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Understand LFP and FEF of Media Converter

Posted on By FS.COM


Understand LFP and FEF of Media Converter

Media converters are flexible and cost-effective devices for implementing and optimizing fiber links in all types of networks. They play an significant role in today’s multi-protocol, mixed media local area networks (LAN). The converters support many different data communication protocols including Ethernet, Fast Ethernet, Gigabit Ethernet, T1/E1, DS3/E3, as well as multiple cabling types such as twisted pair, multimode and single-mode fiber and single-strand fiber optics. This article will focus on using Ethernet-to-fiber media converters with copper Ethernet switches.

With Media Converters in Network

As there are large number of different switch and converter products available on the market, it is quite likely that your network uses switches from different vendors, or perhaps different models of switch from the same vendor. Actually, if your network is already using one switch with a built-in fiber port and you wish to connect the fiber port to a copper-only switch, you can do this by inserting an Ethernet-to-fiber media converter between the two devices. The following picture shows the connection between copper switch and fiber-enable switch by using an Ethernet-to-fiber media converter.

You may think that converting between copper and fiber is the sole purpose of an Ethernet-to-fiber media converter. Actually, there’s a little more to it than that. It is certainly true that the main role of a media converter is to pass data between two devices that are not capable of communicating directly, and in this regard, a good media converter must do its best to remain invisible to the network. So the media converter must be able to “simulate a cable”. When media converters are used in pairs, the cable actually consists of two copper cables and two fiber cables (see the picture below). This increased complexity has motivated the introduction of the “link fault pass-through” and “far end fault” functions.

Understand LFP and FEF of Media Converter

Link Fault Pass-through (LFP): Consider what happens if the copper cable connecting the switch on the left with the media converter gets unplugged unexpectedly. In this case, the switch on the right will have no idea what has happened, and even though the connection has been disconnected the network will continue on its merry way assuming that the connection is still viable. This is where link fault pass-through comes in. The following picture illustrates the detailed operation of LFP.

1.Normal operation of the two media converters.

2.Copper cable to converter A gets disconnected.

3.Converter A disables the connection to converter B.

4.Converter B disables its copper connection.

5.Converter B disables the connection to converter A.

Far End Fault (FEF): Consider what would happen if the fiber cable connecting the media converter on the left to the media converter on the right gets disconnected unexpectedly. In this case, the switch on the left will not be able to transmit data to the switch on the right. However, if the other fiber cable is still viable, the switch on the right will continue transmitting to the switch on the left, which could lead to transmission faults across the network. This is where far end fault comes in. The following picture illustrates the detailed operation of FEF.

1.Normal operation of the two media converters.

2.Fiber cable from converter A to converter B gets disconnected.

3.Converter B disables its copper connection.

4.Converter B disables its fiber connection to converter A.

5.Converter A disables its copper connection.

Use Media Converters in Pairs

In the above illustrations, we showed media converters being used in pairs. Actually, most vendors illustrate LFP and FEF with two media converters used in pairs, as they well should, but nevertheless many engineers disregard this point and use only one Ethernet-to-fiber converter. The problem with using only one media converter is that if the media converter’s “far end fault” and “link fault pass-through” functions are activated, the functions will not work properly. Not only should the media converters be used in pairs, but in addition, you should also choose the same brand and same model. This is because different vendors could use proprietary protocols to run “far end fault” and the “link fault pass-through”. In fact, the same vendor could use different converter ICs in different models, which will result in the different models being incompatible.

Let’s take a closer look at what happens if using LFP with a single Ethernet-to-fiber media converter (see the picture below). In fact, it is easy to understand why using LFP in this situation will cause problems. To begin with, keep in mind that LFP is a function that resides in the media converter. The media converter will send a message to the switch on the right notifying it that the link has failed if the copper connection between the switch on the left and the media converter fails. The problem with this scenario is that the switch will not understand the LFP message, and hence the message will be discarded. Because of this, the switch on the right will continue transmitting data to the media converter with the expectation that the data will be passed on to the switch on the left. The fact that this does not happen can cause the network to fail.

Summary

Media converters remain as invisible as possible by “simulating a cable”, but since some properties of media converters may not be completely compatible with Ethernet switches or routers, such as LFP or FEF, you should consider several factors before deciding to use media converters as part of a network. Keep the following important factors in mind:

 If you intend to enable the “Link Fault Pass-through” function, be sure to use Ethernet-to-fiber media converters as a pair. In addition, remember to select the same brand and model for the two sides of the connection.  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.

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