What Are FEF and LFP on Media Converter

Updated on Dec 7, 2023 by

Fiber media converters are essential for seamlessly bridging different media types, such as twisted pair and fiber optic cabling. Typically deployed in pairs, these converters extend the reach of data transmission between copper-based networks and fiber-optic communications, connecting LANs in separate buildings or locations at each end of a fiber link.

Importance of FEF and LFP Functions

In a paired media converter setup, if the fiber or copper link on one side fails, the device on the other side continues to operate without transmitting data or reporting an error. FEF (Far End Fault) and LFP (Link Fault Pass Through) functions are critical for network reliability, ensuring that administrators are alerted to link failures and can troubleshoot effectively.

Normal Operation of Paired Media Converters.jpg

Figure 1: Normal Operation of Paired Media Converters

FEF on Fiber Media Converter

Commonly seen on media converters, a Far End Fault (FEF) is an indication mechanism used in fiber-optic networking to alert the network that a break or other fault has been detected at the far end of a connection. It is a protocol compliant with IEEE 802.3u standard to sense remote data link errors in a network. The FEF function on media converters makes it easy for network administrators to identify and address faults in fiber media converter links.

When a fiber link error is detected on one end, the media converter on this end will transmit the fault signal over a fiber connection to inform the other media converter at the far end that a fault has occurred. As a result, both copper links connected to the fiber link will be automatically disconnected. Devices such as switches at both ends will indicate the operating status through methods like LEDs. By using media converters that support FEF, network administrators can detect and troubleshoot link faults immediately. This prevents data from being transmitted over a failed link without any alert by disconnecting the faulty link and sending a far-end fault signal to the media converter.

Operation of FEF on Media Converter

1. When Far End Fault is supported and enabled, a loss of RX signal on Media Converter A side will cause the TX end to generate a Far End Fault pattern in order to inform Media ConverterB at the far end of the fiber pair that a fault has occurred.

a fault occurs in the RX end of fiber connection.jpg

Figure 2: Fault at RX End on Media Converter A

2. When Media Converter B has been informed the link fault occurred in the RX end, the transmission of the TX end of Media Converter A will be disabled.

Media Converter A sends a far end fault to Media Converter B.jpg

Figure 3: Disabled Transmission at TX End on Media Converter A

3. Fiber Media Converter A takes down its copper link. On the corresponding Ethernet switch, the LED indicator shows a link disconnected status.

Disconnected Status LED on the Left-sided Switch

Figure 4: Disconnected Status LED on the Left-sided Switch

4. On the other side, Media Converter B takes down its copper link, too. The connected switch LED indicator shows a link downstate.

Disconnected Status LED on the Right-sided Switch

Figure 5: Disconnected Status LED on the Right-sided Switch

5. When the RX end on Media Converter A detects a signal again, the TX end on it will automatically return to normal operation.

LFP on Media Converter

LFP (Link Fault Pass Through) ensures that a link fault on one side (local side) of a media converter is transmitted to the media converter on the other side (remote side). This function is crucial for monitoring the copper links connected to media converters from the local device.

In simpler terms, if there's an issue with the copper cable linked to a media converter, the Link Pass-Through feature ensures that the problem will lead to the disabling of both the related fiber link and the copper connection on the remote end as well. Through this method, the media converters can immediately alarm network administrators the problem of the link and offer an efficient solution to monitor the network, which results in quicker issue resolution and reduces the risk of potential downtime caused by connectivity issues.

Operation of LFP on Media Converter

1. Let's assume that there is a disruption on the left-sided copper link, which could be a basic problem such as a disconnected cable, or it could escalate to a more challenging problem such as a hardware failure within the switch.

Copper cable to Media Convert A gets failed

Figure 6: A Disruption on the Left-sided Copper Link

2. With its LFP function, Media Converter A notifies Media Converter B of the copper link failure and disables the fiber connection to Media Converter B.

LFP Media Converter A notifies Media Converter B

Figure 7: The Disabled Fiber Connection

3. After receiving the link failure signal, Media Converter B will disable its copper connection, and the switch connected to it will display the link down state on the corresponding LED indicator.

Media Converter B disables its copper connection

Figure 8: The Disabled Connection on the Right-sided Copper Link

Application Notes on FEF and LFP

With the understanding that FEF is an indication mechanism for remote link failures, while LFP is a fault state transfer mechanism used at both ends of the link, here are some application-specific things to keep in mind:

• Use Matching Media Converters: Ensure that you use Ethernet-to-fiber media converters as a pair, and both devices must support FEF and LFP functions. It is important to select the same brand and model for both sides of the connection to ensure compatibility and seamless operation.

• Network Device Configuration: Other devices in the network may need to be configured to recognize and handle FEF signals, as well as fault statuses communicated through the Link Pass-Through mechanism. Review the configuration options for these devices to ensure they are properly set up to recognize and respond to these signals.

• Pre-Implementation Testing: Before implementing FEF and LFP functions on a working network, thoroughly test these features to ensure they operate correctly and do not disrupt network performance.

• Seek Expert Advice: Consult with networking experts or your system vendor to get professional advice on setting up and optimizing your network with FEF and LFP-enabled media converters. This will help in achieving better connectivity and transmission results.

Both FEF and LFP functions on media converters are crucial for diagnosing and troubleshooting network outages, ensuring efficient network operation of the links.


Understanding FEF (Far End Fault) and LFP (Link Fault Pass Through) on media converters is crucial for maintaining network integrity and reliability. These features alert administrators to link failures, allowing proactive management. FEF informs both ends about fiber link errors for timely resolution, while LFP communicates local copper link issues to the remote end, prompting a shutdown to signal a problem. To use these tools effectively, employ compatible Ethernet-to-fiber media converters and ensure proper network configuration. Testing and consulting with system vendors are recommended for seamless integration. Integrating FEF and LFP minimizes network downtime and data loss, which is vital for mission-critical systems relying on robust communication infrastructure.

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