I was just wondering if FTTP/H can be delivered by overhead lines, or does it always get delivered underground?
Here's one (first picture below) of BT's access infrastructure pictures for FTTP. This particular one is relatively recent, shows 2 levels of splitter, and a lot of aerial fibre in different forms:
- 96-fibre lightweight aerial cable, with a figure-8 configuration, where a steel strengthening element runs in a parallel sheath
- 36-fibre ultra-lightweight aerial cable, where the steel strengthening elements run within the sheath
- aerial blown fibre tubing, with integral strengthening elements.
This shows a lot of integration with pole-mounted boxes too ...
I'm thinking its underground only?
I think there are two separate elements of the existing network to consider:
- Distribution lines: How does the 25-100-pair cable get from the PCP to the DP in the street outside your house
- Drop lines: How does the 1-2-pair cable get from the DP to your house.
Either of these can be underground or overhead; it partially depends on the age and location as to the method chosen.
The combinations you tend to get are:
- Underground distribution (cables run chamber -> chamber -> chamber) with overhead drop lines (pole next to chamber; DP at top of pole; aerial drop-line runs overhead)
- Underground distribution, with underground drop-lines (strictly called lead-in wires). Ducting to hoes can be a number of forms
- Overhead distribution (cables run pole -> pole -> pole) with overhead drop lines.
Distribution can be a mix, swapping from underground to overhead as conditions require, or as later segments of the network preferred different ways of working.
The first picture shows a lot of options on the distribution side.
The second (rather older) infrastructure diagram shows more of the original thinking, but also shows the different drop-wire/lead-in wire options.
What if BT/OR slowly changed the overhead lines on the poles to fibre.. then could a FTTP/H be delivered that way?
They don't have authorisation to remove copper, so they have to
add fibre instead of swapping copper out.
That is fine, and possible: we have some photo's from exactly this kind of deployment. See
this post, and the link to imgur. When you see the photos of the pole-top, you'll see the thick cables running off left and right - those are copper. You'll also see thinner cables running left and right too, which are fibre. The remaining photos show the various cables dropping down the pole.
The downside of adding fibre to the poles ... is that poles have engineering limits on the amount of force they can be put on by large number of cables. If a pole is full, or full on one-side, then no fibre can be added.
If they hit that restriction on the drop-wire, I believe they have an option to remove the old copper drop-wire, and replace it with one that has both copper and fibre. I don't think there are any options if the pole is fully-loaded with distribution cables.
Hmm.. well with that information.. I wonder why BT don't work on improving the overhead system of installation. Less planning permission and better access to any maintenance too.
A couple of reasons:
- You might get better access for maintenance, but you need more maintenance. Being out in the open, ravaged by the wind and sun, makes the cable more likely to suffer from damage. Underground is better protected.
- I'm not sure it really is less planning permission. Planning requirements have gradually required all infrastructure to go underground as much as possible. There has been a temporary softening from the government to allow broadband infrastructure use poles, but that doesn't changes people's attitudes.
People don't like overhead infrastructure. Do you?
Two images of Austin, Texas...
-
https://goo.gl/maps/TFy4umXu9842-
https://goo.gl/maps/bbxqhfPCoWkThe underground ducts could be used for installing power cables to backup the cabinets and supply power. Maybe they could team up with the electricity companies to do this.
So you'd now need to both clear and fix the underground ducts, *and* set up the fibre in the most maintenance-intensive way? I'm not sure that is the best option.
As for co-operation with electricity companies ... there are a lot of "shared use" poles around, with both mains electricity and copper. Unfortunately, it makes for much stricter access conditions to be able to do any maintenance work, and also seem to be banned from any new infrastructure. Is it worth long-term pain for the short-term gain?
I understand that there are starting to be some agreements to allow fibre onto these shared-use poles, so there is obviously a changing belief that it is worth doing.
For the BT historians, I'd be interested to know, which method came first, overhead or underground telephone lines?
For people who love details, there has been a study done by Analysys Mason to figure out spare capacity in ducts and poles (for PIA purposes), but it gives a good picture of the way that ducts and poles have been used historically.
See
This survey document.
Pages 10-12 show the different types of distribution, showing that overhead
distribution, tended to be not used for new work after 1968, but that overhead
drop-wires continue to today.
Page 16 tells us that there are 24% of DPs are underground, and 47% are overhead - but it doesn't break the overhead ones down into the ways that the distribution is fed.
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