CWDM Optical Attenuators

[bunker]

I have a client that wants to conduct a pilot for CWDM. He has a pair of 3550 switches and would like to just get a pair of CWDM gbics and hook up the two 3550s in a simple point to point configuration. This is basically a lab environment in one rack. The SC cables will be short, like 3m so I understand that I will need some optical attenuators.

My question to the group here is what kind of attenuators(reflective, absorption, inline, variable etc) and what is the recommended Db level to attenuate at for the CWDM gbics in a lab environment?

I understand that I will need two attenuators(one for TX light and one for the RX light) or one unit with both TX/RX included. I have found some from several vendors and some are variable inline attenuators meaning I can just turn a screw to affect the light’s intensity.

Does anyone have any specific attenuators used in a similar environment or know of the proper Db level I should attenuate using the CWDM gbics so I can purchase the proper attenuators.

I have seen the other thread regarding the DWDM attenuators listed in this forum so any additional help would be greatly appreciated….

Regards…

[didyap]

For short distances such as the test configuration in a lab, optical attenuators are required. For point to point CWDM networks, you could attenuate the trunk Rx on the OADM or attenuate the access Tx on the OADM. 10 dB inline attenuators are generally used.

[bunker]

Thanks didyap,

there is no OADM in between. Just a PtP tx/rx between the two 3550s. So, my question is do I need two 10db inline attenuators for each piece of fiber cable between the two gbics?

Also, why 10db? Is there any specific calculation or chart based on distance and attenuation levels?

Thanks again.. Regards..

[Rene Frank]

To calculate the min and max attenuation the information is written in the datasheet of the CWDM GBICs.

To calculate the minimum attenuation:

-7dB max input power on receiver

+5dB max output power for the transmitter

Calculate: +5 - (-7) = 12dB

This means that you should use at least 12dB minimum attenuation to be sure not to overload the receiver.

To calculate the maximum attenuation:

-31dB min reveicer signal

+1dB min transmitter output

Calculate: +1 - (-31) = 32dB

This means that you should have not more attenuation than 32dB, to be sure the receiver can detect the signal. The line and connector attenuation included. So a good value for the attenuator in your lab is 15dB or 20dB. As you already said, you must include in both fiber a attenuator,normally near to the receiver port.

Regards

Rene