6100 BSC Programming Manual

6100 BSC Concepts and Context
Some special conditions, and some applications, call for
variations on the general protocol. The means for accomplishing
such variations are described as "Data Transfer Variations,"
below.
RELINQUISHING THE LINE. When a station has completed its
transmissions, it issues the following sequence:
SYN SYN ... SYN EOT PAD
The application does not supply this sequence. Rather, BSC sends
it automatically when a READ request follows a series of WRITEs.
(A READ after a series of WRITE requests implies that the station
is finished writing.) See Section 3 for more information.
The station that issues the EOT must wait for at least three
seconds to let the other station bid for the line. This wait is
not enforced by BSC, but is a protocol convention. If the other
station has not made a bid within three seconds, the first
station is entitled to make another bid for the line.
A station that wishes to refuse the line, instead of relying on
the three-second timeout, can issue the sequence:
SYN SYN ... SYN EOT PAD
This sequence has the same meaning as the one that it echoes: "I
have nothing to send. You may bid for the line." After sending
this sequence, the station waits (by convention) for at least
three seconds, during which the first station may bid for the
line. The application makes a CONTROL request to send this EOT
sequence; it does not supply the EOT sequence in its output
buffer. It should not issue a line bid until at least three
seconds have elapsed.
The advantage of refusing the line, instead of waiting for the
timeout, is to reduce the time during which the line is idle.
Idle time almost always hurts application throughput.
Figure 1-11 shows the transfer of control from one station to
another. One station bids for the line, transmits data, and
finally relinquishes the line. Then the other station does the
same. The first station refuses the chance to make another line
bid, so the second station claims the line and sends another
series of messages.
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