What Are FEF and LFP on Media Converter

Updated on Dec 7, 2023 by

Fiber media converters play a crucial role in bridging diverse media types seamlessly, such as twisted pair and fiber optic cabling. Typically deployed in pairs, media converters are often used to connect LANs in separate buildings or locations at each end of a fiber link, bridging the gap between copper-based networks and fiber-optic communications, thereby extending the reach of data transmission.

However, in such a paired media converter network, if the fiber or copper link on one side fails, the device on the other side will continue to work even though no data is transmitted, and will not report an error to the system administrator. This is where the FEF (Far End Fault) and LFP (Link Fault Pass Through) functions on media converters become critical for network reliability.

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. With the FEF function on media converter, it is very easy for the network administrator to detect the faults on the 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. Then, both copper links connected to the fiber link will be automatically disconnected, and the connected devices such as switch on both ends will indicate the operating status by means like LEDs. By using media converters that support FEF, the fault on the link can be detected and troubleshot immediately, preventing data from being transmitted to a failed link without any alert, by cutting down the failed link and sending a far end fault 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

The LFP (Link Fault Pass Through) means the link fault of media converter on one side (local side) will be passed to the media converter on the other side (remote side). It is used for monitoring the copper links connected to the 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:

• If you intend to enable the FEF and LFP function on media converter, be sure to use Ethernet-to-fiber media converters as a pair and both devices must support FEF and LFP. In addition, remember to select the same brand and model for the two sides of the connection.

• 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. The configuration options for the devices need to be reviewed to ensure that they are properly set up to recognize and respond to these signals.

• Test the functions you intend to use before actually implementing the functions on a working network.

• Be sure to seek expert advice from your system vendor.


Understanding the functions of FEF (Far End Fault) and LFP (Link Fault Pass Through) on media converters is crucial for maintaining network integrity and reliability. These features provide administrators with immediate alerts and mechanisms to handle link failures proactively. FEF informs both ends of a network about fiber link errors, allowing for timely fault isolation and resolution, while LFP ensures that any local copper link problems are communicated to the remote end, prompting a shutdown that signals a network issue. To effectively leverage these tools, it is essential to employ compatible Ethernet-to-fiber media converters and have proper network configuration. Prior testing and consulting with system vendors are also recommended to ensure a seamless integration and operation of these functionalities. By integrating FEF and LFP, network downtime can be minimized and data loss can be mitigated, which is vital for any mission-critical system relying on robust communication infrastructure.

Related Articles

What Is Media Converter and How to Choose It?

How to Use Fiber Media Converter in Your Network?

Single-Mode vs. Multimode Media Converters: Differences Explained

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