Fibre line loss

Can anyone help me understand fibre line loss (dB's) please, this is in the context of FTTP.

For example

Does fibre have a line loss\attenuation per kilometer in the same way that copper does? (if so do we know what that is for fibre used by Openreach).

Does each splice joint introduce line loss (even if its a good splice)?

When an engineer does a light test (say at the CBT or ONT) how do they calculate if the result is as expected? (how do they determine what it should be)

What is a bad line loss result?

The fibre lines if installed to standards should be able to run 20km so you should not be worrying about the light loss figures

It is either constructed well enough to work or not.

Edited by MrSaffron (Sat 08-Dec-18 12:18:53)

I would like to understand fibre line loss for my own technical understanding not for any specific issues.

AFAIK light can travel for up to 40km in fibre without requiring booster(s).

But in answer to the OP, yes there is light loss in a FTTP circuit at various points. During the testing phase of my FTTP line and also later on when there was a fault*, i saw Openreach go around the various points (ONT/CSP/DP/splitter/agg node) measuring light loss. I believe there is a certain tolerance wrt light loss, though I have no idea what the actual numbers are.

But at the end of the day, if your FTTP service works then you have nothing to worry about.

*the "fault" actually turned out to be a human error as someone had messed up my line profile at the exchange - a line reconfig cured this but only after the Engineer had spent 8 hrs outside in the pouring rain tracing the fibre all the way back to the exchange.

Edited by deleted (Sat 08-Dec-18 12:02:05)

https://amzn.to/2PpwiCd

As others have said there is degradation of the light level as the signal travels through fibre. How far the signal goes depends on the optics deployed.

The speed you get over fibre is fixed - so it will always run at the same speed regardless of the loss. When the loss gets to a low enough level then the fibre will stop working - there's no adaptive speed function that means it would continue to work at a lower speed, like there is with xDSL technologies.

Dect

As other have said if it works it doesn't affect FTTP.

There is loss caused by the distance in the fibre and by each splice.

Splices are affected if the fibre was dirty, if not a clean cut etc. just like a torch lens affects light quality.

The loss in the fibre is affected by the SFP used ( the bit that goes on the ends) these are rated for different distances. eg short medium long, SX,MX,LX (say up to 100meters,80m-10k , 8k to 100k etc.) and the quality of fibre. Look online if you want more technical detail on what is available now as my knowledge is slightly out of date in a fast moving field

Using a higher spec one at a shorter distance will blow the distant end and they should match both ends

Over the distances for FTTP this is all irrelevant as it will work or not. There is NO degradation of the service until it breaks, so you get the full speed you buy or nothing over the access FTTP segment. There is the possibility of Contention over the segment from the splitter to the Head end and congestion of backhaul, core and to the content provider will still affect your actual speeds.

Hope this gives you the overview you wanted.

These are a good starting point
http://www.thefoa.org/tech/lossbudg.htm
http://www.thefoa.org/tech/loss-est.htm
http://www.thefoa.org/tech/Linkspec.htm

. I believe there is a certain tolerance wrt light loss, though I have no idea what the actual numbers are.

So for the Openreach PON product measuring @1490 the light loss reading ought to be within -10 to -25db

Have attended a couple of faults where initial tests at the ONT showed readings towards the top end of the scale -23db the reported issue being intermittent lengthy loss of service.

Both were cleared by locating a bum splice, once redone the final reading was -14db

Well done dect for sticking with it. At least you got some sensible replies!

Yes, there is an end-to-end budget for attenuation. Usually not an issue.

In our case (our local FTTP network which has <32 homes) Openreach came and replaced the main 32 way optical splitter as it was apparently 2dB out of specification and causing issues with signal headroom on new downstream installations (making the splice joint quality too critical).

Poor chap sat there for a good chunk of time redoing each of the 32 down-stream fibres to the new splitter but was happy to chat about it (i'd wandered out as our internet was down, and the main chamber with splitter is just around the corner from our house, in the expectation of someone working there).

Edited by deleted (Sat 08-Dec-18 19:16:28)

Thanks to everyone for your replies they have been very helpful and are much appreciated.

Why was he redoing those 32 splices, if the whole splitter is high, then it�s the splices before the splitter that are the issue, not those after.

In reply to a post by Zarjaz:

Why was he redoing those 32 splices, if the whole splitter is high, then it�s the splices before the splitter that are the issue, not those after.

He was replacing the splitter itself. Didn't look like a job for the feint hearted!

In reply to a post by kitcat:

The loss in the fibre is affected by the SFP used ( the bit that goes on the ends) these are rated for different distances. eg short medium long, SX,MX,LX (say up to 100meters,80m-10k , 8k to 100k etc.) and the quality of fibre. Look online if you want more technical detail on what is available now as my knowledge is slightly out of date in a fast moving field

Using a higher spec one at a shorter distance will blow the distant end and they should match both ends

No the SFP used does not effect the attenuation. The longer reaching and invariable more expensive SFP's use more powerful lasers, which is why they are more expensive. IT should be noted that the longer reaching SFP's present a laser safety risk and you should never look into the SFP and/or cable. Especially as they all oporate in the infre-red region and you can't see jack anyway, Use the camera from a mobile phone to see if you have light.

Also modern SFP's will reduce the laser power do you don't burn out the photodiode at the receiving end. Older GBIC's didn't do that so you often had to but in attenuators for testing purposes or if the link was not long enough etc.

So the component itself was faulty ? Interesting, you�d hope these items were tested before being sent out from the manufacturer wouldn�t you ?

Use the camera from a mobile phone to see if you have light.

