Fibre backbone Noise Acceptable Limits

[Grommit]

Fibre backbone Noise Acceptable Limits

 

We have a fibre going to one of our buildings approx 500m run..

 

It got tested and there was 31db of Noise ..

 

Now our Network works fine and there is no issues.. but the CCTV company says that this is unacceptable

 

What is the acceptable limits for fibre as I have googled it but can't seem to find a decent answer...

 

Is 31db OK ?

 

And does anyone have a professional link :-)

[Geoff]

You need an minimum SNR margin of 6dB.

[FN-GM]

if the network is working fine just leave it, don't fix something that isn't broken

[Grommit]

You need an minimum SNR margin of 6dB.

 

Sorry Geoff.. i don't understand... is 31Db OK or not ? :?

 

if the network is working fine just leave it, don't fix something that isn't broken

 

I agree.. but the CCTV is taking 2 cores of my fibre and they say their stuff isn't working and they say it's because of the 31db, and that it has to be re spliced......

 

Is this true ?

[FN-GM]

I'm not to sure by self I’m not a fibre expert but I know somebody who is if nobody else knows I will ring him later.

[Geoff]

SNR margin is the different between the Signal strength and the Noise.

 

So with a noise level of 20db and a signal strength is 30db the SNR margin is 10dB

[CyberNerd]

What Loss Should You Get When Testing Cables?

While it is difficult to generalise, here are some guidelines:

 

-For each connector, figure 0.5 dB loss (0.7 max)

-For each splice, figure 0.2 dB

-For multimode Fibre, the loss is about 3 dB per km for 850 nm sources, 1 dB per km for 1300 nm. This roughly translates into a loss of 0.1 dB per 100 feet for 850 nm, 0.1 dB per 300 feet for 1300 nm.

-For singlemode Fibre, the loss is about 0.5 dB per km for 1300 nm sources, 0.4 dB per km for 1550 nm. This roughly translates into a loss of 0.1 dB per 600 feet for 1300 nm, 0.1 dB per 750 feet for 1300 nm.

So for the loss of a cable plant, calculate the approximate loss as:

 

(0.5 dB x number of connectors) + (0.2 dB x number of splices) + Fibre loss on the total length of cable

 

 

 

http://www.techoptics.com/pages/Fiber%20Optics%20-%20Testing.html

 

the maximum loss the equipment can tollerate depends upon what your using, we have max 2-5db loss in any run.

If the figure you quote is 30db loss in the run then that is far too much.

[Oops_my_bad]

Isn't it something like anything from -4dB to +4dB on multimode?

[andresvillegas]

It depends on the receiver. The receiving units usually have some specifications that dictate how much power they should receive to stablish proper communication. Usually 25 db is considered the limit on multimode networks. For single mode, a slightly greater loss is acceptable