ACC HDLC-NRM (SDLC) User's Guide

Chapter 3 35
Using HDLC-NRM (SDLC) Protocols
Error Handling
If frames pass the above checks, then they are also checked for various
command/response exception conditions as follows:
• Unrecognized or unimplemented control f ield;
• Frame too long (longer than maximum configured I-field);
• Invalid receive s equence number, N(R).
If any of these command/response exception conditions are detected,
different recovery actions will be taken, depending on whether the
receiving station is primary or secondary. The ISO 4335 standard in
section 7.3.2.2 defines a three byte information field which is used to
convey information about the command response reject condition. The
format of the reason code is given in a later section of this chapter.
If the receiving station is a primary station, the station will:
• Notify the application program with an unsolicited status message
indicating “Invalid terminal response”. This unsolicited message will
contain the three byte reject reason code.
• Reset the mode of t he station, using the SNRM command.
If the receiving station is a secondary, the station will:
• Notify the application program with an unsolicited status message
indicating “Frame reject transmitted”. This message will contain
three bytes of reject reas on code.
• The secondary station will request the resetting of the mode, by
sending a Frame Reject (FRMR) frame to the primary, whenever i t is
polled.
When a primary station receives a Frame Reject frame, it will:
• Notify the application program with an unsolicited status message
indicating “Frame reject received”. This message w ill contain the
three bytes of reason code, as received in the FRMR frame from the
remote s e condary s tation.
• Reset the mode of t he station, using the SNRM command.