Forward Error Correction (FEC) and Bit Error Rate (BER)

Optical interfaces on PTX Series support monitoring the condition of an optical 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

• P1-PTX-24-10G-W-SFPP

The PICs use forward error correction (FEC) to correct bit errors in the received data. As long as the pre-FEC BER is below the FEC limit, all bit errors are successfully identified and corrected and, therefore, no packet loss occurs. The system monitors the pre-FEC BER on each port. This gives an early warning of link degradation. By configuring an appropriate pre-FEC BER threshold and interval, you enable the PIC to take preemptive action before the FEC limit is reached. If you combine the pre-FEC BER threshold logic with MPLS fast reroute, then you can prevent or minimize packet loss.

You must specify both the signal degradation threshold (ber-threshold-signal-degrade) and the interval (interval) for the interface. The threshold defines the BER criteria for a signal degrade condition and the interval defines the minimum duration over which the BER must exceed the threshold before an alarm is raised. The relationship between the threshold and the interval is illustrated in Figure 1. After an alarm is raised, if the BER returns to a level below the threshold clear value (ber-threshold-clear), the alarm is cleared.

Figure 1: Pre-FEC BER Monitoring

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 optical 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.

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.

When a received signal degrade alarm is active and backward fast reroute is enabled, a specific flag is inserted into the trasmitted optics overhead. The remote PIC at the opposite end of the link monitors the optics overhead, thus enabling both ends to initiate traffic rerouting in the event of a signal degrade condition. When the signal degrade condition is cleared, the overhead flag is returned to a normal state.

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. Each PIC has a set FEC limit—it is intrinsic to the FEC decoder implementation.

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.

This section describes FEC modes supported on different routers at the [edit interfaces interface-name otn-options] level.

MX Series Routers

PTX Series Routers

Table 6 lists the FEC modes that are supported on PTX Series routers at the [edit interfaces interface-name otn-options] hierarchy level.

Table 7 lists the FEC modes that are supported on ACX6360 routers at the [edit interfaces interface-name optics-options] hierarchy level.

Table 8 lists the forward error correction (FEC) modes that are supported on ACX5448-D routers. You can configure the FEC modes at the [edit interfaces interface-name optics-options] hierarchy level.