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Community Member

## Is attenuation cumulative with inline attenuators at both ends?

Dear Netprof,

Apologies if this seems to be the same sort of question as per my previous posts, but need to determine that by adding an inline 5 dB attenuator at each end of a link if the attenuation value is cumulative or only one way (i.e. just 5 or 10 dB)?

My fibre provider has now accurate power readings regarding one of the three fibre links they are supplying. Since the original estimation, the link distance is now 14.3 as opposed to 20 Km provided in the estimation, also the actual attenuation is far less than the estimate.

The details are;

dc1 -> dc2 (14.3 km)

dc1 will use ZX (via a WS-G5487 in a 7204VXR) and dc2 we will use ZX (via a WS-G5487 in another 7204VXR)

The power / attenuation values provided by the fibre supplier are as follows;

Fibre 1 10.47 (1310 nm)

7.53 (1550 nm)

Fibre 2 10.10 (1310 nm)

7.0 (1550 nm)

I have reviewed the G5487 specs and the receive/transmit values are;

Transmit 0 – 5.2

Therefore if my maths is right hopefully I should by adding 5 dB attenuators to each end should have the one of the following attenuation values?

w/5 dB w/10 dB

-6.8 -11.8

-10.27 -15.27

3 REPLIES
Hall of Fame Super Silver

## Re: Is attenuation cumulative with inline attenuators at both en

The G5487 1000BaseZX GBIC operates at 1550 nm wavelength. According to the spec sheet I reviewed (http://www.cisco.com/en/US/partner/products/hw/modules/ps5455/products_data_sheet09186a008014cb5e.html), it transmits somewhere between 0 and +5 dBm of power. That gives you an expected receive power level of -7.53 to -2.53 on fibre #1 and -7.0 to -2.0 on fibre #2.

Therefore you could potentially run a little bit higher than the receiver's upper sensitivity level. 5 dB of inline attenuation should do it. In line attenuators are for a single fiber (typical assemblies: http://www.fiberdyne.com/products/buildoutpage29.html) and typically attached at the receive end - once you verify via actual measurement that the received optical power level requires attenuation.

Community Member

## Re: Is attenuation cumulative with inline attenuators at both en

Many thanks for your assistance, is there any physical issues if the signal is not attenuated enough?

Hall of Fame Super Silver

## Re: Is attenuation cumulative with inline attenuators at both en

If the recevived signal power level exceeds the receiver's specifications, it could burn it out, rendering it useless. It would not be covered under warranty since you were using it in a way not intended by the product specifications.

In reality, you likely have a couple of dBm "wiggle room", but why risk thousands of dollars of optics for want of tens of dollars atenuators?

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