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 HDLC-Frame Protocol User's Guide > Chapter 3 Using HDLC.FRAME Protocol

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Non-E1/T1 ACC

If the "DSIG" configuration bit is set then the UP/DOWN state of the HDLC.FRAME terminal is dependent on the state of the CTS and DCD signals. When CTS and DCD are both asserted, the HDLC.FRAME terminal is UP.

If the "DSIG" configuration bit is clear then the state of the HDLC.FRAME terminal is not dependent on the state of the CTS and DCD signals. In this case the HDLC.FRAME terminal is always UP.

E1/T1 ACC

The E1/T1 ACC determines the UP/DOWN state of each port by the state of the frame synchronization mechanism. When the hardware establishes E1 or T1 frame synchronization on a port then all HDLC.FRAME terminals on that port are immediately placed in the UP state. If the hardware cannot detect received E1 or T1 frames without error, then all HDLC.FRAME terminals on that port are placed in the DOWN state. The sensitivity of the ACC to errors on the E1 or T1 lines is affected by the "qdown" port option.

The "qdown" port configuration option is documented in the ACC Utilities Guide chapter on TTGEN. It affects the way an E1 or T1 port is set DOWN when frame synchronization is lost.

  • Without the "qdown" option, the frame synchronization of an E1 or T1 port is checked every second. If frame synchronization is lost at 4 consecutive checkpoints, then the port is set DOWN. This ensures that higher level protocols are not disturbed by transient errors on the line. This is the recommended configuration.

  • If the "qdown" option is set then the port is set DOWN whenever the hardware detects a loss of synchronization. It is possible for the hardware to alternately re-establish and lose synchronization extremely rapidly with a poor quality line, making it difficult for higher layer protocols to operate. The "qdown" bit should be used only where line quality is known to be very good and with upper layer protocols that need immediate notification of any line problems (SS7 for example).

If an E1/T1 port is configured for external clock, and has no cable connected, it is possible for the hardware to detect framing due to crosstalk from other E1/T1 lines, so HDLC.FRAME terminals on the port may be indicated as UP.



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