6100 BSC Programming Manual

6100 BSC Concepts and Context
MESSAGE BLOCKING. In the two formats shown so far, the whole
message was packaged in one unit, with one block check sequence
and one occasion for acknowledgement by the remote station. BSC
also lets you divide a message into blocks. There are several
reasons you might choose to divide (or "block") a message:
• The remote device might have a buffer size less than that
of the complete message.
• By dividing the message into blocks, you provide for one
block check sequence per block. Thus you get better error
control than if you made the whole message one unit.
• By dividing the message into blocks and transmitting the
blocks separately, you give the remote station several chances
to reply. The remote station can receive and process the
message a block at a time, instead of having to accept and
process the whole message at once. If a block is in error or
if the station is too busy to receive more data, your
application finds out right away and avoids fruitless
transmissions.
There is a possible disadvantage of separate transmission blocks.
Depending on the rate at which the remote station can operate,
sending a message in several transmissions can slow down the
application. The delay includes the time it takes for the line
to turn around twice, for the remote station to acknowledge each
block, and for the next block to be dispatched. This factor
becomes less important if the remote station is slow, i.e.,
if it couldn't handle the data all at once without overruns and
reduction in throughput.
In general, the ideal block size is a function of line quality
and turnaround time. On a good line, one can use large blocks
to avoid frequent line turnaround delays. On a noisy line,
large blocks are impractical, because the error rate increases
with the length of the block.
There are two ways to block a message. One way divides the
message into blocks for separate transmission. The other
divides the message without turning the line around between the
blocks. The two ways are illustrated in Figure 1-6.
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