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Can your fiber-optic cable go the distance?

Why is it possible to overextend fiber-optic cable lengths and get good performance? CCIE Greg Ferro explains.


What is the maximum distance of Gigabit Ethernet over multimode fiber?

I have had a number of people ask questions about fiber-optic cabling. In particular, some people have been overextending their cable lengths and it has been working well. But why? Let's look into it.

The laser light is injected into the cable at a given "launch power." As the laser signal moves over the cable, it will be subject to a certain amount of loss. For example, where the transmitter connects to the patch lead, there will be a small amount of loss at the connector interface, and again where the patch lead meets the building fiber. There will also be some loss due to the cable itself. When it gets to the receiver, the signal must be strong enough to be "read." This is known as the "receiver sensitivity."

So let's consider how much laser power you need. I will look at a 1000BaseLX Gigabit Ethernet and then look at 1000BaseSX Gigabit Ethernet and see what happens.

From you manufacturer manuals you can find the following information about an 1000BaseLX laser transmitter and receiver standards that would look something like this. Power is expresses in decibels with reference to one milliwatt, or dBm. Don't worry about what the numbers actually mean. You will see why in a minute.

Laser transmitter characteristics:
Minimum launch power -9.5 dBm
Maximum launch power -3 dBm

Receiver characteristics:
Minimum receiver sensitivity -20 dBm
Maximum input power -3 dBm

From this we can deduce that the worst-case scenario would be when the launch power from the laser is -9.5 dBm and the receiver sensitivity is -20 dBm; this gives us a worst case of 11.5 dBm of "headroom" for guaranteed function. In any given circumstance, launch power is affected by the quality of the fiber connector, fiber cable and the laser installed -- not much we can do there. Receiver sensitivity is determined by the amount of laser power received at the other end, which is degraded by such factors as patch leads, fiber length, fusion splice or mechanical splice, and types of connectors.

The interesting thing is that even on circuits where it is conceivable that we have more 11.5 dBm of loss, things may work, most likely because we are getting good launch characteristics (up to -3 dBm of launch) giving up to 6.5 dBm more headroom. Potentially, the receiver could work on an even lower input power. Thus, the recommended length of 550 m for 1000BaseLX on 62.5-um multimode fiber could be longer, depending on how good the end-to-end performance of the cabling system is and the quality of your lasers.

Note that for 1000BaseSX the figures are different. This is why the shorter cable length comes into play. There is less power budget to work with, but in a given situation we may be able to exceed the standard and get away with it.

Laser transmitter characteristics:
Minimum launch power -10 dBm
Maximum launch power -4 dBm

Receiver characteristics:
Minimum receiver sensitivity -17 dBm
Maximum input power 0 dBm

The trick here is that the overall power budget (the difference between launch and power receive) can fluctuate with temperature and other environmental issues, and this may cause intermittent errors. Or, you may only have the minimum amount of power being received and it isn't reliable. The gamble with overrunning the standard lengths is that we risk intermittent and indeterminate faults, often with no apparent cause.

My experience has been that you can overrun cable by about 40% without too much risk. But make sure that you have minimum loss in your cabling by using a knowledgeable cable installer, good vendor equipment and good design.

You can test your fiber-optics and work out its performance with the right tools. Your cabling company should know how to do this. If not, go and find the right company. In fact, if your cabling company can't understand this article, likely they won't understand how to test it.

This was last published in November 2002

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