ACC HDLC-NRM (SDLC) User's Guide

Using HDLC-NRM (SDLC) Protocols
Error Handling
Chapter 3 35
If frames pass the above checks, then they are also checked for various
command/response exception conditions as follows:
• Unrecognized or unimplemented control field;
• Frame too long (longer than maximum configured I-field);
• Invalid receive sequence 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 the 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 reason code.
• The secondary station will request the resetting of the mode, by
sending a Frame Reject (FRMR) frame to the primary, whenever it 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 will contain the
three bytes of reason code, as received in the FRMR frame from the
remote secondary station.
• Reset the mode of the station, using the SNRM command.