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    Understanding Pre-FEC BER Monitoring and BER Thresholds

    Optical transport network (OTN) interfaces on PTX Series Packet Transport Routers support monitoring the condition of an OTN link by using the pre-forward error correction (pre-FEC) bit error rate (BER). The following PICs support pre-FEC BER monitoring:

    • P1-PTX-2-100G-WDM

    • P2-100GE-OTN

    • PTX-5-100G-WDM

    • P1-PTX-24-10G-W-SFPP

    With pre-FEC BER monitoring enabled, when the configured pre-FEC BER signal degrade threshold is reached, the PIC stops forwarding packets to the remote interface and raises an interface alarm. Ingress packets continue to be processed. If pre-FEC BER monitoring is used with MPLS fast reroute or another link protection method, then traffic is rerouted to a different interface.

    You can also configure backward fast reroute to insert the local pre-FEC status into transmitted OTN frames, notifying the remote interface of signal degradation. The remote interface can use the information to reroute traffic to a different interface. If you use pre-FEC BER monitoring together with backward fast reroute, then notification of signal degradation and rerouting of traffic occurs in less time than that required through a Layer 3 protocol.

    Include the signal-degrade-monitor-enable and backward-frr-enable statements at the [edit interfaces interface-name otn-options preemptive-fast-reroute] hierarchy level to enable pre-FEC BER monitoring and backward fast reroute.

    Note: When you configure pre-FEC BER signal degrade monitoring, we recommend that you configure both the signal-degrade-monitor-enable and the backward-frr-enable statements.

    You can also configure the pre-FEC BER thresholds that raise or clear a signal degrade alarm and the time interval for the thresholds. If the BER thresholds and interval are not configured, the default values are used.

    The pre-FEC BER signal degrade threshold value defines a specific amount of system margin relative to the BER correction limit (or FEC limit) of the PIC’s receive FEC decoder. The FEC limit is fixed on each PIC—it is intrinsic to the FEC decoder implementation.

    Note: The following examples use Q2–factor measurements (also known as Q–factor). Q2–factor is expressed in units of decibels relative to a Q2–factor of zero (dBQ). Q2–factor enables you to describe system margin in linear terms in contrast to BER values, which are nonlinear in nature. After you determine the thresholds, you must convert the threshold values from Q2–factor to BER to enter them in the CLI by using scientific notation. BER can be converted to Q2–factor by using the following equation:

    Q2–factor = 20 * log10 (sqrt(2) * erfcinv(2 * BER))

    Tip: To convert between Q2–factor and BER in a spreadsheet program, you can approximate the values by using the following formulas:

    • To calculate Q2–factor:

      = 20 * LOG10(–NORMSINV(BER))
    • To calculate BER:

      = 1 – NORMSDIST(10^(0.05 * Q2–factor))

    Table 1 shows the relationship between the fixed FEC limit, the configurable signal degrade threshold, and the configurable clear threshold for different PICs. In this example, approximately 1 dBQ of system margin has been set between the FEC limit, signal degrade threshold, and clear threshold.

    Table 1: Example—Signal Degrade and Clear Threshold Values at 1 dBQ

    PIC

    FEC Type

    FEC Limit

    Signal Degrade Threshold

    Clear Threshold

    Q2–FactorBERQ2–FactorBERQ2–FactorBER

    P1-PTX-2-100G-WDM

    SD-FEC

    6.7 dBQ

    1.5E–2

    7.7 dBQ

    7.5E–3

    8.7 dBQ

    3.0E–3

    P2-100GE-OTN

    G.709 GFEC

    11.5 dBQ

    8.0E–5

    12.5 dBQ

    1.1E–5

    13.5 dBQ

    1.0E–6

    P1-PTX-24-10G-W-SFPP

    G.975.1 I.4 (UFEC)

    9.1 dBQ

    2.2E–3

    10.1 dBQ

    6.9E–4

    11.1 dBQ

    1.6E–4

    G.975.1 I.7 (EFEC)

    9.6 dBQ

    1.3E–3

    10.6 dBQ

    3.6E–4

    11.6 dBQ

    7.5E–5

    G.709 GFEC

    11.5 dBQ

    8.0E–5

    12.5 dBQ

    1.1E–5

    13.5 dBQ

    1.0E–6

    To adjust the signal degrade threshold, you must first decide on a new system margin target and then calculate the respective BER value (using the equation to convert from Q2–factor to BER). Table 2 shows the values if 3 dBQ of system margin relative to the FEC limit is desired for the signal degrade threshold (while maintaining the clear threshold at 1 dBQ relative to the signal degrade threshold).

    Note: The choice of system margin is subjective, as you might want to optimize your thresholds based on different link characteristics and fault tolerance and stability objectives. For guidance about configuring pre-FEC BER monitoring and BER thresholds, contact your Juniper Networks representative.

    Table 2: Example—Signal Degrade and Clear Thresholds After Configuration

    PIC

    FEC Type

    FEC Limit

    Signal Degrade Threshold

    Clear Threshold

    Q2–FactorBERQ2–FactorBERQ2–FactorBER

    P1-PTX-2-100G-WDM

    SD-FEC

    6.7 dBQ

    1.5E–2

    9.7 dBQ

    1.1E–3

    10.7 dBQ

    2.9E–4

    P2-100GE-OTN

    G.709 GFEC

    11.5 dBQ

    8.0E–5

    14.5 dBQ

    4.9E–8

    15.5 dBQ

    1.1E–9

    P1-PTX-24-10G-W-SFPP

    G.975.1 I.4 (UFEC)

    9.1 dBQ

    2.2E–3

    12.1 dBQ

    2.8E–5

    13.1 dBQ

    3.1E–6

    G.975.1 I.7 (EFEC)

    9.6 dBQ

    1.3E–3

    12.6 dBQ

    1.1E–5

    13.6 dBQ

    9.1E–7

    G.709 GFEC

    11.5 dBQ

    8.0E–5

    14.5 dBQ

    4.8E–8

    15.5 dBQ

    1.1E–9

    Include the ber-threshold-signal-degrade, ber-threshold-clear, and interval statements at the [edit interfaces interface-name otn-options signal-degrade] hierarchy level to configure the BER thresholds and time interval.

    Note: Configuring a high BER threshold for signal degradation and a long interval might cause the internal counter register to be saturated. Such a configuration is ignored by the router, and the default values are used instead. A system log message is logged for this error.

    Modified: 2017-09-13