Dear

We had an outage today affected one of our network . Most of the terminals were down . some areanyhow surviving Up SNR is very low.
If i increase the tx power from imonitor probe , terminal are coming up but UP C/N is low.

What could be the possible reason for this outage. Line card has warning (CRC)

Shij

What can you see using a spectrum analyser to look at the return link TDMA frequency slot?.  I would suspect an adjacent satellite or cross pol interferer.  Have a look at the cross pol spectrum and see if there is a new carrier that has appeared opposite to your TDMA receive slot.  Whatever, chase immediately before the installers have time to leave the site !

Anyone operating a network should have a spectrum analyser continuously watching the return links. From time to time, use max hold to get an overall used spectrum envelope to detect spreading bursts and interference.  Do single sweep zero span many times and look for distorted, high or low level bursts. Print copies of the normal spectrum, both copol and cross pol.

Best regards, Eric.  

I agree with Eric in the fact that you should have visibility on your returns at all times. I would recommend running a max and min trace on the TDMA returns to look for energy underneath.  The max hold is good for maintaining situational awareness on customers who might be set high in intial tx power...and the minimum hold is great for situational awareness of the noise floor under the TDMA.   

Regarding your issue:  

Where all nodes reporting low upstream SNR?  Every one of them?

Concur with Eric that my initial assessment would be rogue energy from the copol or crosspol.  

Is there any chance that someone at the earthstation (hub location) may have been working on the downlink RF/IF chain?  Maybe removing an attenuation pad?  Swapping a downconverter?