Optical Attenuator for LX/LH

Unanswered Question
Feb 13th, 2007

Hi,

My optical link (single-mode cable) have a short distance (30 meters). I will use LX/LH GBIC. Must I use optical Attenuator, or it is recommended just for ZX interface ?

I have this problem too.
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pciaccio Tue, 02/13/2007 - 03:56

If you are using an LX/LH GBIC and your length is fairly short I would recommend that you place a 3dB, or 6dB attenuator inline on your RCV paths. The best way is to check the optical levels (if you have a test set). You want to be in the -10dB to -25dB levels. These are optimal levels . If you have a high dB level you can run errors and even cause the GBIC to fail.Remember in the dB world every 3dB higher is double the optical power and every 3dB lower is half the optical power....Good Luck...

tighinean Tue, 02/20/2007 - 23:25

Dear pciaccio, can you give a link on Cisco website containing the detailed information statements you said?

Thanks

We have successfully used patch distances of 2 meters:

See:

The minimum cabling distance for 1000BaseSX and 1000BaseLX/LH GBICs is 6.5 feet (2 meters).

When using the 1000BaseLX/LH GBIC with 62.5-micron diameter MMF, you must install a mode-conditioning patch cord between the MMF fiber-optic network and the GBIC. The mode-conditioning patch cord (CAB-GELX-625 or equivalent) is required to comply with IEEE standards. The IEEE found that link distances could not be met with certain types of fiber-optic cable cores. The solution is to launch light from the laser at a precise offset from the center by using the mode-conditioning patch cord. At the output of the patch cord, the LX/LH GBIC is compliant with the IEEE 802.3z standard for 1000BaseLX.

You must insert a 10-dB inline optical attenuator between the single-mode fiber-optic network and the receiving port on the 1000BaseZX GBIC at each end of the link if the link length is less than 15.5 miles (25 km).

You must insert a 5-dB inline optical attenuator between the single-mode fiber-optic network and the receiving port on the 1000BaseZX GBIC at each end of the link if the link is greater than 15.5 miles (25 km), but less than 31 miles (50 km).

http://www.cisco.com/en/US/products/hw/switches/ps679/products_module_installation_guide_chapter09186a008007f4bb.html

mcotherman Wed, 10/09/2013 - 04:51

Link is 404....  You would think before Cisco took a page down, they would see what pages refer to it on your own website....  probably kinda important for an "internet of everything", dontcha think?

Joseph W. Doherty Wed, 10/09/2013 - 06:28

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No, you don't need an attenuator for LX optics if it exceeds a length of 2m.  (ZX optics can drive up to 70 Km, so they are much, much more powerful than LX optics, which is why you need them for shorter distances.)

To add to Btw's post, the distances are not exact.  What you really are insuring that the received power is within the expected power range.  This will vary based on the cables itself.  Ideally you would test the cable end-to-end to know what the received power is.  You can also predict expected power loss if you know all the components end-to-end.

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