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Help with installing ZX GBICs under 25 km

Dear Support,

Sorry but new to optical networking, but need some assistance on an upgrade that we are performing.

We currently have two data centres that hold our servers and are planning to upgrade from LES circuits to Gigabit links, and triangulate our topology. (effectively a ring topology)

I have already spoken to our proposed fibre provider and they have provide me with rough distances, but due to one of the data centres being more than 10 km away we need to use ZX based GBICs. Summary of distances below;

HQ -> dc1 (1 – 2 km)

HQ -> dc2 (15 km)

Dc1 -> dc2 (20 km)

From our HQ to dc1 we will use LH (via a GLC-LH-SM= in a 2970G) and a to dc2 we will use ZX (via a GLC-ZX-SM= in another 2970G)

From DC1 to HQ we will use LH (via a WS-G5486 in a 7204VXR) and a to dc2 we will use ZX (via a WS-G5487 in another 7204VXR)

From DC2 to HQ we will use ZX (via a WS-G5487 in a 7204VXR) and a to dc1 we will use ZX (via a WS-G5487 in another 7204VXR)

Hope this doesn’t confuse things more.

Now to the question.

From the Cisco web site it mentions the following;

When shorter distances of single-mode fiber are used, it might be necessary to insert an in-line optical attenuator in the link to avoid overloading the receiver:

• A 5-dB or 10-dB inline optical attenuator should be inserted between the fiber-optic cable plant and the receiving port on the Cisco 1000BASE-ZX GBIC at each end of the link whenever the fiber-optic cable span is less than 15.5 miles (25 km).

We haven’t yet had the fibre survey performed, but once they have advised me of the attenuation on the link, how should I calculate what level of “in-line optical attenuator” I should need or should all reputable fibre providers advise me of this detail.

Thanks in advance.

Regards, Adrian

1 ACCEPTED SOLUTION

Accepted Solutions
New Member

Re: Help with installing ZX GBICs under 25 km

This is something I see commonly that causes much confusion among people. Some optical equipment is rated in distance, while a far more useful rating is the Tx and Rx levels of the equipment.

ZX lasers transmit between +5dBm and 0dBm.

ZX receivers will receive a signal between 0dBm and -23dBm.

Imagine you have a ZX laser transmitting at +5dBm, and you need to go 2km. Assuming the fiber span introduces 2db of attenuation (from the span length, splices, and patch panel connections), you will now be hitting the receiver at +3dBm (transmit at +5 -2db of attenuation). That would be too powerful and would overdrive your receiver because the most powerful signal the receiver can see is 0dBm. Any attenuator that gets the signal down to 0dBm would work, but I would aim for the middle or so and add at least a 10dB attenuator, probably a 15dB.

The best thing would be for your fiber provider to test each of your fiber paths with a power meter and give you the loss on both 1310nm and 1550nm wavelengths. With that information, you will no longer have to guess about which SFP you need just because of the distance. In fact, if your 20km span is in good shape and with few splices, you may be able to get away with LH lasers for that span and save some money.

Ideally, you want to fall in about the middle of the power budget. If you’re too close either way, you could run into issues if your fiber provider ever has to perform repairs which add more loss or change the route and remove some loss.

Document showing the Transmit/Receive levels of various SFP modules:

http://www.cisco.com/en/US/products/hw/modules/ps5455/products_data_sheet0900aecd8033f885.html

A good Cisco document on calculating optical power budget:

http://www.cisco.com/warp/public/127/max_att_27042.pdf

Please let me know if I can answer any more questions on this issue.

Josh

2 REPLIES
New Member

Re: Help with installing ZX GBICs under 25 km

This is something I see commonly that causes much confusion among people. Some optical equipment is rated in distance, while a far more useful rating is the Tx and Rx levels of the equipment.

ZX lasers transmit between +5dBm and 0dBm.

ZX receivers will receive a signal between 0dBm and -23dBm.

Imagine you have a ZX laser transmitting at +5dBm, and you need to go 2km. Assuming the fiber span introduces 2db of attenuation (from the span length, splices, and patch panel connections), you will now be hitting the receiver at +3dBm (transmit at +5 -2db of attenuation). That would be too powerful and would overdrive your receiver because the most powerful signal the receiver can see is 0dBm. Any attenuator that gets the signal down to 0dBm would work, but I would aim for the middle or so and add at least a 10dB attenuator, probably a 15dB.

The best thing would be for your fiber provider to test each of your fiber paths with a power meter and give you the loss on both 1310nm and 1550nm wavelengths. With that information, you will no longer have to guess about which SFP you need just because of the distance. In fact, if your 20km span is in good shape and with few splices, you may be able to get away with LH lasers for that span and save some money.

Ideally, you want to fall in about the middle of the power budget. If you’re too close either way, you could run into issues if your fiber provider ever has to perform repairs which add more loss or change the route and remove some loss.

Document showing the Transmit/Receive levels of various SFP modules:

http://www.cisco.com/en/US/products/hw/modules/ps5455/products_data_sheet0900aecd8033f885.html

A good Cisco document on calculating optical power budget:

http://www.cisco.com/warp/public/127/max_att_27042.pdf

Please let me know if I can answer any more questions on this issue.

Josh

New Member

Re: Help with installing ZX GBICs under 25 km

Hi Josh,

Appreciate your help on this, thanks again.

regards,

Adrian

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