6100 BSC Programming Manual

6100 BSC Concepts and Context
DISCONNECTING THE LINE. To sever a dialup connection, a
station issues the following sequence:
SYN SYN ... SYN DLE EOT PAD
Normally, this sequence occurs when all data transfers are
complete. You can also use it at other times to terminate the
session, for example, if an error condition makes it useless
to proceed. The application makes a CONTROL request to
send the disconnect sequence; it does not supply the sequence
in its output buffer.
If the sequence DLE EOT occurs in any context, even in a
message, it will disconnect the line. If the line is
switched, its activity ceases and DTR drops; if the line is
leased, its activity ceases but DTR doesn't drop.
DATA TRANSFER VARIATIONS. There are several possible variations
on the protocol shown in Figure 1-10. For example, a station can
solicit retransmission of a block or reply, or indicate that it
is temporarily unable to send or receive. The next few pages
describe some of these features of BSC.
Temporary Text Delay (TTD). After receiving a positive reply to
a line bid or a message block, the transmitting station must send
its next block within a certain interval. The "receive timeout"
of the other station dictates the interval; for a Tandem station,
the receive timeout is a SYSGEN parameter which you can modify
with CMI or the SET CONFIGURATION request.
If a station cannot send a block within the timeout interval of
the other station, it can send a TTD sequence to retain its
access to the line. The sequence must be issued within two
seconds of the last positive reply, and has the form:
SYN SYN ... SYN STX ENQ PAD
STX ENQ is equivalent to TTD. The application does not supply
the sequence in its output buffer; rather, it issues a CONTROL
request with the TTD option.
The receiving station sends a NAK to acknowledge a TTD sequence.
Then it waits for the expected block. This activity is
transparent to the local application if the station is controlled
by 6100 BSC.
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