6100 BSC Programming Manual

6100 BSC Concepts and Context
The ETB and ITB characters are visible to the application. The
sending application includes them in its output buffer; the
receiving application finds them in its input buffer. The block
check characters do not appear in any application buffer.
The blocks of a message need not be of uniform length.
SYN WITHIN THE MESSAGE. This section has already described the
SYN pattern that starts a transmission. There are other
situations in which a station transmits SYN characters. Remember
that SYN is never visible to the application, but that you would
see it if you used a line monitor to observe data on the line.
During a transmission BSC inserts two consecutive SYN characters
every second (except in one case, to be mentioned shortly).
These characters help the sending and receiving stations to stay
synchronized. The receiving station strips the SYN characters
from the data stream as they arrive.
If a message contains ITB characters, BSC transmits two SYN
characters after every intermediate block; the SYN characters
immediately follow the block check sequence. The receiving
station strips the SYN characters from the data stream.
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