Attenuation calculation on zx interface

[Massimiliano Tognon]

Hi to all, i've read many times many discussion, but some of them are very old about the question.

i see the page on cisco web site speaking about zx

http://www.cisco.com/c/en/us/products/collateral/interfaces-modules/gigabit-ethernet-gbic-sfp-modules/product_data_sheet0900aecd8033f885.html

but i haven't understand how to calculate the attenuator i need.

we have made attenuation measurement on the dark fiber we have, this is the results:

we have a lenght from site A to site B equal to 11.372 kms. and the attenuation are as follows:

site A ---> site B                          4,625 db

site A <--- site B                          4.385 db

we have GLC-ZX-SM as optical transceiver.

Which kind of attenuator should insert to be 100% sure it works ?

Many thanks for any help

Massimiliano Tognon

[vivek srivastava]

Hi,

Firstly, the attenuation is the signal loss over the distance. as you have already calculated. However I must say this is not something which is going to work practically. You must have in-between signal optimizer (or similer device) to boost the signals.

Now comes to Cisco SFPs model which you have can be used for max. distance of 70Km with below limitation on optical power

Product	Transmit Power Range (dBm)	Receive Power Range (dBm)	Maximum Channel insertion loss in dB (by fiber type)*	Transmit and Receive Wavelength Range (nm)
1000BASE-ZX	+5 to 0	-3 to -23	21 (any SMF)	1500 to 1580

It means that your end devices (having GLC-ZX-SM) should receive the power in above range at both end. Also the distance from the device to first-hop signal boostor (must have ZX SFP) can go upto 70Km.

If anyhow you recieve power more than -3dBm (for example say -2dBm) then you need to use a ATTENUATOR say -10dBm so that the receive power will go down to -12dBm which is under the limit i.e. in-between -3 to -23dBm. Same is true for transmit power as well. Cisco devices shows a worning if transmit or receive power is not in the limt. You can see this in "sh inter gix/x trans" command.

Hope it answer your question..

Vivek

[Massimiliano Tognon]

the attenuation i gave on the post was the fiber attenuation due to lenght and fiber connection (i think), this was measured by telecom that installed the dark fiber.

So with 4.3 db attenuation on the fiber, you think that with a 10db attenuator the zx interface will work fine on this scenario ?

many thanks in advance for the answer

Massimiliano

[vivek srivastava · Accepted solution]

If there is dark fiber connection (without any signal booster) with distance and attenualtion you have provided then its not going to work with GLC-ZX-SM or any other Cisco SFP (as far as i know).

As I explained previously, the attenualtion is calculated between directly connected active devices.

For example, if your device is going to connect to the 1st signal booster (or similer device) and the distance between them is apprx 35Km, then first you need to measure the power from power meter. If receive power is under the range -3 to -23dBm then you don't need any attenuation and you have to use a 0dBm attenuator for paching only. If power is more (for example say -2dBm) then you need to use a attenuator in-between say -10dBm (other options also available with -3dBm or -5dBm) so that the receive power will go down to -12dBm which is under the limit i.e. in-between -3 to -23dBm.