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ACC HDLC-NRM (SLDC) User's Guide > Chapter 3 Using HDLC-NRM (SDLC) Protocols
Error Handling |
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Several mechanisms are used to detect and recover from error conditions. The principal method of error recovery used by the HDLC-NRM (SDLC) protocol is known as checkpointing. This technique, which is more fully described in ISO 4335, ensures that a received frame with the poll or final bit set acknowledges all frames sent prior to or including the last frame sent with the final or poll bit (respectively) set. If this is not the case, then there is a need for re-transmission of the unacknowledged frames. If this condition occurs more times than the configured retry limit, then the protocol software will attempt to either request a reset of the link (if a secondary) or reset the mode with SNRM (if a primary), to recover from the error condition. Another error which can occur is the inability to transmit frames. This is usually because of a hardware exception condition. Such a condition is usually caused by a cable problem between the mux panel and the Data Communications Equipment (DCE). However, there are several other conditions which can cause such an error, such as an absent or slow transmit clock signal, or no Clear to Send signal. Because this condition is detected by a timer expiring, it can also be detected because a very low line speed is being used. When this error occurs, an immediate exception condition is sent to the application via an unsolicited status message with reason code “Cable or local modem fault”. In accordance with the ISO standards, a number of error conditions associated with frames are detected and handled. Short frames (less than Address and Control field) are discarded. Frames ending in an abort sequence, or frames received with FCS errors are also discarded. If frames pass the above checks, then they are also checked for various command/response exception conditions as follows:
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:
If the receiving station is a secondary, the station will:
When a primary station receives a Frame Reject frame, it will:
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