchange for a line with
SYSGEN, CMI, or a SET CONFIGURATION request.
As soon as a telephone connection occurs, the stations perform ID
exchange as shown in Figure 1-8. All characters shown, except
the SYN and PAD characters, appear in the sender's application
buffer and eventually reach the receiver's application buffer.
The calling station sends a sequence of the form:
SYN SYN ... SYN id ENQ PAD
The underscore shows the contents of the application buffer. The
sequence identifies the calling station and asks the other
station to identify itself. The number of initial SYN characters
depends on the SYSGEN entry for the line. The ID, if present, is
from 2 to 15 characters; if it has two characters, they must be
the same. Different devices can have specific device ID
requirements; for instance, for an IBM 3780 workstation the first
two characters must be in the range 60 to 7F hexadecimal, and the
rest may be from 20 through 7F hexadecimal (ASCII). ENQ is 05
hexadecimal in ASCII, 2D hexadecimal in EBCDIC.
1-19