6100 BSC Programming Manual

6100 BSC Concepts and Context
Error Control
The following means of error control have been mentioned in this
section:
• Block check characters (BCC)
• Timeouts
• Negative acknowledgements and sequences requesting
retransmission of a block (NAK) or reply (ENQ)
• Means for aborting a transmitted block (ENQ) or a series of
transmissions (TTD followed by EOT)
• Means for indicating inability to send or receive data (TTD,
WACK)
All of these topics, except the timeouts, were described in some
detail. There is more information on block checking in the
context of line configuration options. The next few paragraphs
summarize the important timeouts in BSC protocol.
IDLE SYN CHARACTERS. BSC inserts a pair of SYN characters at
one-second intervals within a transmission. If a block is
transparent, the characters are DLE SYN, instead of SYN SYN.
BSC also transmits a SYN when no other character is ready to
send.
In some devices, a timer is set every time a SYN sequence
arrives. If three seconds elapse before the next SYN, the device
sends a NAK or takes some other action for recovery. BSC doesn't
use this timeout but can communicate with devices that do.
Idle SYN characters are invisible to the application.
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