...... or better still, put your light meter on it ??

True, but that requires going and digging the light meter out from wherever it is currently hiding, finding the right patch lead and possibly coupler and hoping the battery has not run out. Alternatively I can just whip my mobile out and test to see if I have light, and reserve the light meter for when I have light but I am still not getting link.

You mean you are installing some part of our FTTP circuits and not doing a proper light test? Do you work for Openreach, or one of the shoddy sub-contractors who are the cause of a high proportion of faults?

Such as perhaps the 32-connection splitter cockup here?

Edited by RobertoS (Mon 10-Dec-18 10:02:50)

[what Bob said]

dect,

Don't believe all the half-truths given here as answers. In the 1980s and 90s I worked professionally in the area of fibre-optics for data communications and even designed and specified composite fibre bundles myself. I can therefore inform you without hesitation that optical fibre does indeed have a per-unit distance loss. Indeed, all media, not just optical media, exhibit unit distance loss in the propagation of electromagnetic energy. There's a formula for it, based on the speed-of-light in a vacuum and which has a multiplication factor in the case of optical fibre that takes into account the refractive index of the core, the impurities in the medium, and unit distance losses through the cladding. Back in the 80s/90s, so-called singlemode fibre had a typical loss of around 0.5dB max per km, though this did, and does, also depend on wavelength. I don't think much has changed in that respect in the interim. Maybe that figure has, with more advanced manufacturing over the years, dropped a little? A good-quality singlemode, which certainly in those days was being used by BT for longhaul datacomms/phonelines, was perhaps 0.2 - 0.35dB per km. Transit losses in the fibre itself have nothing to do with launch power, of course.

All fibre splices and all jointing that involves optical connectors also have associated losses, and even a good-quality, in-the-field fusion-splice will render an attenuation loss of typically 0.2dB, and mated screw-together connectors typically 0.5dB. Yes, I had experience doing fusion-splicing too.

Engineers with the requisite professional equipment can check the integrity of an optical line by launching a calibrated optical test signal into it and viewing the graphical results on an optical time-domain reflectometer (OTDR). The latter is a piece of sophisticated kit vaguely resembling a portable oscilloscope. Essentially, the OTDR plots the attenuation (loss of the light energy) of the light-signal (of a particular wavelength) against distance, so you can see precisely what's happening along the entire length. Beside the unit distance loss of the fibre itself, the TDR will also show in the plot each fusion-splice loss as a small step. Calibrated scaling on the TDR enables the engineer to read off either the individual losses or the overall end-to-end loss. Prior to the advent of fibre, ie. the days of communication on long lines that involved copper-only, there were just 'plain vanilla' TDR's for copper lines. BT would use them for finding exactly where jointing issues in copper lines existed in hard-to-access situations. In fact, later fibre-based ones were just an optical version of the same principle being employed, that of transmission-line theory.

Although fusion-splicing has become more common these days it still requires a good deal of skill to lessen the misalignment between the two butt ends of the fibre at the required joint. The fibre ends need careful preparation too, as the fibre will have a micro-thin inherent cladding or outer coating on it and this needs to be stripped off at the proposed joint and thoroughly cleaned, before setting the two butt ends into a jig on the splicer, that holds them in place for their fusing together. Any contamination of the joint, as it's being done, results in a high loss and bad joint that then has to be re-done. The operator uses a microscope to check the cleanliness of the ends and to see that they're in optimum alignment in the jig, before pressing the button to evoke an 'arc-welding' of the two butt-ends. To protect the joint from thereon in, a cover already slid on to the fibre is then brought over the finished joint and is crimped on or otherwise sealed on.

Hope this puts you in the picture.

In reply to a post by RobertoS:

You mean you are installing some part of our FTTP circuits and not doing a proper light test? Do you work for Openreach, or one of the shoddy sub-contractors who are the cause of a high proportion of faults?

Such as perhaps the 32-connection splitter cockup here?

Nope I work as an admin for a HPC (otherwise known as a supercomputer) at a UK university. I have been working with fibre optic for ethernet, fibre channel and Infiniband now for around 15 years.

What I find is that if I have an issue with link and I am not sure if I am getting light or whether the patch lead is a straight or crossover then a mobile phone is great for a quick check. Much quicker than going and digging the light meter out for an actual loss measurement. Most of the time the issue is either a duff patch cable, a patch cable is not seated properly or a dead laser.

The closest I have come to a fibre install was stringing some pre-terminated 24 core cable between a couple of rows of racks and that was about a decade ago. On the other hand I regularly deal with fibre optic links. Right now I am about to go over to the data centre to try and work out why my new 30m active optic 40Gbps QSFP leads don't work. They where supposed to replace the hack with two 15m ones and an Infiniband switch in the middle but I am not getting link I am hoping I have not damage the cables but they allegedly support a 7.5mm minimum bend radius which I am confident I have not exceeded installing them.

Hi meditator

Many thanks for your detailed response, it was a very interesting read and has given me a better understand.

I think the point is that sure the signal degrades over distance with fibre optic. However unlike xDSL technology where the degraded signal leads to a lower data rate, with a fibre optic link you either have the link and it operates at full speed, or you don't have a link at all. Further given the much lower losses of the signal with single mode fibre you can be 20km from the exchange and still get full speed. This is a fundamental difference and why people tend to describe fibre links as suffering no loss.

However note it is not quite true that you either get link or you don't. If you are right on the cusp of signal loss you may get link but then suffer a lot of RX/TX errors and only get a fraction of the advertised link speeds. Don't ask how I know...