Media converters can support several protocols, including Ethernet, Fast Ethernet, and Gigabit Ethernet. Media converters range physically from small standalone devices and PC cards, to large chassis systems. Some media converters also support Power Over Ethernet (PoE). In the picture, the PoE media converter delivers power to, and receives data from, the IP camera using a standard copper cable. The data is converted and sent through the fiber link to the switch.

A common use of media converters is to connect multiple LANs in separate buildings or geographical locations. In the paired media converters setup, two media converters are used, one on each end of the fibre link. The paired media converters allow the copper-based LANs in the office and factory to communicate using the fibre link, even if the physical locations are distant (fibre-optic cable can transmit data over distances up to 100 kilometres).

However, in any fibre-copper topology with paired media converters, there is a disadvantage. If the copper link on one side of the fibre connection fails, the devices on the other side (such as switches) have no way of knowing that the link is down. They will continue to operate even though no data can be transmitted, and will not report an error to the system administrator. In case the copper link on the left has failed, the switch and computer on the right will continue to operate as if the link were working, even though no data can be successfully transmitted.

To overcome this shortcoming, Link Fault Pass Through (LFPT) provides constant monitoring of the links connected to the media converters. If either of the copper links fails, the media converter will pass the fail state on throughout the link, taking down the middle fibre link as well as the copper link on the opposite end. This prevents the connected switches from sending packets that would end up lost, and makes detection and troubleshooting much easier. If a redundant connection exists, the network can switch over to it immediately.

For faults that occur on the fibre link, Far-End Fault (FEF) is an IEEE 802.3u standard that allows both ends of a pair of fibres to be informed when there is a problem with one fibre. Without Far End Fault, it is impossible for a media converter’s fibre interface to detect a problem that affects only its TX (transmit) fibre. When a fault is detected, the media converter will transmit a Far-End Fault signal pattern over the fibre connection in order to inform the media converter at the far end of the fibre pair that the fault has occurred. Both copper links connected to the fibre link will be automatically taken down. This not only aids detection and troubleshooting, but also prevents data from being transmitted to a non-functioning link.

LFTP must be used with media converters arranged in pairs, and both devices must support LFPT. If only one media converter is used, Far End Fault and LFPT functions will not work properly, because switches will not understand the LFPT message. Furthermore, it is recommended to use the same brand of media converter for each pair, because different vendors sometimes use proprietary LFPT and FEF protocols.

Media converters are flexible and cost-effective devices for implementing and optimizsng fibre links in all types of networks. They play an important role in modern local and wide area networks, as these networks often employ multiple protocols and media types. Link Fault Pass Through and Far End Fault are essential tools that aid network administrators in diagnosing and troubleshooting network outages.